I. Colin Prentice

Land use has generally been considered a local environmental issue, but it is becoming a force of global importance. Worldwide changes to forests, farmlands, waterways, and air are being driven by the need to provide food, fiber, water, and shelter to more than six billion people. Global croplands, pastures, plantations, and urban areas have expanded in recent decades, accompanied by large increases in energy, water, and fertilizer consumption, along with considerable losses of biodiversity. Such changes in land use have enabled humans to appropriate an increasing share of the planet's resources, but they also potentially undermine the capacity of ecosystems to sustain food production, maintain freshwater and forest resources, regulate climate and air quality, and ameliorate infectious diseases. We face the challenge of managing trade-offs between immediate human needs and maintaining the capacity of the biosphere to provide goods and services in the long term.

The Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ) combines process-based, large-scale representations of terrestrial vegetation dynamics and land-atmosphere carbon and water exchanges in a modular framework. Features include feedback through canopy conductance between photosynthesis and transpiration and interactive coupling between these ‘fast’ processes and other ecosystem processes including resource competition, tissue turnover, population dynamics, soil organic matter and litter dynamics and fire disturbance. Ten plants functional types (PFTs) are differentiated by physiological, morphological, phenological, bioclimatic and fire-response attributes. Resource competition and differential responses to fire between PFTs influence their relative fractional cover from year to year. Photosynthesis, evapotranspiration and soil water dynamics are modelled on a daily time step, while vegetation structure and PFT population densities are updated annually. Simulations have been made over the industrial period both for specific sites where field measurements were available for model evaluation, and globally on a 0.5°° × 0.5°° grid. Modelled vegetation patterns are consistent with observations, including remotely sensed vegetation structure and phenology. Seasonal cycles of net ecosystem exchange and soil moisture compare well with local measurements. Global carbon exchange fields used as input to an atmospheric tracer transport model (TM2) provided a good fit to observed seasonal cycles of CO2 concentration at all latitudes. Simulated inter-annual variability of the global terrestrial carbon balance is in phase with and comparable in amplitude to observed variability in the growth rate of atmospheric CO2. Global terrestrial carbon and water cycle parameters (pool sizes and fluxes) lie within their accepted ranges. The model is being used to study past, present and future terrestrial ecosystem dynamics, biochemical and biophysical interactions between ecosystems and the atmosphere, and as a component of coupled Earth system models.

A model to predict global patterns in vegetation physiognomy was developed from physiological considera- tions influencing the distributions of different functional types of plant. Primary driving variables are mean coldest- month temperature, annual accumulated temeprature over 5C, and a drought index incorporating the seasonality of precipitation and the available water capacity of the soil. The model predicts which plant types can occur in a given environment, and selects the potentially dominant types from among them. Biomes arise as combinations of domi- nant types. Global environmental data were supplied as monthly means of temperature, precipitation and sunshine (interpolated to a global 0.5 grid, with a lapse-rate correc-

This work presents a new dynamic global vegetation model designed as an extension of an existing surface‐vegetation‐atmosphere transfer scheme which is included in a coupled ocean‐atmosphere general circulation model. The new dynamic global vegetation model simulates the principal processes of the continental biosphere influencing the global carbon cycle (photosynthesis, autotrophic and heterotrophic respiration of plants and in soils, fire, etc.) as well as latent, sensible, and kinetic energy exchanges at the surface of soils and plants. As a dynamic vegetation model, it explicitly represents competitive processes such as light competition, sapling establishment, etc. It can thus be used in simulations for the study of feedbacks between transient climate and vegetation cover changes, but it can also be used with a prescribed vegetation distribution. The whole seasonal phenological cycle is prognostically calculated without any prescribed dates or use of satellite data. The model is coupled to the IPSL‐CM4 coupled atmosphere‐ocean‐vegetation model. Carbon and surface energy fluxes from the coupled hydrology‐vegetation model compare well with observations at FluxNet sites. Simulated vegetation distribution and leaf density in a global simulation are evaluated against observations, and carbon stocks and fluxes are compared to available estimates, with satisfying results.

Summary The possible responses of ecosystem processes to rising atmospheric CO 2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics. The models are driven by the IPCC IS92a scenario of rising CO 2 ( Wigley et al . 1991 ), and by climate changes resulting from effective CO 2 concentrations corresponding to IS92a, simulated by the coupled ocean atmosphere model HadCM2‐SUL. Simulations with changing CO 2 alone show a widely distributed terrestrial carbon sink of 1.4–3.8 Pg C y −1 during the 1990s, rising to 3.7–8.6 Pg C y −1 a century later. Simulations including climate change show a reduced sink both today (0.6–3.0 Pg C y −1 ) and a century later (0.3–6.6 Pg C y −1 ) as a result of the impacts of climate change on NEP of tropical and southern hemisphere ecosystems. In all models, the rate of increase of NEP begins to level off around 2030 as a consequence of the ‘diminishing return’ of physiological CO 2 effects at high CO 2 concentrations. Four out of the six models show a further, climate‐induced decline in NEP resulting from increased heterotrophic respiration and declining tropical NPP after 2050. Changes in vegetation structure influence the magnitude and spatial pattern of the carbon sink and, in combination with changing climate, also freshwater availability (runoff). It is shown that these changes, once set in motion, would continue to evolve for at least a century even if atmospheric CO 2 concentration and climate could be instantaneously stabilized. The results should be considered illustrative in the sense that the choice of CO 2 concentration scenario was arbitrary and only one climate model scenario was used. However, the results serve to indicate a range of possible biospheric responses to CO 2 and climate change. They reveal major uncertainties about the response of NEP to climate change resulting, primarily, from differences in the way that modelled global NPP responds to a changing climate. The simulations illustrate, however, that the magnitude of possible biospheric influences on the carbon balance requires that this factor is taken into account for future scenarios of atmospheric CO 2 and climate change.

Global change will alter the supply of ecosystem services that are vital for human well-being. To investigate ecosystem service supply during the 21st century, we used a range of ecosystem models and scenarios of climate and land-use change to conduct a Europe-wide assessment. Large changes in climate and land use typically resulted in large changes in ecosystem service supply. Some of these trends may be positive (for example, increases in forest area and productivity) or offer opportunities (for example, "surplus land" for agricultural extensification and bioenergy production). However, many changes increase vulnerability as a result of a decreasing supply of ecosystem services (for example, declining soil fertility, declining water availability, increasing risk of forest fires), especially in the Mediterranean and mountain regions.

The last 6000 years are of particular interest to the understanding of the Earth System because the boundary conditions of the climate system did not change dramatically (in comparison to larger glacial–interglacial changes), and because abundant, detailed regional palaeoclimatic proxy records cover this period. We use selected proxy-based reconstructions of different climate variables, together with state-of-the-art time series of natural forcings (orbital variations, solar activity variations, large tropical volcanic eruptions, land cover and greenhouse gases), underpinned by results from General Circulation Models (GCMs) and Earth System Models of Intermediate Complexity (EMICs), to establish a comprehensive explanatory framework for climate changes from the Mid-Holocene (MH) to pre-industrial time. The redistribution of solar energy, due to orbital forcing on a millennial timescale, was the cause of a progressive southward shift of the Northern Hemisphere (NH) summer position of the Intertropical Convergence Zone (ITCZ). This was accompanied by a pronounced weakening of the monsoon systems in Africa and Asia and increasing dryness and desertification on both continents. The associated summertime cooling of the NH, combined with changing temperature gradients in the world oceans, likely led to an increasing amplitude of the El Niño Southern Oscillation (ENSO) and, possibly, increasingly negative North Atlantic Oscillation (NAO) indices up to the beginning of the last millennium. On decadal to multi-century timescales, a worldwide coincidence between solar irradiance minima, tropical volcanic eruptions and decadal to multi-century scale cooling events was not found. However, reconstructions show that widespread decadal to multi-century scale cooling events, accompanied by advances of mountain glaciers, occurred in the NH (e.g., in Scandinavia and the European Alps). This occurred namely during the Little Ice Age (LIA) between AD ∼1350 and 1850, when the lower summer insolation in the NH, due to orbital forcing, coincided with solar activity minima and several strong tropical volcanic eruptions. The role of orbital forcing in the NH cooling, the southward ITCZ shift and the desertification of the Sahara are supported by numerous model simulations. Other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics, but there is also evidence that changes in the ocean heat transport took place during the LIA at high northern latitudes, with possible additional implications for climates of the Southern Hemisphere (SH).

Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

Here we present a new terrestrial biosphere model (the Integrated Biosphere Simulator ‐ IBIS) which demonstrates how land surface biophysics, terrestrial carbon fluxes, and global vegetation dynamics can be represented in a single, physically consistent modeling framework. In order to integrate a wide range of biophysical, physiological, and ecological processes, the model is designed around a hierarchical, modular structure and uses a common state description throughout. First, a coupled simulation of the surface water, energy, and carbon fluxes is performed on hourly timesteps and is integrated over the year to estimate the annual water and carbon balance. Next, the annual carbon balance is used to predict changes in the leaf area index and biomass for each of nine plant functional types, which compete for light and water using different ecological strategies. The resulting patterns of annual evapotranspiration, runoff, and net primary productivity are in good agreement with observations. In addition, the model simulates patterns of vegetation dynamics that qualitatively agree with features of the natural process of secondary succession. Comparison of the model's inferred near‐equilibrium vegetation categories with a potential natural vegetation map shows a fair degree of agreement. This integrated modeling framework provides a means of simulating both rapid biophysical processes and long‐term ecosystem dynamics that can be directly incorporated within atmospheric models.

Abstract This study tests the ability of five Dynamic Global Vegetation Models (DGVMs), forced with observed climatology and atmospheric CO 2 , to model the contemporary global carbon cycle. The DGVMs are also coupled to a fast ‘climate analogue model’, based on the Hadley Centre General Circulation Model (GCM), and run into the future for four Special Report Emission Scenarios (SRES): A1FI, A2, B1, B2. Results show that all DGVMs are consistent with the contemporary global land carbon budget. Under the more extreme projections of future environmental change, the responses of the DGVMs diverge markedly. In particular, large uncertainties are associated with the response of tropical vegetation to drought and boreal ecosystems to elevated temperatures and changing soil moisture status. The DGVMs show more divergence in their response to regional changes in climate than to increases in atmospheric CO 2 content. All models simulate a release of land carbon in response to climate, when physiological effects of elevated atmospheric CO 2 on plant production are not considered, implying a positive terrestrial climate‐carbon cycle feedback. All DGVMs simulate a reduction in global net primary production (NPP) and a decrease in soil residence time in the tropics and extra‐tropics in response to future climate. When both counteracting effects of climate and atmospheric CO 2 on ecosystem function are considered, all the DGVMs simulate cumulative net land carbon uptake over the 21st century for the four SRES emission scenarios. However, for the most extreme A1FI emissions scenario, three out of five DGVMs simulate an annual net source of CO 2 from the land to the atmosphere in the final decades of the 21st century. For this scenario, cumulative land uptake differs by 494 Pg C among DGVMs over the 21st century. This uncertainty is equivalent to over 50 years of anthropogenic emissions at current levels.

The equilibrium terrestrial biosphere model BIOME3 simulates vegetation distribution and biogeochemistry, and couples vegetation distribution directly to biogeochemistry. Model inputs consist of latitude, soil texture class, and monthly climate (temperature, precipitation, and sunshine) data on a 0.5° grid. Ecophysiological constraints determine which plant functional types (PFTs) may potentially occur. A coupled carbon and water flux model is then used to calculate, for each PFT, the leaf area index (LAI) that maximizes net primary production (NPP), subject to the constraint that NPP must be sufficient to maintain this LAI. Competition between PFTs is simulated by using the optimal NPP of each PFT as an index of competitiveness, with additional rules to approximate the dynamic equilibrium between natural disturbance and succession driven by light competition. Model output consists of a quantitative vegetation state description in terms of the dominant PFT, secondary PFTs present, and the total LAI and NPP for the ecosystem. Canopy conductance is treated as a function of the calculated optimal photosynthetic rate and water stress. Regional evapotranspiration is calculated as a function of canopy conductance, equilibrium evapotranspiration rate, and soil moisture using a simple planetary boundary layer parameterization. This scheme results in a two‐way coupling of the carbon and water fluxes through canopy conductance, allowing simulation of the response of photosynthesis, stomatal conductance, and leaf area to environmental factors including atmospheric CO 2 . Comparison with the mapped distribution of global vegetation shows that the model successfully reproduces the broad‐scale patterns in potential natural vegetation distribution. Comparison with NPP measurements, and with an FPAR (fractional absorbed photosynthetically active radiation) climatology based on remotely sensed greenness measurements, provides further checks on the model's internal logic. The model is envisaged as a tool for integrated analysis of the impacts of changes in climate and CO 2 on ecosystem structure and function.

The concurrent effects of increasing atmospheric CO 2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process‐based terrestrial biosphere models. Over the long‐term(1920–1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long‐term analysis based on ice core and atmospheric CO 2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO 2 . During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr −1 , which is within the uncertainty of analysis based on CO 2 and O 2 budgets. Three of the four models indicated (in accordance with O 2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long‐term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO 2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Niño/Southern Oscillation (ENSO)‐scale variability in the atmospheric CO 2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO 2 suggested that the observed trend may be a consequence of CO 2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process‐based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand‐level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO 2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system.

Abstract Ecosystem processes are important determinants of the biogeochemistry of the ocean, and they can be profoundly affected by changes in climate. Ocean models currently express ecosystem processes through empirically derived parameterizations that tightly link key geochemical tracers to ocean physics. The explicit inclusion of ecosystem processes in models will permit ecological changes to be taken into account, and will allow us to address several important questions, including the causes of observed glacial–interglacial changes in atmospheric trace gases and aerosols, and how the oceanic uptake of CO 2 is likely to change in the future. There is an urgent need to assess our mechanistic understanding of the environmental factors that exert control over marine ecosystems, and to represent their natural complexity based on theoretical understanding. We present a prototype design for a Dynamic Green Ocean Model (DGOM) based on the identification of (a) key plankton functional types that need to be simulated explicitly to capture important biogeochemical processes in the ocean; (b) key processes controlling the growth and mortality of these functional types and hence their interactions; and (c) sources of information necessary to parameterize each of these processes within a modeling framework. We also develop a strategy for model evaluation, based on simulation of both past and present mean state and variability, and identify potential sources of validation data for each. Finally, we present a DGOM‐based strategy for addressing key questions in ocean biogeochemistry. This paper thus presents ongoing work in ocean biogeochemical modeling, which, it is hoped will motivate international collaborations to improve our understanding of the role of the ocean in the climate system.

Abstract BIOME 6000 is an international project to map vegetation globally at mid‐Holocene (6000 14 C yr bp ) and last glacial maximum (LGM, 18,000 14 C yr bp ), with a view to evaluating coupled climate‐biosphere model results. Primary palaeoecological data are assigned to biomes using an explicit algorithm based on plant functional types. This paper introduces the second Special Feature on BIOME 6000. Site‐based global biome maps are shown with data from North America, Eurasia (except South and Southeast Asia) and Africa at both time periods. A map based on surface samples shows the method’s skill in reconstructing present‐day biomes. Cold and dry conditions at LGM favoured extensive tundra and steppe. These biomes intergraded in northern Eurasia. Northern hemisphere forest biomes were displaced southward. Boreal evergreen forests (taiga) and temperate deciduous forests were fragmented, while European and East Asian steppes were greatly extended. Tropical moist forests (i.e. tropical rain forest and tropical seasonal forest) in Africa were reduced. In south‐western North America, desert and steppe were replaced by open conifer woodland, opposite to the general arid trend but consistent with modelled southward displacement of the jet stream. The Arctic forest limit was shifted slighly north at 6000 14 C yr bp in some sectors, but not in all. Northern temperate forest zones were generally shifted greater distances north. Warmer winters as well as summers in several regions are required to explain these shifts. Temperate deciduous forests in Europe were greatly extended, into the Mediterranean region as well as to the north. Steppe encroached on forest biomes in interior North America, but not in central Asia. Enhanced monsoons extended forest biomes in China inland and Sahelian vegetation into the Sahara while the African tropical rain forest was also reduced, consistent with a modelled northward shift of the ITCZ and a more seasonal climate in the equatorial zone. Palaeobiome maps show the outcome of separate, independent migrations of plant taxa in response to climate change. The average composition of biomes at LGM was often markedly different from today. Refugia for the temperate deciduous and tropical rain forest biomes may have existed offshore at LGM, but their characteristic taxa also persisted as components of other biomes. Examples include temperate deciduous trees that survived in cool mixed forest in eastern Europe, and tropical evergreen trees that survived in tropical seasonal forest in Africa. The sequence of biome shifts during a glacial‐interglacial cycle may help account for some disjunct distributions of plant taxa. For example, the now‐arid Saharan mountains may have linked Mediterranean and African tropical montane floras during enhanced monsoon regimes. Major changes in physical land‐surface conditions, shown by the palaeobiome data, have implications for the global climate. The data can be used directly to evaluate the output of coupled atmosphere‐biosphere models. The data could also be objectively generalized to yield realistic gridded land‐surface maps, for use in sensitivity experiments with atmospheric models. Recent analyses of vegetation‐climate feedbacks have focused on the hypothesized positive feedback effects of climate‐induced vegetation changes in the Sahara/Sahel region and the Arctic during the mid‐Holocene. However, a far wider spectrum of interactions potentially exists and could be investigated, using these data, both for 6000 14 C yr bp and for the LGM.

Subfossil pollen and plant macrofossil data derived from 14C-dated sediment profiles can provide quantitative information on glacial and interglacial climates. The data allow climate variables related to growing-season warmth, winter cold, and plant-available moisture to be reconstructed. Continental-scale reconstructions have been made for the mid-Holocene (MH, around 6 ka) and Last Glacial Maximum (LGM, around 21 ka), allowing comparison with palaeoclimate simulations currently being carried out as part of the fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change. The synthesis of the available MH and LGM climate reconstructions and their uncertainties, obtained using modern-analogue, regression and model-inversion techniques, is presented for four temperature variables and two moisture variables. Reconstructions of the same variables based on surface-pollen assemblages are shown to be accurate and unbiased. Reconstructed LGM and MH climate anomaly patterns are coherent, consistent between variables, and robust with respect to the choice of technique. They support a conceptual model of the controls of Late Quaternary climate change whereby the first-order effects of orbital variations and greenhouse forcing on the seasonal cycle of temperature are predictably modified by responses of the atmospheric circulation and surface energy balance.

1 Advances in dynamic ecosystem modelling have made a number of different approaches to vegetation dynamics possible. Here we compare two models representing contrasting degrees of abstraction of the processes governing dynamics in real vegetation. 2 Model (a) (GUESS) simulates explicitly growth and competition among individual plants. Differences in crown structure (height, depth, area and LAI) influence relative light uptake by neighbours. Assimilated carbon is allocated individually by each plant to its leaf, fine root and sapwood tissues. Carbon allocation and turnover of sapwood to heartwood in turn govern height and diameter growth. 3 Model (b) (LPJ) incorporates a ‘dynamic global vegetation model’ (DGVM) architecture, simulating growth of populations of plant functional types (PFTs) over a grid cell, integrating individual-level processes over the proportional area (foliar projective cover, FPC) occupied by each PFT. Individual plants are not simulated, but are replaced by explicit parameterizations of their growth and interactions. 4 The models are identical in their representation of core physiological and biogeochemical processes. Both also use the same set of PFTs, corresponding to the major woody plant groups in Europe, plus a grass type. 5 When applied at a range of locations, broadly spanning climatic variation within Europe, both models successfully predicted PFT composition and succession within modern natural vegetation. However, the individual-based model performed better in areas where deciduous and evergreen types coincide, and in areas subject to pronounced seasonal water deficits, which would tend to favour grasses over drought-intolerant trees. 6 Differences in model performance could be traced to their treatment of individual-level processes, in particular light competition and stress-induced mortality. 7 Our results suggest that an explicit individual-based approach to vegetation dynamics may be an advantage in modelling of ecosystem structure and function at the resolution required for regional- to continental-scale studies.

We present a model of the dust cycle that successfully predicts dust emissions as determined by land surface properties, monthly vegetation and snow cover, and 6‐hourly surface wind speeds for the years 1982–1993. The model takes account of the role of dry lake beds as preferential source areas for dust emission. The occurrence of these preferential sources is determined by a water routing and storage model. The dust source scheme also explicitly takes into account the role of vegetation type as well as monthly vegetation cover. Dust transport is computed using assimilated winds for the years 1987–1990. Deposition of dust occurs through dry and wet deposition, where subcloud scavenging is calculated using assimilated precipitation fields. Comparison of simulated patterns of atmospheric dust loading with the Total Ozone Mapping Spectrometer satellite absorbing aerosol index shows that the model produces realistic results from daily to interannual timescales. The magnitude of dust deposition agrees well with sediment flux data from marine sites. Emission of submicron dust from preferential source areas are required for the computation of a realistic dust optical thickness. Sensitivity studies show that Asian dust source strengths are particularly sensitive to the seasonality of vegetation cover.

Forests are likely to show complex transient responses to rapid changes in climate. The model described here simulates the dynamics of forest landscapes in a changing environment with simple phenomenological equations for tree growth processes and local environmental feedbacks. Tree establishment and growth rates are modified by species-specific functions describing the effects of winter and summer temperature limitations, accumulated annual foliage net assimilation and sapwood respiration as functions of temperature, CO2 fertilization, and growing-season drought. These functions provide external conditions for the simulation of patch-scale forest dynamics by a forest succession model, FORSKA, in which all of the trees on each 0.1 ha patch interact by competition for light and nutrients. The landscape is simulated as an array of such patches. The probability of disturbance on a patch is a power function of time since disturbance. Forest structure, composition and biomass simulated for the landscape average in boreal and temperate deciduous forests approach reasonable equilibrium values in 200–400 years. A climatic warning scenario is applied to central Sweden, where temperature and precipitation increases are shown to interact with each other and with soil water capacity in determining the transient and equilibrium responses of the forest landscape to climate change.

Response surfaces describing the empirical dependence of surface pollen percentages of 13 taxa on three standard climatic variables (mean July temperature, mean January temperature, and mean annual precipitation) in eastern North America were used to infer past climates from palynological data. Inferred climates at 3000—yr intervals from 18 000 years ago to the present, based on six taxa (spruce, birch, northern pines, oak, southern pines, and prairie forbs), were used to generate time series of simulated isopoll maps for these taxa and seven others (hickory, fir, beech, hemlock, elm, alder, and sedge). The simulations captured the essential features of the observed isopoll maps for both sets of taxa, including differences in migration patterns during the past 10 000 yr that have previously been attributed to differential migration lag. These results establish that the continental—scale vegetation patterns have responded to continuous changes in climate from the last glacial maximum to the present, with lags ≤ 1500 yr. The inferred climatic changes include seasonality changes consistent with orbitally controlled changes in insolation, and shifts in temperature and moisture gradients that are consistent with modelled climatic interactions of the insolation changes with the shrinking Laurentide ice sheet. These results pose new ecological questions about the processes by which vegetated landscapes approach dynamic equilibrium with their changing environment.

A coupled physical‐biogeochemical climate model that includes a dynamic global vegetation model and a representation of a coupled atmosphere‐ocean general circulation model is driven by the nonintervention emission scenarios recently developed by the Intergovernmental Panel on Climate Change (IPCC). Atmospheric CO 2 , carbon sinks, radiative forcing by greenhouse gases (GHGs) and aerosols, changes in the fields of surface‐air temperature, precipitation, cloud cover, ocean thermal expansion, and vegetation structure are projected. Up to 2100, atmospheric CO 2 increases to 540 ppm for the lowest and to 960 ppm for the highest emission scenario analyzed. Sensitivity analyses suggest an uncertainty in these projections of −10 to +30% for a given emission scenario. Radiative forcing is estimated to increase between 3 and 8 W m −2 between now and 2100. Simulated warmer conditions in North America and Eurasia affect ecosystem structure: boreal trees expand poleward in high latitudes and are partly replaced by temperate trees and grasses at lower latitudes. The consequences for terrestrial carbon storage depend on the assumed sensitivity of climate to radiative forcing, the sensitivity of soil respiration to temperature, and the rate of increase in radiative forcing by both CO 2 and other GHGs. In the most extreme cases, the terrestrial biosphere becomes a source of carbon during the second half of the century. High GHG emissions and high contributions of non‐CO 2 agents to radiative forcing favor a transient terrestrial carbon source by enhancing warming and the associated release of soil carbon.

1. Net primary production (NPP) by terrestrial ecosystems appears to be proportional to absorbed photosynthetically active radiation (APAR) on a seasonal and annual basis. This observation has been used in 'diagnostic' models that estimate NPP from remotely sensed vegetation indices. In 'prognostic' process-based models carbon fluxes are more commonly integrated with respect to leaf area index assuming invariant leaf photosynthetic parameters. This approach does not lead to a proportional relationship between NPP and APAR. However, leaf nitrogen content and Rubisco activity are known to vary seasonally and with canopy position, and there is evidence that this variation takes place in such a way as to nearly optimize total canopy net photosynthesis. 2. Using standard formulations for the instantaneous response of leaf net photosynthesis to APAR, we show that the optimized canopy net photosynthesis is proportional to APAR. This theory leads to reasonable values for the maximum (unstressed) light-use efficiency of gross and net primary production of C-3 plants at current ambient CO2, comparable with empirical estimates for agricultural crops and forest plantations. 3. By relating the standard formulations to the Collatz-Farquhar model of photosynthesis, we show that a range of observed physiological responses to temperature and CO2 can be understood as consequences of the optimization. These responses include the CO2 fertilization response and stomatal closure in C-3 plants, the increase of leaf N concentration with decreasing growing season temperature, and the downward acclimation of leaf respiration and N content with increasing ambient CO2. The theory provides a way to integrate diverse experimental observations into a general framework for modelling terrestrial primary production. (Less)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid‐Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

Abstract Pollen data from China for 6000 and 18,000 14 C yr bp were compiled and used to reconstruct palaeovegetation patterns, using complete taxon lists where possible and a biomization procedure that entailed the assignment of 645 pollen taxa to plant functional types. A set of 658 modern pollen samples spanning all biomes and regions provided a comprehensive test for this procedure and showed convincing agreement between reconstructed biomes and present natural vegetation types, both geographically and in terms of the elevation gradients in mountain regions of north‐eastern and south‐western China. The 6000 14 C yr bp map confirms earlier studies in showing that the forest biomes in eastern China were systematically shifted northwards and extended westwards during the mid‐Holocene. Tropical rain forest occurred on mainland China at sites characterized today by either tropical seasonal or broadleaved evergreen/warm mixed forest. Broadleaved evergreen/warm mixed forest occurred further north than today, and at higher elevation sites within the modern latitudinal range of this biome. The northern limit of temperate deciduous forest was shifted c. 800 km north relative to today. The 18,000 14 C yr bp map shows that steppe and even desert vegetation extended to the modern coast of eastern China at the last glacial maximum, replacing today’s temperate deciduous forest. Tropical forests were excluded from China and broadleaved evergreen/warm mixed forest had retreated to tropical latitudes, while taiga extended southwards to c . 43°N.

Abstract Advances in dynamic ecosystem modelling have made a number of different approaches to vegetation dynamics possible. Here we compare two models representing contrasting degrees of abstraction of the processes governing dynamics in real vegetation. Model (a) (GUESS) simulates explicitly growth and competition among individual plants. Differences in crown structure (height, depth, area and LAI) influence relative light uptake by neighbours. Assimilated carbon is allocated individually by each plant to its leaf, fine root and sapwood tissues. Carbon allocation and turnover of sapwood to heartwood in turn govern height and diameter growth. Model (b) (LPJ) incorporates a ‘dynamic global vegetation model’ (DGVM) architecture, simulating growth of populations of plant functional types (PFTs) over a grid cell, integrating individual‐level processes over the proportional area (foliar projective cover, FPC) occupied by each PFT. Individual plants are not simulated, but are replaced by explicit parameterizations of their growth and interactions. The models are identical in their representation of core physiological and biogeochemical processes. Both also use the same set of PFTs, corresponding to the major woody plant groups in Europe, plus a grass type. When applied at a range of locations, broadly spanning climatic variation within Europe, both models successfully predicted PFT composition and succession within modern natural vegetation. However, the individual‐based model performed better in areas where deciduous and evergreen types coincide, and in areas subject to pronounced seasonal water deficits, which would tend to favour grasses over drought‐intolerant trees. Differences in model performance could be traced to their treatment of individual‐level processes, in particular light competition and stress‐induced mortality. Our results suggest that an explicit individual‐based approach to vegetation dynamics may be an advantage in modelling of ecosystem structure and function at the resolution required for regional‐ to continental‐scale studies.

Abstract The global vegetation response to climate and atmospheric CO 2 changes between the last glacial maximum and recent times is examined using an equilibrium vegetation model (BIOME4), driven by output from 17 climate simulations from the Palaeoclimate Modelling Intercomparison Project. Features common to all of the simulations include expansion of treeless vegetation in high northern latitudes; southward displacement and fragmentation of boreal and temperate forests; and expansion of drought‐tolerant biomes in the tropics. These features are broadly consistent with pollen‐based reconstructions of vegetation distribution at the last glacial maximum. Glacial vegetation in high latitudes reflects cold and dry conditions due to the low CO 2 concentration and the presence of large continental ice sheets. The extent of drought‐tolerant vegetation in tropical and subtropical latitudes reflects a generally drier low‐latitude climate. Comparisons of the observations with BIOME4 simulations, with and without consideration of the direct physiological effect of CO 2 concentration on C 3 photosynthesis, suggest an important additional role of low CO 2 concentration in restricting the extent of forests, especially in the tropics. Global forest cover was overestimated by all models when climate change alone was used to drive BIOME4, and estimated more accurately when physiological effects of CO 2 concentration were included. This result suggests that both CO 2 effects and climate effects were important in determining glacial‐interglacial changes in vegetation. More realistic simulations of glacial vegetation and climate will need to take into account the feedback effects of these structural and physiological changes on the climate.

Abstract. For the first time, a model that simulates methane emissions from northern peatlands is incorporated directly into a dynamic global vegetation model. The model, LPJ-WHyMe (LPJ Wetland Hydrology and Methane), was previously modified in order to simulate peatland hydrology, permafrost dynamics and peatland vegetation. LPJ-WHyMe simulates methane emissions using a mechanistic approach, although the use of some empirical relationships and parameters is unavoidable. The model simulates methane production, three pathways of methane transport (diffusion, plant-mediated transport and ebullition) and methane oxidation. A sensitivity test was conducted to identify the most important factors influencing methane emissions, followed by a parameter fitting exercise to find the best combination of parameter values for individual sites and over all sites. A comparison of model results to observations from seven sites resulted in normalised root mean square errors (NRMSE) of 0.40 to 1.15 when using the best site parameter combinations and 0.68 to 1.42 when using the best overall parameter combination.

Abstract. Natural methane (CH4) emissions from wet ecosystems are an important part of today's global CH4 budget. Climate affects the exchange of CH4 between ecosystems and the atmosphere by influencing CH4 production, oxidation, and transport in the soil. The net CH4 exchange depends on ecosystem hydrology, soil and vegetation characteristics. Here, the LPJ-WHyMe global dynamical vegetation model is used to simulate global net CH4 emissions for different ecosystems: northern peatlands (45°–90° N), naturally inundated wetlands (60° S–45° N), rice agriculture and wet mineral soils. Mineral soils are a potential CH4 sink, but can also be a source with the direction of the net exchange depending on soil moisture content. The geographical and seasonal distributions are evaluated against multi-dimensional atmospheric inversions for 2003–2005, using two independent four-dimensional variational assimilation systems. The atmospheric inversions are constrained by the atmospheric CH4 observations of the SCIAMACHY satellite instrument and global surface networks. Compared to LPJ-WHyMe the inversions result in a~significant reduction in the emissions from northern peatlands and suggest that LPJ-WHyMe maximum annual emissions peak about one month late. The inversions do not put strong constraints on the division of sources between inundated wetlands and wet mineral soils in the tropics. Based on the inversion results we diagnose model parameters in LPJ-WHyMe and simulate the surface exchange of CH4 over the period 1990–2008. Over the whole period we infer an increase of global ecosystem CH4 emissions of +1.11 Tg CH4 yr−1, not considering potential additional changes in wetland extent. The increase in simulated CH4 emissions is attributed to enhanced soil respiration resulting from the observed rise in land temperature and in atmospheric carbon dioxide that were used as input. The long-term decline of the atmospheric CH4 growth rate from 1990 to 2006 cannot be fully explained with the simulated ecosystem emissions. However, these emissions show an increasing trend of +3.62 Tg CH4 yr−1 over 2005–2008 which can partly explain the renewed increase in atmospheric CH4 concentration during recent years.

Climate is an important control on biomass burning, but the sensitivity of fire to changes in temperature and moisture balance has not been quantified. We analyze sedimentary charcoal records to show that the changes in fire regime over the past 21,000 yrs are predictable from changes in regional climates. Analyses of paleo‐ fire data show that fire increases monotonically with changes in temperature and peaks at intermediate moisture levels, and that temperature is quantitatively the most important driver of changes in biomass burning over the past 21,000 yrs. Given that a similar relationship between climate drivers and fire emerges from analyses of the interannual variability in biomass burning shown by remote‐sensing observations of month‐by‐month burnt area between 1996 and 2008, our results signal a serious cause for concern in the face of continuing global warming.

Biome reconstruction from pollen and plant macrofossil data provides an objective method to reconstruct past vegetation. Biomes for Africa and the Arabian peninsula have been mapped for 6000 years bp and provide a new standard for the evaluation of simulated palaeovegetation distributions. A test using modern pollen data shows the robustness of the biomization method, which is able to predict the major vegetation types with a high confidence level. The application of the procedure to the 6000 years data set (pollen and plant macrofossil analyses) shows systematic differences from the present that are consistent with the numerous previous regional and continental interpretations, while providing a more extensive and more objective basis for such interpretations. Madagascar, eastern, southern and central Africa show only minor changes in terms of biomes, compared to present. Major changes in biome distributions occur north of 15°N, with steppe in many low‐elevation sites that are now desert, and temperate xerophytic woods/scrub and warm mixed forest in the Saharan mountains. These shifts in biome distributions imply significant changes in climate, especially precipitation, between 6000 years and present, reflecting a change in monsoon extent combined with a southward expansion of Mediterranean influence.

Palaeoclimates across Europe for 6000 y BP were estimated from pollen data using the modern pollen analogue technique constrained with lake-level data. The constraint consists of restricting the set of modern pollen samples considered as analogues of the fossil samples to those locations where the implied change in annual precipitation minus evapotranspiration (P–E) is consistent with the regional change in moisture balance as indicated by lakes. An artificial neural network was used for the spatial interpolation of lake-level changes to the pollen sites, and for mapping palaeoclimate anomalies. The climate variables reconstructed were mean temperature of the coldest month (T c ), growing degree days above 5 °C (GDD), moisture availability expressed as the ratio of actual to equilibrium evapotranspiration (α), and P–E. The constraint improved the spatial coherency of the reconstructed palaeoclimate anomalies, especially for P–E. The reconstructions indicate clear spatial and seasonal patterns of Holocene climate change, which can provide a quantitative benchmark for the evaluation of palaeoclimate model simulations. Winter temperatures (T c ) were 1–3 K greater than present in the far N and NE of Europe, but 2–4 K less than present in the Mediterranean region. Summer warmth (GDD) was greater than present in NW Europe (by 400–800 K day at the highest elevations) and in the Alps, but >400 K day less than present at lower elevations in S Europe. P–E was 50–250 mm less than present in NW Europe and the Alps, but α was 10–15% greater than present in S Europe and P–E was 50–200 mm greater than present in S and E Europe.

Global change research needs data sets describing past states of the Earth system. Vegetation distributions for specified ‘time slices’ (with known forcings, such as changes in insolation patterns due to the Earth's orbital variations, changes in the extent of ice‐sheets, and changes in atmospheric trace‐gas composition) should provide a benchmark for coupled climate‐biosphere models. Pollen and macrofossil records from dated sediments give spatially extensive coverage of data on vegetation distribution changes. Applications of such data have been delayed by the lack of a global synthesis. The BIOME 6000 project of IGBP aims at a synthesis for 6000 years bp. Success depends on community‐wide participation for data compilation and quality assurance, and on a robust methodology for assigning palaeorecords to biomes. In the method summarized here, taxa are assigned to one or more plant functional types (PFTs) and biomes reconstructed using PFT‐based definitions. By involving regional experts in PFT assignments, one can combine data from different floras without compromising global consistency in biome assignments. This article introduces a series of articles that substantially extend the BIOME 6000 data set. The list of PFTs and the reconstruction procedure itself are evolving. Some compromises (for example, restricted taxon lists in some regions) limit the precision of biome assignments and will become obsolete as primary data are put into community data bases. This trend will facilitate biome mapping for other time slices. Co‐evolution of climate‐biosphere modelling and palaeodata synthesis and analysis will continue.

Contributing Authors D. Archer, M.R. Ashmore, O. Aumont, D. Baker, M. Battle, M. Bender, L.P. Bopp, P. Bousquet, K. Caldeira, P. Ciais, P.M. Cox, W. Cramer, F. Dentener, I.G. Enting, C.B. Field, P. Friedlingstein, E.A. Holland, R.A. Houghton, J.I. House, A. Ishida, A.K. Jain, I.A. Janssens, F. Joos, T. Kaminski, C.D. Keeling, R.F. Keeling, D.W. Kicklighter, K.E. Kohfeld, W. Knorr, R. Law, T. Lenton, K. Lindsay, E. Maier-Reimer, A.C. Manning, R.J. Matear, A.D. McGuire, J.M. Melillo, R. Meyer, M. Mund, J.C. Orr, S. Piper, K. Plattner, P.J. Rayner, S. Sitch, R. Slater, S. Taguchi, P.P. Tans, H.Q. Tian, M.F. Weirig, T. Whorf, A. Yool

Numerous studies have underscored the importance of terrestrial ecosystems as an integral component of the Earth's climate system. This realization has already led to efforts to link simple equilibrium vegetation models with Atmospheric General Circulation Models through iterative coupling procedures. While these linked models have pointed to several possible climate–vegetation feedback mechanisms, they have been limited by two shortcomings: (i) they only consider the equilibrium response of vegetation to shifting climatic conditions and therefore cannot be used to explore transient interactions between climate and vegetation; and (ii) the representations of vegetation processes and land-atmosphere exchange processes are still treated by two separate models and, as a result, may contain physical or ecological inconsistencies. Here we present, as a proof concept, a more tightly integrated framework for simulating global climate and vegetation interactions. The prototype coupled model consists of the GENESIS (version 2) Atmospheric General Circulation Model and the IBIS (version 1) Dynamic Global Vegetation Model. The two models are directly coupled through a common treatment of land surface and ecophysiological processes, which is used to calculate the energy, water, carbon, and momentum fluxes between vegetation, soils, and the atmosphere. On one side of the interface, GENESIS simulates the physics and general circulation of the atmosphere. On the other side, IBIS predicts transient changes in the vegetation structure through changes in the carbon balance and competition among plants within terrestrial ecosystems. As an initial test of this modelling framework, we perform a 30 year simulation in which the coupled model is supplied with modern CO2 concentrations, observed ocean temperatures, and modern insolation. In this exploratory study, we run the GENESIS atmospheric model at relatively coarse horizontal resolution (4.5° latitude by 7.5° longitude) and IBIS at moderate resolution (2° latitude by 2° longitude). We initialize the models with globally uniform climatic conditions and the modern distribution of potential vegetation cover. While the simulation does not fully reach equilibrium by the end of the run, several general features of the coupled model behaviour emerge. We compare the results of the coupled model against the observed patterns of modern climate. The model correctly simulates the basic zonal distribution of temperature and precipitation, but several important regional biases remain. In particular, there is a significant warm bias in the high northern latitudes, and cooler than observed conditions over the Himalayas, central South America, and north-central Africa. In terms of precipitation, the model simulates drier than observed conditions in much of South America, equatorial Africa and Indonesia, with wetter than observed conditions in northern Africa and China. Comparing the model results against observed patterns of vegetation cover shows that the general placement of forests and grasslands is roughly captured by the model. In addition, the model simulates a roughly correct separation of evergreen and deciduous forests in the tropical, temperate and boreal zones. However, the general patterns of global vegetation cover are only approximately correct: there are still significant regional biases in the simulation. In particular, forest cover is not simulated correctly in large portions of central Canada and southern South America, and grasslands extend too far into northern Africa. These preliminary results demonstrate the feasibility of coupling climate models with fully dynamic representations of the terrestrial biosphere. Continued development of fully coupled climate-vegetation models will facilitate the exploration of a broad range of global change issues, including the potential role of vegetation feedbacks within the climate system, and the impact of climate variability and transient climate change on the terrestrial biosphere.

Abstract Process‐based models can be classified into: (a) terrestrial biogeochemical models (TBMs), which simulate fluxes of carbon, water and nitrogen coupled within terrestrial ecosystems, and (b) dynamic global vegetation models (DGVMs), which further couple these processes interactively with changes in slow ecosystem processes depending on resource competition, establishment, growth and mortality of different vegetation types. In this study, four models – RHESSys, GOTILWA+, LPJ‐GUESS and ORCHIDEE – representing both modelling approaches were compared and evaluated against benchmarks provided by eddy‐covariance measurements of carbon and water fluxes at 15 forest sites within the EUROFLUX project. Overall, model‐measurement agreement varied greatly among sites. Both modelling approaches have somewhat different strengths, but there was no model among those tested that universally performed well on the two variables evaluated. Small biases and errors suggest that ORCHIDEE and GOTILWA+ performed better in simulating carbon fluxes while LPJ‐GUESS and RHESSys did a better job in simulating water fluxes. In general, the models can be considered as useful tools for studies of climate change impacts on carbon and water cycling in forests. However, the various sources of variation among models simulations and between models simulations and observed data described in this study place some constraints on the results and to some extent reduce their reliability. For example, at most sites in the Mediterranean region all models generally performed poorly most likely because of problems in the representation of water stress effects on both carbon uptake by photosynthesis and carbon release by heterotrophic respiration ( R h ). The use of flux data as a means of assessing key processes in models of this type is an important approach to improving model performance. Our results show that the models have value but that further model development is necessary with regard to the representation of the some of the key ecosystem processes.

Fossil pollen data supplemented by tree macrofossil records were used to reconstruct the vegetation of the Former Soviet Union and Mongolia at 6000 years. Pollen spectra were assigned to biomes using the plant‐functional‐type method developed by Prentice et al . (1996). Surface pollen data and a modern vegetation map provided a test of the method. This is the first time such a broad‐scale vegetation reconstruction for the greater part of northern Eurasia has been attempted with objective techniques. The new results confirm previous regional palaeoenvironmental studies of the mid‐Holocene while providing a comprehensive synopsis and firmer conclusions. West of the Ural Mountains temperate deciduous forest extended both northward and southward from its modern range. The northern limits of cool mixed and cool conifer forests were also further north than present. Taiga was reduced in European Russia, but was extended into Yakutia where now there is cold deciduous forest. The northern limit of taiga was extended (as shown by increased Picea pollen percentages, and by tree macrofossil records north of the present‐day forest limit) but tundra was still present in north‐eastern Siberia. The boundary between forest and steppe in the continental interior did not shift substantially, and dry conditions similar to present existed in western Mongolia and north of the Aral Sea.

Seventeen simulations of the Last Glacial Maximum (LGM) climate have been performed using atmospheric general circulation models (AGCM) in the framework of the Paleoclimate Modeling Intercomparison Project (PMIP). These simulations use the boundary conditions for CO2, insolation and ice-sheets; surface temperatures (SSTs) are either (a) prescribed using CLIMAP data set (eight models) or (b) computed by coupling the AGCM with a slab ocean (nine models). The present-day (PD) tropical climate is correctly depicted by all the models, except the coarser resolution models, and the simulated geographical distribution of annual mean temperature is in good agreement with climatology. Tropical cooling at the LGM is less than at middle and high latitudes, but greatly exceeds the PD temperature variability. The LGM simulations with prescribed SSTs underestimate the observed temperature changes except over equatorial Africa where the models produce a temperature decrease consistent with the data. Our results confirm previous analyses showing that CLIMAP (1981) SSTs only produce a weak terrestrial cooling. When SSTs are computed, the models depict a cooling over the Pacific and Indian oceans in contrast with CLIMAP and most models produce cooler temperatures over land. Moreover four of the nine simulations, produce a cooling in good agreement with terrestrial data. Two of these model results over ocean are consistent with new SST reconstructions whereas two models simulate a homogeneous cooling. Finally, the LGM aridity inferred for most of the tropics from the data, is globally reproduced by the models with a strong underestimation for models using computed SSTs.

Mean July temperatures across Europe 6000 years before present were reconstructed from palynological data by the transfer function method. Reconstructed summer temperatures were warmer than those at present over most of Europe with the greatest heating, more than 2 degrees C, in the midcontinent and the far north. This pattern is explained by high summer insolation and a weak zonal insolation gradient 6000 years before present and the effective heating of the landmass relative to ocean and coastal areas. A strong land-sea pressure gradient may in turn have increased westerly air flow into southern Europe, which is consistent with cooler reconstructed summer temperatures in the Mediterranean region, and reduced the environmental lapse rate in the central European mountains.

Conflicting hypotheses are investigated for the observed atmospheric CO 2 increase of 20 ppm between 8 ka BP and pre‐industrial time. The carbon component of the Bern Carbon Cycle Climate (Bern CC) model, which couples the Lund‐Potsdam‐Jena Dynamic Global Vegetation Model to an atmosphere‐ocean‐sediment component, is driven by climate fields from time‐slice simulations of the past 21 ka with the Hadley Centre Unified Model or the NCAR Climate System Model. The entire Holocene ice core record of CO 2 is matched within a few ppm for the standard model setup, and results are broadly consistent with proxy data of atmospheric 13 CO 2 , mean ocean δ 13 C, and pollen data, within their uncertainties. Our analysis suggests that a range of mechanisms, including calcite compensation in response to earlier terrestrial uptake, terrestrial carbon uptake and release, SST changes, and coral reef buildup, contributed to the 20 ppm rise. The deep sea δ 13 C record constrains the contribution of the calcite compensation mechanism to 4–10 ppm. Terrestrial carbon inventory changes related to climate and CO 2 forcing, the greening of the Sahara, peat buildup, and land use have probably influenced atmospheric CO 2 by a few ppm only. The early Holocene CO 2 decrease is quantitatively explained by terrestrial uptake and calcite compensation in response to terrestrial uptake during the glacial‐interglacial transition. The recent hypothesis by Ruddiman [2003] that anthropogenic land use caused a 40 ppm CO 2 anomaly over the past 8 ka, preventing the climate system from entering a new glacial, would imply an anthropogenic emission of 700 GtC and a decrease in atmospheric δ 13 C of 0.6 permil. This is not compatible with the ice core δ 13 C record and would require an upward revision of land use emission estimates by a factor of 3 to 4.

Abstract There is scope for land‐use changes to increase or decrease CO 2 concentrations in the atmosphere over the next century. Here we make simple but robust calculations of the maximum impact of such changes. Historical land‐use changes (mostly deforestation) and fossil fuel emissions have caused an increase in atmospheric concentration of CO 2 of 90 ppm between the pre‐industrial era and year 2000. The projected range of CO 2 concentrations in 2100, under a range of emissions scenarios developed for the IPCC, is 170–600 ppm above 2000 levels. This range is mostly due to different assumptions regarding fossil fuel emissions. If all of the carbon so far released by land‐use changes could be restored to the terrestrial biosphere, atmospheric CO 2 concentration at the end of the century would be about 40–70 ppm less than it would be if no such intervention had occurred. Conversely, complete global deforestation over the same time frame would increase atmospheric concentrations by about 130–290 ppm. These are extreme assumptions; the maximum feasible reforestation and afforestation activities over the next 50 years would result in a reduction in CO 2 concentration of about 15–30 ppm by the end of the century. Thus the time course of fossil fuel emissions will be the major factor in determining atmospheric CO 2 concentrations for the foreseeable future.

Abstract Lake-level data can be used to refine palaeoclimate reconstructions based on pollen data. This approach is illustrated for the European Holocene. Estimates of P-PET (precipitation minus potential evapotranspiration) were first inferred from modern pollen analogues. The pollen-based estimates were then compared with the status of lakes within a 5° radius. Analogues with P-PET anomalies inconsistent with the lake-level changes were rejected. The "constrained" sets of analogues were used to estimate continental-scale patterns of annual mean temperature and annual precipitation at 3000-yr intervals. Estimated temperature anomalies differed only slightly from the unconstrained reconstructions. Estimated precipitation anomalies, however, showed improved spatial coherence and increased regional contrast and were occasionally reversed in sign. The effect of the constraint was to impose a rational selection among almost equally similar modern pollen analogues with similar temperatures but widely varying moisture regimes. The resulting maps showed clear, spatially coherent patterns of change in precipitation as well as temperature, suitable for comparison with climate-model results. Further improvement of these maps will become possible as a more extensive coverage of lake-level data is obtained.

We coupled a global biome model iteratively with an atmospheric general circulation model to study the possible role of vegetation in the climate system, at the time of glacial inception 115,000 years ago. Orbital forcing alone was not sufficient to initiate glaciation when other components of the climate system were kept as present (atmospheric composition, oceans, biosphere and cryosphere). Summers were however cold enough to induce major vegetation shifts in high northern latitudes. Southward migration of the boreal forest/tundra limit helped to create favourable conditions for continental ice‐sheet growth, with increasing snow depth and duration in Labrador, Arctic Canada and northern/western Fennoscandia. These results support a role for biogeophysical feedback in initiating glaciations.

Abstract Numerous studies have underscored the importance of terrestrial ecosystems as an integral component of the Earth's climate system. This realization has already led to efforts to link simple equilibrium vegetation models with Atmospheric General Circulation Models through iterative coupling procedures. While these linked models have pointed to several possible climate–vegetation feedback mechanisms, they have been limited by two shortcomings: (i) they only consider the equilibrium response of vegetation to shifting climatic conditions and therefore cannot be used to explore transient interactions between climate and vegetation; and (ii) the representations of vegetation processes and land‐atmosphere exchange processes are still treated by two separate models and, as a result, may contain physical or ecological inconsistencies. Here we present, as a proof concept, a more tightly integrated framework for simulating global climate and vegetation interactions. The prototype coupled model consists of the GENESIS (version 2) Atmospheric General Circulation Model and the IBIS (version 1) Dynamic Global Vegetation Model. The two models are directly coupled through a common treatment of land surface and ecophysiological processes, which is used to calculate the energy, water, carbon, and momentum fluxes between vegetation, soils, and the atmosphere. On one side of the interface, GENESIS simulates the physics and general circulation of the atmosphere. On the other side, IBIS predicts transient changes in the vegetation structure through changes in the carbon balance and competition among plants within terrestrial ecosystems. As an initial test of this modelling framework, we perform a 30 year simulation in which the coupled model is supplied with modern CO 2 concentrations, observed ocean temperatures, and modern insolation. In this exploratory study, we run the GENESIS atmospheric model at relatively coarse horizontal resolution (4.5° latitude by 7.5° longitude) and IBIS at moderate resolution (2° latitude by 2° longitude). We initialize the models with globally uniform climatic conditions and the modern distribution of potential vegetation cover. While the simulation does not fully reach equilibrium by the end of the run, several general features of the coupled model behaviour emerge. We compare the results of the coupled model against the observed patterns of modern climate. The model correctly simulates the basic zonal distribution of temperature and precipitation, but several important regional biases remain. In particular, there is a significant warm bias in the high northern latitudes, and cooler than observed conditions over the Himalayas, central South America, and north‐central Africa. In terms of precipitation, the model simulates drier than observed conditions in much of South America, equatorial Africa and Indonesia, with wetter than observed conditions in northern Africa and China. Comparing the model results against observed patterns of vegetation cover shows that the general placement of forests and grasslands is roughly captured by the model. In addition, the model simulates a roughly correct separation of evergreen and deciduous forests in the tropical, temperate and boreal zones. However, the general patterns of global vegetation cover are only approximately correct: there are still significant regional biases in the simulation. In particular, forest cover is not simulated correctly in large portions of central Canada and southern South America, and grasslands extend too far into northern Africa. These preliminary results demonstrate the feasibility of coupling climate models with fully dynamic representations of the terrestrial biosphere. Continued development of fully coupled climate‐vegetation models will facilitate the exploration of a broad range of global change issues, including the potential role of vegetation feedbacks within the climate system, and the impact of climate variability and transient climate change on the terrestrial biosphere.

Question: What plant properties might define plant functional types (PFTs) for the analysis of global vegetation responses to climate change, and what aspects of the physical environment might be expected to predict the distributions of PFTs? Methods: We review principles to explain the distribution of key plant traits as a function of bioclimatic variables. We focus on those whole-plant and leaf traits that are commonly used to define biomes and PFTs in global maps and models. Results: Raunkiær's plant life forms (underlying most later classifications) describe different adaptive strategies for surviving low temperature or drought, while satisfying requirements for reproduction and growth. Simple conceptual models and published observations are used to quantify the adaptive significance of leaf size for temperature regulation, leaf consistency for maintaining transpiration under drought, and phenology for the optimization of annual carbon balance. A new compilation of experimental data supports the functional definition of tropical, warm-temperate, temperate and boreal phanerophytes based on mechanisms for withstanding low temperature extremes. Chilling requirements are less well quantified, but are a necessary adjunct to cold tolerance. Functional traits generally confer both advantages and restrictions; the existence of trade-offs contributes to the diversity of plants along bioclimatic gradients. Conclusions: Quantitative analysis of plant trait distributions against bioclimatic variables is becoming possible; this opens up new opportunities for PFT classification. A PFT classification based on bioclimatic responses will need to be enhanced by information on traits related to competition, successional dynamics and disturbance.

Simulations of the climatic response to mid-Holocene (6 ka BP) orbital forcing with two coupled ocean–atmosphere models (FOAM and CSM) show enhancement of monsoonal precipitation in parts of the American Southwest, Central America and northernmost South America during Northern Hemisphere summer. The enhanced onshore flow that brings precipitation into Central America is caused by a northward displacement of the inter-tropical convergence zone, driven by cooling of the equatorial and warming of the northern subtropical and mid-latitude ocean. Ocean feedbacks also enhance precipitation over the American Southwest, although the increase in monsoon precipitation there is largely driven by increases in land-surface temperature. The northward shift in the equatorial precipitation band that causes enhanced precipitation in Central America and the American Southwest has a negative feedback effect on monsoonal precipitation in northern South America. The simulations demonstrate that mid-Holocene aridity in the mid-continent of North America is dynamically linked to the orbitally induced enhancement of the summer monsoon in the American Southwest, with a spatial structure (wet in the Southwest and dry in the mid-continent) similar to that found in strong monsoon years today. Changes in winter precipitation along the west coast of North America, in Central America and along the Gulf Coast, caused by southward-displacement of the westerly storm tracks, indicate that changes in the Northern Hemisphere winter monsoon also play a role in regional climate changes during the mid-Holocene. Although the simulations with FOAM and CSM differ in detail, the general mechanisms and patterns are common to both. The model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation, lake status and aeolian data from the Americas.

Biomization provides an objective and robust method of assigning pollen spectra to biomes so that pollen data can be mapped and compared directly with the output of biomgeographic models. We have tested the applicability of this procedure, originally developed for Europe, to assign modern surface samples from China to biomes. The procedure successfully delineated the major vegetation types of China. When the same procedure was applied to fossil pollen samples for 6000 years ago, the reconstructions showed systematic differences from present, consistent with previous interpretations of vegetation changes since the mid‐Holocene. In eastern China, the forest zones were systematically shifted northwards, such that cool mixed forests displaced taiga in northeastern China, while broad‐leaved evergreen forest extended c . 300 km and temperate deciduous forest c . 500–600 km beyond their present northern limits. In northwestern China, the area of desert and steppe vegetation was reduced compared to present. On the Tibetan Plateau, forest vegetation extended to higher elevations than today and the area of tundra was reduced. These shifts in biome distributions imply significant changes in climate since 6000 years ago that can be interpreted qualitatively as a response to orbital forcing and its secondary effects on the Asian monsoon.

Global patterns of potential natural vegetation were simulated for present and last glacial maximum (LGM) climates. The LGM simulation showed good agreement with available evidence, most importantly in the humid tropics. Simple calculations based on these simulations indicate that terrestrial carbon storage increased by 300-700 Pg C after the LGM. The range is due to uncertainties in the mean carbon storage values for different biomes, and in the amount of carbon in boreal peats. These results are consistent with the global change in ocean 8'3C, inferred from measurements on benthic foraminifera, reflecting the increased storage of isotopically light carbon on land.

Two previous reconstructions of palaeovegetation across the whole of China were performed using a simple classification of plant functional types (PFTs). Now a more explicit, global PFT classification scheme has been developed, and a substantial number of additional pollen records have become available. Here we apply the global scheme of PFTs to a comprehensive set of pollen records available from China to test the applicability of the global scheme of PFTs in China, and to obtain a well-founded reconstruction of changing palaeovegetation patterns. A total of 806 pollen surface samples, 188 mid-Holocene (MH, 6000 14C yr BP) and 50 last glacial maximum (LGM, 18,000 14C yr BP) pollen records were used to reconstruct vegetation patterns in China, based on a new global classification system of PFTs and a standard numerical technique for biome assignment (biomization). The biome reconstruction based on pollen surface samples showed convincing agreement with present potential natural vegetation. Coherent patterns of change in biome distribution between MH, LGM and present are observed. In the MH, cold and cool-temperate evergreen needleleaf forests and mixed forests, temperate deciduous broadleaf forest, and warm-temperate evergreen broadleaf and mixed forest in eastern China were shifted northward by 200–500 km. Cold-deciduous forest in northeastern China was replaced by cold evergreen needleleaf forest while in central northern China, cold-deciduous forest was present at some sites now occupied by temperate grassland and desert. The forest–grassland boundary was 200–300 km west of its present position. Temperate xerophytic shrubland, temperate grassland and desert covered a large area on the Tibetan Plateau, but the area of tundra was reduced. Treeline was 300–500 m higher than present in Tibet. These changes imply generally warmer winters, longer growing seasons and more precipitation during the MH. Westward shifts of the forest–shrubland–grassland and grassland–desert boundaries imply greater moisture availability in the MH, consistent with a stronger summer monsoon. During the LGM, in contrast, cold-deciduous forest, cool-temperate evergreen needleleaf forest, cool mixed forests, warm-temperate evergreen broadleaf and mixed forest in eastern China were displaced to the south by 300–1000 km, while temperate deciduous broadleaf forest, pure warm-temperate evergreen forest, tropical semi-evergreen and evergreen broadleaf forests were restricted or absent from the mainland of southern China, implying colder winters than present. Strong shifts of temperate xerophytic shrubland, temperate grassland and desert to the south and east in northern and western China and on the Tibetan Plateau imply drier conditions than present.

Abstract A global scale Dynamic Nitrogen scheme (DyN) has been developed and incorporated into the Lund–Posdam–Jena (LPJ) dynamic global vegetation model (DGVM). The DyN is a comprehensive process‐based model of the cycling of N through and within terrestrial ecosystems, with fully interactive coupling to vegetation and C dynamics. The model represents the uptake, allocation and turnover of N in plants, and soil N transformations including mineralization, N 2 fixation, nitrification and denitrification, NH 3 volatilization, N leaching, and N 2 , N 2 O and NO production and emission. Modelled global patterns of site‐scale nitrogen fluxes and reservoirs are highly correlated to observations reported from different biomes. The simulation of site‐scale net primary production and soil carbon content was improved relative to the original LPJ, which lacked an interactive N cycle, especially in the temporal and boreal regions. Annual N uptake by global natural vegetation was simulated as 1.084 Pg N yr −1 , with lowest values <1 g N m −2 yr −1 (polar desert) and highest values in the range 24–36.5 g N m −2 yr −1 (tropical forests). Simulated global patterns of annual N uptake are consistent with previous model results by Melillo et al. The model estimates global total nitrogen storage potentials in vegetation (5.3 Pg N), litter (4.6 Pg N) and soil (≥67 Pg as organic N and 0.94 Pg as inorganic N). Simulated global patterns of soil N storage are consistent with the analysis by Post et al. although total simulated N storage is less. Deserts were simulated to store 460 Tg N (up to 0.262 kg N m −2 ) as NO 3 − , contributing 80% of the global total NO 3 − inventory of 580 Tg N. This model result is in agreement with the findings of a large NO 3 − pool beneath deserts. Globally, inorganic soil N is a small reservoir, comprising only 1.6% of the global soil N content to 1.5 m soil depth, but the ratio has a very high spatial variability and in hot desert regions, inorganic NO 3 − is estimated to be the dominant form of stored N in the soil.

[1] Peatlands and permafrost are important components of the carbon cycle in the northern high latitudes. The inclusion of these components into a dynamic global vegetation model required changes to physical land surface routines, the addition of two new peatland-specific plant functional types, incorporation of an inundation stress mechanism, and deceleration of decomposition under inundation. The new model, LPJ-WHy v1.2, was used to simulate net ecosystem production (NEP), net primary production (NPP), heterotrophic respiration (HR), and soil carbon content. Annual peatland NEP matches observations even though the seasonal amplitude is overestimated. This overestimation is caused by excessive NPP values, probably due to the lack of nitrogen or phosphorus limitation in LPJ-WHy. Introduction of permafrost reduces circumpolar (45–90°N) NEP from 1.65 to 0.96 Pg C a−1 and leads to an increase in soil carbon content of almost 40 Pg C; adding peatlands doubles this soil carbon increase. Peatland soil carbon content and hence HR depend on model spin-up duration and are crucial for simulating NEP. These results highlight the need for a regional peatland age map to help determine spin-up times. A sensitivity experiment revealed that under future climate conditions, NPP may rise more rapidly than HR resulting in increases in NEP.

Changes in climate and land use, caused by socio-economic changes, greenhouse gas emissions, agricultural policies and other factors, are known to affect both natural and managed ecosystems, and will likely impact on the European terrestrial carbon balance during the coming decades. This study presents a comprehensive European Union wide (EU15 plus Norway and Switzerland, EU*) assessment of potential future changes in terrestrial carbon storage considering these effects based on four illustrative IPCC-SRES storylines (A1FI, A2, B1, B2). A process-based land vegetation model (LPJ-DGVM), adapted to include a generic representation of managed ecosystems, is forced with changing fields of land-use patterns from 1901 to 2100 to assess the effect of land-use and cover changes on the terrestrial carbon balance of Europe. The uncertainty in the future carbon balance associated with the choice of a climate change scenario is assessed by forcing LPJ-DGVM with output from four different climate models (GCMs: CGCM2, CSIRO2, HadCM3, PCM2) for the same SRES storyline. Decrease in agricultural areas and afforestation leads to simulated carbon sequestration for all land-use change scenarios with an average net uptake of 17–38 Tg C/year between 1990 and 2100, corresponding to 1.9–2.9% of the EU*s CO2 emissions over the same period. Soil carbon losses resulting from climate warming reduce or even offset carbon sequestration resulting from growth enhancement induced by climate change and increasing atmospheric CO2 concentrations in the second half of the twenty-first century. Differences in future climate change projections among GCMs are the main cause for uncertainty in the cumulative European terrestrial carbon uptake of 4.4–10.1 Pg C between 1990 and 2100.

ABSTRACT Aim To implement plant hydraulic architecture within the Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ–DGVM), and to test the model against a set of observational data. If the model can reproduce major patterns in vegetation and ecosystem processes, we consider this to be an important linkage between plant physiology and larger‐scale ecosystem dynamics. Location The location is global, geographically distributed. Methods A literature review was carried out to derive model formulations and parameter values for representing the hydraulic characteristics of major global plant functional types (PFTs) in a DGVM. After implementing the corresponding formulations within the LPJ–DGVM, present‐day model output was compared to observational data. Results The model reproduced observed broad‐scale patterns in potential natural vegetation, but it failed to distinguish accurately between different types of grassland and savanna vegetation, possibly related to inadequate model representations of water fluxes in the soil and wildfire effects. Compared to a version of the model using an empirical formulation for calculating plant water supply without considering plant hydraulic architecture, the new formulation improved simulated patterns of vegetation in particular for dry shrublands. Global‐scale simulation results for runoff and actual evapotranspiration (AET) corresponded well to available data. The model also successfully reproduced the magnitude and seasonal cycle of AET for most EUROFLUX forests, while modelled variation in NPP across a large number of sites spanning several biomes showed a strong correlation with estimates from field measurements. Main conclusions The model was generally confirmed by comparison to observational data. The novel model representation of water flow within plants makes it possible to resolve mechanistically the effects of hydraulic differences between plant functional groups on vegetation structure, water cycling, and competition. This may be an advantage when predicting ecosystem responses to nonextant climates, in particular in areas dominated by dry shrubland vegetation.

A growing number of studies investigated the feedback between the carbon cycle and the climate system. Modeling studies evolved from analysis based on simple land or ocean carbon cycle models to comprehensive Earth System Models accounting for state-of-the-art climate models coupled to land and ocean biogeochemical models. So far, there is a general agreement that climate change negatively affects the oceanic uptake of carbon. On land there was a similar agreement until recently where new studies showed that warming could reduce nitrogen limitation to growth, reducing the amplitude, or even changing the sign of, the land feedback. In parallel, alternative approaches used the observational record of atmospheric CO2 and temperature, on time scales ranging from interannual to millennial, to estimate the climate–carbon cycle feedback. These studies confirmed that at the global scale, warming leads to a release of CO2 from the land/ocean system to the atmosphere. Whether these observations can strongly constrain the magnitude of the feedback under future climate change is still under investigation.

Abstract. Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs): six types of phytoplankton, three types of zooplankton, and heterotrophic procaryotes. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing macrozooplankton (e.g. krill), and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean high-nutrient low-chlorophyll (HNLC) region during summer. When model simulations do not include macrozooplankton grazing explicitly, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there is no iron deposition from dust. When model simulations include a slow-growing macrozooplankton and trophic cascades among three zooplankton types, the high-chlorophyll summer bias in the Southern Ocean HNLC region largely disappears. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community, despite iron limitation of phytoplankton community growth rates. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.

We present simulations of the dust cycle during present and glacial climate states, using a model, which explicitly simulates the control of dust emissions as a function of seasonal and interannual changes in vegetation cover. The model produces lower absolute amounts of dust emissions and deposition than previous simulations of the Last Glacial Maximum (LGM) dust cycle. However, the simulated 2‐ to 3‐fold increase in emissions and deposition at the LGM compared to today, is in agreement with marine‐ and ice‐core observations, and consistent with previous simulations. The mean changes are accompanied by a prolongation of the length of the season of dust emissions in most source regions. The increase is most pronounced in Asia, where LGM dust emissions are high throughout the winter, spring and summer rather than occurring primarily in spring as they do today. Changes in the seasonality of dust emissions, and hence atmospheric loading, interact with changes in the seasonality of precipitation, and hence of the relative importance of wet and dry deposition processes at high northern latitudes. As a result, simulated dust deposition rates in the high northern latitudes show high interannual variability. Our results suggest that the high dust concentration variability shown by the Greenland ice core records during the LGM is a consequence of changes in atmospheric circulation and precipitation locally rather than a result of changes in the variability of dust emissions.

We ran a sequence of climate model experiments for 6000 years ago, with land‐surface conditions based on a realistic map of palaeovegetation, lakes and wetlands, to quantify the effects of land‐surface feedbacks in the Saharan region. Vegetation‐induced albedo and moisture flux changes produced year‐round warming, forced the monsoon to 17°–25°N two months earlier, and shifted the precipitation belt ≈300 km northwards compared to the effects of orbital forcing alone. The addition of lakes and wetlands produced localised changes in evaporation and precipitation, but caused no further extension of the monsoon belt. Diagnostic analyses with biome and continental hydrology models showed that the combined land‐surface feedbacks, although substantial, could neither maintain grassland as far north as observed (≈26°N) nor maintain Lake “MegaChad” (330,000 km²).

Results of an intercomparison among terrestrial biogeochemical models (TBMs) are reported, in which one diagnostic and five prognostic models have been run with the same long‐term climate forcing. Monthly fields of net ecosystem production (NEP), which is the difference between net primary production (NPP) and heterotrophic respiration R H , at 0.5° resolution have been generated for the terrestrial biosphere. The monthly estimates of NEP in conjunction with seasonal CO 2 flux fields generated by the seasonal Hamburg Model of the Oceanic Carbon Cycle (HAMOCC3) and fossil fuel source fields were subsequently coupled to the three‐dimensional atmospheric tracer transport model TM2 forced by observed winds. The resulting simulated seasonal signal of the atmospheric CO 2 concentration extracted at the grid cells corresponding to the locations of 27 background monitoring stations of the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory network is compared with measurements from these sites. The Simple Diagnostic Biosphere Model (SDBM1), which is tuned to the atmospheric CO 2 concentration at five monitoring stations in the northern hemisphere, successfully reproduced the seasonal signal of CO 2 at the other monitoring stations. The SDBM1 simulations confirm that the north‐south gradient in the amplitude of the atmospheric CO 2 signal results from the greater northern hemisphere land area and the more pronounced seasonality of radiation and temperature in higher latitudes. In southern latitudes, ocean‐atmosphere gas exchange plays an important role in determining the seasonal signal of CO 2 . Most of the five prognostic models (i.e., models driven by climatic inputs) included in the intercomparison predict in the northern hemisphere a reasonably accurate seasonal cycle in terms of amplitude and, to some extent, also with respect to phase. In the tropics, however, the prognostic models generally tend to overpredict the net seasonal exchanges and stronger seasonal cycles than indicated by the diagnostic model and by observations. The differences from the observed seasonal signal of CO 2 may be caused by shortcomings in the phenology algorithms of the prognostic models or by not properly considering the effects of land use and vegetation fires on CO 2 fluxes between the atmosphere and terrestrial biosphere.

Ecological ApplicationsVolume 10, Issue 6 p. 1553-1573 Invited Feature The Carbon Balance of the Terrestrial Biosphere: Ecosystem Models and Atmospheric Observations I. Colin Prentice, I. Colin Prentice Max Planck Institute for Biogeochemistry, P.O. Box 100164, D-07701 Jena, Germany E-mail: colin.prentice@bgc-jena.mpg.deSearch for more papers by this authorMartin Heimann, Martin Heimann Max Planck Institute for Biogeochemistry, P.O. Box 100164, D-07701 Jena, GermanySearch for more papers by this authorStephen Sitch, Stephen Sitch Climate Impacts Group, Institute of Ecology, Lund University, S-22362 Lund, Sweden, and Potsdam Institute for Climate Impact Research (PIK), Telegrafenberg, P.O. Box 601203, D-14412 Potsdam, GermanySearch for more papers by this author I. Colin Prentice, I. Colin Prentice Max Planck Institute for Biogeochemistry, P.O. Box 100164, D-07701 Jena, Germany E-mail: colin.prentice@bgc-jena.mpg.deSearch for more papers by this authorMartin Heimann, Martin Heimann Max Planck Institute for Biogeochemistry, P.O. Box 100164, D-07701 Jena, GermanySearch for more papers by this authorStephen Sitch, Stephen Sitch Climate Impacts Group, Institute of Ecology, Lund University, S-22362 Lund, Sweden, and Potsdam Institute for Climate Impact Research (PIK), Telegrafenberg, P.O. Box 601203, D-14412 Potsdam, GermanySearch for more papers by this author First published: 01 December 2000 https://doi.org/10.1890/1051-0761(2000)010[1553:TCBOTT]2.0.CO;2Citations: 100 Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Precise measurements in air are helping to clarify the fate of CO2 released by human activities. Oxygen-to-nitrogen ratios in firn (the transition state from snow to ice) and archived air samples indicate that the terrestrial biosphere was approximately carbon-neutral on average during the 1980s. CO2 release by forest clearance during this period must have been compensated for by CO2 sinks elsewhere on land. Direct atmospheric O2:N2 measurements became available during the 1990s. These measurements indicate net terrestrial CO2 uptake of ∼2 Pg C/yr. From the north–south O2:N2 gradient, it has been inferred that about this amount was taken up by terrestrial ecosystems in the northern nontropics while additional CO2 released by tropical-forest clearance must have been compensated for by additional, tropical, terrestrial CO2 sinks. These and other atmospheric observations provide independent tests of carbon-cycle reconstructions made with process-based terrestrial ecosystem models. Such models can account for major features of the atmospheric-CO2 record, including the amplitude and phase of the seasonal cycle of atmospheric-CO2 concentration at different latitudes, and much of the interannual variability in the rate of increase of atmospheric CO2. Models also predict direct effects of rising atmospheric-CO2 concentration on primary production, modified by feedbacks at the plant and ecosystem levels. These effects translate into a global carbon sink the right order of magnitude to compensate for forest clearance during the 1980s. The modeled sink depends on continuously increasing CO2 to maintain disequilibrium between primary production and carbon storage. There are still substantial differences among the carbon-balance estimates made by different models, reflecting limitations in current understanding of ecosystem-level responses to atmospheric-CO2 concentration, especially with regard to the interactions of C and N cycling and interactions with land-use change. Scenario calculations nevertheless agree that if atmospheric CO2 continues its rise unchecked then the terrestrial sink will start to decline by the middle of the next century, for reasons including saturation of the direct CO2 effect on photosynthesis. Citing Literature Volume10, Issue6December 2000Pages 1553-1573 RelatedInformation

Abstract Aim This paper documents reconstructions of the vegetation patterns in Australia, Southeast Asia and the Pacific (SEAPAC region) in the mid‐Holocene and at the last glacial maximum (LGM). Methods Vegetation patterns were reconstructed from pollen data using an objective biomization scheme based on plant functional types. The biomization scheme was first tested using 535 modern pollen samples from 377 sites, and then applied unchanged to fossil pollen samples dating to 6000 ± 500 or 18,000 ± 1000 14 C yr bp . Results 1. Tests using surface pollen sample sites showed that the biomization scheme is capable of reproducing the modern broad‐scale patterns of vegetation distribution. The north–south gradient in temperature, reflected in transitions from cool evergreen needleleaf forest in the extreme south through temperate rain forest or wet sclerophyll forest (WSFW) and into tropical forests, is well reconstructed. The transitions from xerophytic through sclerophyll woodlands and open forests to closed‐canopy forests, which reflect the gradient in plant available moisture from the continental interior towards the coast, are reconstructed with less geographical precision but nevertheless the broad‐scale pattern emerges. 2. Differences between the modern and mid‐Holocene vegetation patterns in mainland Australia are comparatively small and reflect changes in moisture availability rather than temperature. In south‐eastern Australia some sites show a shift towards more moisture‐stressed vegetation in the mid‐Holocene with xerophytic woods/scrub and temperate sclerophyll woodland and shrubland at sites characterized today by WSFW or warm‐temperate rain forest (WTRF). However, sites in the Snowy Mountains, on the Southern Tablelands and east of the Great Dividing Range have more moisture‐demanding vegetation in the mid‐Holocene than today. South‐western Australia was slightly drier than today. The single site in north‐western Australia also shows conditions drier than today in the mid‐Holocene. Changes in the tropics are also comparatively small, but the presence of WTRF and tropical deciduous broadleaf forest and woodland in the mid‐Holocene, in sites occupied today by cool‐temperate rain forest, indicate warmer conditions. 3. Expansion of xerophytic vegetation in the south and tropical deciduous broadleaf forest and woodland in the north indicate drier conditions across mainland Australia at the LGM. None of these changes are informative about the degree of cooling. However the evidence from the tropics, showing lowering of the treeline and forest belts, indicates that conditions were between 1 and 9 °C (depending on elevation) colder. The encroachment of tropical deciduous broadleaf forest and woodland into lowland evergreen broadleaf forest implies greater aridity. Main conclusions This study provides the first continental‐scale reconstruction of mid‐Holocene and LGM vegetation patterns from Australia, Southeast Asia and the Pacific (SEAPAC region) using an objective biomization scheme. These data will provide a benchmark for evaluation of palaeoclimate simulations within the framework of the Palaeoclimate Modelling Intercomparison Project.

Previous climate model simulations have shown that the configuration of the Earth's orbit during the early to mid‐Holocene (approximately 10–5 kyr) can account for the generally warmer‐than‐present conditions experienced by the high latitudes of the northern hemisphere. New simulations for 6 kyr with two atmospheric/mixed‐layer ocean models (Community Climate Model, version 1, CCMl, and Global ENvironmental and Ecological Simulation of Interactive Systems, version 2, GENESIS 2) are presented here and compared with results from two previous simulations with GENESIS 1 that were obtained with and without the albedo feedback due to climate‐induced poleward expansion of the boreal forest. The climate model results are summarized in the form of potential vegetation maps obtained with the global BIOME model, which facilitates visual comparisons both among models and with pollen and plant macrofossil data recording shifts of the forest‐tundra boundary. A preliminary synthesis shows that the forest limit was shifted 100–200 km north in most sectors. Both CCMl and GENESIS 2 produced a shift of this magnitude. GENESIS 1 however produced too small a shift, except when the boreal forest albedo feedback was included. The feedback in this case was estimated to have amplified forest expansion by approximately 50%. The forest limit changes also show meridional patterns (greatest expansion in central Siberia and little or none in Alaska and Labrador) which have yet to be reproduced by models. Further progress in understanding of the processes involved in the response of climate and vegetation to orbital forcing will require both the deployment of coupled atmosphere‐biosphere‐ocean models and the development of more comprehensive observational data sets.

(...) Relationships of leaf area to sapwood area, stem respiration to sapwood volume and vigour to growth efficiency provide equations for tree growth, leaf-area dynamics and canopy structure. The model simulates mixed-age, mixed-species populations of trees on a 1000-m 2 patch. The test site, Fiby urskog (central Sweden), has been unmanaged since a catastrophic storm in 1795. (...)

14C-dated pollen and lake-level data from Europe are used to assess the spatial patterns of climate change between 6000 yr BP and present, as simulated by the NCAR CCM1 (National Center for Atmospheric Research, Community Climate Model, version 1) in response to the change in the Earth’s orbital parameters during this perod. First, reconstructed 6000 yr BP values of bioclimate variables obtained from pollen and lake-level data with the constrained-analogue technique are compared with simulated values. Then a 6000 yr BP biome map obtained from pollen data with an objective biome reconstruction (biomization) technique is compared with BIOME model results derived from the same simulation. Data and simulations agree in some features: warmer-than-present growing seasons in N and C Europe allowed forests to extend further north and to higher elevations than today, and warmer winters in C and E Europe prevented boreal conifers from spreading west. More generally, however, the agreement is poor. Predominantly deciduous forest types in Fennoscandia imply warmer winters than the model allows. The model fails to simulate winters cold enough, or summers wet enough, to allow temperate deciduous forests their former extended distribution in S Europe, and it incorrectly simulates a much expanded area of steppe vegetation in SE Europe. Similar errors have also been noted in numerous 6000 yr BP simulations with prescribed modern sea surface temperatures. These errors are evidently not resolved by the inclusion of interactive sea-surface conditions in the CCM1. Accurate representation of mid-Holocene climates in Europe may require the inclusion of dynamical ocean–atmosphere and/or vegetation–atmosphere interactions that most palaeoclimate model simulations have so far disregarded.

The magnitude and location of terrestrial carbon sources and sinks remains subject to large uncertainties. Estimates of terrestrial CO2 fluxes from ground-based inventory measurements typically find less carbon uptake than inverse model calculations based on atmospheric CO2 measurements, while a wide range of results have been obtained using models of different types. However, when full account is taken of the processes, pools, time scales and geographic areas being measured, the different approaches can be understood as complementary rather than inconsistent, and can provide insight as to the contribution of various processes to the terrestrial carbon budget. For example, quantitative differences between atmospheric inversion model estimates and forest inventory estimates in northern extratropical regions suggest that carbon fluxes to soils (often not accounted for in inventories), and into non-forest vegetation, may account for about half of the terrestrial uptake. A consensus of inventory and inverse methods indicates that, in the 1980s, northern extratropical land regions were a large net sink of carbon, and the tropics were approximately neutral (albeit with high uncertainty around the central estimate of zero net flux). The terrestrial flux in southern extratropical regions was small. Book-keeping model studies of the impacts of land-use change indicated a large source in the tropics and almost zero net flux for most northern extratropical regions; similar land use change impacts were also recently obtained using process-based models. The difference between book-keeping land-use change model studies and inversions or inventories was previously interpreted as a “missing” terrestrial carbon uptake. Land-use change studies do not account for environmental or many management effects (which are implicitly included in inventory and inversion methods). Process-based model studies have quantified the impacts of CO2 fertilisation and climate change in addition to land use change, and found that these environmental effects are in the right order of magnitude to account for the “missing” terrestrial carbon uptake. Despite recent carbon losses due to fire and insect attack in Canada and Russia, the northern extratropical regions generally have been a net carbon sink, only partially due to land-use changes such as abandonment of agricultural land. In the tropics, inventory data and flux measurements in extant forests support the existence of an environmental or management sink that counterbalances the effect of deforestation. Woody encroachment in savannas may also be a significant (but as yet poorly quantified) cause of tropical carbon uptake.

Abstract. A coupled carbon and water flux model (BIOME2) captures the broad‐scale environmental controls on the natural distribution of vegetation structural and phenological types in Australia. Model input consists of latitude, soil type, and mean monthly climate (temperature, precipitation, and sunshine hours) data on a 1/10° grid. Model output consists of foliage projective cover (FPC) for the quantitative combination of plant types that maximizes net primary production (NPP). The model realistically simulates changes in FPC along moisture gradients as a consequence of the trade‐off between light capture and water stress. A two‐layer soil hydrology model also allows simulation of the competitive balance between grass and woody vegetation including the strong effects of soil texture.

Sensitivity experiments with a simple water-balance model were used to constrain the possible climatic causes of distinct Holocene patterns of lake-level variation in different regions of Europe. Lakes in S Sweden were low at 9 ka, high around 6.5 ka, low again around 4 ka and are high now. Lakes in Estonia show similar but weaker trends. Lakes in S France were highest around 9 ka, lowest around 4 ka, intermediate now. Lakes in Greece were also high around 9 ka but continued rising until 7.5 ka, then fell gradually from 5 ka with a brief high phase around 3 ka, and are low now. The model was forced with insolation anomalies deduced from orbital variations, temperature anomalies inferred from the pollen record and cloudiness anomalies derived from changes in the position of the subtropical anticyclone (inferred from reconstructed changes in the equator-to-pole temperature gradient), in order to evaluate the effects of resultant evaporation changes on catchment water balance. The resulting simulated changes in runoff (precipitation minus actual evapotranspiration) were slight, and frequently opposite to the observed trends. Larger changes in precipitation are plausible and are required to explain the data. The required precipitation increase in N Europe from 9 ka (low) to 6 ka (high) is suggested by GCM experiments to have been a consequence of interacting insolation and residual ice-sheet effects on the atmospheric circulation over the North Atlantic. The explanation of other observed changes, including the drying trend during the Holocene in S Europe, has not been provided by GCM experiments to date. Explanations may lie in changes in mesoscale circulation, sea-surface temperature patterns and the coupling between these phenomena that may not follow orbital changes in any simple way.

We show the implications of the commonly observed age-related decline in aboveground productivity of forests, and hence forest age structure, on the carbon dynamics of European forests in response to historical changes in environmental conditions. Size-dependent carbon allocation in trees to counteract increasing hydraulic resistance with tree height has been hypothesized to be responsible for this decline. Incorporated into a global terrestrial biosphere model (the Lund-Potsdam-Jena model, LPJ), this hypothesis improves the simulated increase in biomass with stand age. Application of the advanced model, including a generic representation of forest management in even-aged stands, for 77 European provinces shows that model-based estimates of biomass development with age compare favorably with inventory-based estimates for different tree species. Model estimates of biomass densities on province and country levels, and trends in growth increment along an annual mean temperature gradient are in broad agreement with inventory data. However, the level of agreement between modeled and inventory-based estimates varies markedly between countries and provinces. The model is able to reproduce the present-day age structure of forests and the ratio of biomass removals to increment on a European scale based on observed changes in climate, atmospheric CO2 concentration, forest area, and wood demand between 1948 and 2000. Vegetation in European forests is modeled to sequester carbon at a rate of 100 Tg C/yr, which corresponds well to forest inventory-based estimates.

Autoecological data for north European trees were compiled from literature, field observations and comparisons of species distributions with bioclimatic maps. When compared with an independent dataset for the same species in montane south-central Europe, individual descriptors showed widely varying between-set correlations. Qualitative life-history descriptors were usually consistent between datasets; inconsistent shade-tolerance assignments (Acer platanoides, Fraxinus excelsior) may relfect overlooked changes in tolerance with age. Species' estimated bioclimatic limits were poorly correlated between datasets. Forest succession models will gain reliability by eliminating unreliable descriptors such as maximum tree diamter, dealing appropriately with ontogenetic changes in key attrobutes such as shade tolerance, and incorporating climate-response functions based on explicit physiological constraints.

Abstract. Atmospheric CO2 concentration has varied from minima of 170–200 ppm in glacials to maxima of 280–300 ppm in the recent interglacials. Photosynthesis by C3 plants is highly sensitive to CO2 concentration variations in this range. Physiological consequences of the CO2 changes should therefore be discernible in palaeodata. Several lines of evidence support this expectation. Reduced terrestrial carbon storage during glacials, indicated by the shift in stable isotope composition of dissolved inorganic carbon in the ocean, cannot be explained by climate or sea-level changes. It is however consistent with predictions of current process-based models that propagate known physiological CO2 effects into net primary production at the ecosystem scale. Restricted forest cover during glacial periods, indicated by pollen assemblages dominated by non-arboreal taxa, cannot be reproduced accurately by palaeoclimate models unless CO2 effects on C3-C4 plant competition are also modelled. It follows that methods to reconstruct climate from palaeodata should account for CO2 concentration changes. When they do so, they yield results more consistent with palaeoclimate models. In conclusion, the palaeorecord of the Late Quaternary, interpreted with the help of climate and ecosystem models, provides evidence that CO2 effects at the ecosystem scale are neither trivial nor transient.

Abstract. Exchange of non-CO2 trace gases between the land surface and the atmosphere plays an important role in atmospheric chemistry and climate. Recent studies have highlighted its importance for interpretation of glacial-interglacial ice-core records, the simulation of the pre-industrial and present atmosphere, and the potential for large climate-chemistry and climate-aerosol feedbacks in the coming century. However, spatial and temporal variations in trace gas emissions and the magnitude of future feedbacks are a major source of uncertainty in atmospheric chemistry, air quality and climate science. To reduce such uncertainties Dynamic Global Vegetation Models (DGVMs) are currently being expanded to mechanistically represent processes relevant to non-CO2 trace gas exchange between land biota and the atmosphere. In this paper we present a review of important non-CO2 trace gas emissions, the state-of-the-art in DGVM modelling of processes regulating these emissions, identify key uncertainties for global scale model applications, and discuss a methodology for model integration and evaluation.

Abstract The last interglaciation (substage 5e) provides an opportunity to examine the effects of extreme orbital changes on regional climates. We have made two atmospheric general circulation model experiments: P+T+ approximated the northern hemisphere seasonality maximum near the beginning of 5e; P-T- approximated the minimum near the end of 5e. Simulated regional climate changes have been translated into biome changes using a physiologically based model of global vegetation types. Major climatic and vegetational changes were simulated for the northern hemisphere extratropics, due to radiational effects that were both amplified and modified by atmospheric circulation changes and sea-ice feedback. P+T+ showed mid-continental summers up to 8°C warmer than present. Mid-latitude winters were 2-4°C cooler than present but in the Arctic, summer warmth reduced sea-ice extent and thickness, producing winters 2-8°C warmer than present. The tundra and taiga biomes were displaced poleward, while warm-summer steppes expanded in the mid latitudes due to drought. P-T- showed summers up to 5°C cooler than present, especially in mid latitudes. Sea ice and snowpack were thicker and lasted longer; polar desert, tundra, and taiga biomes were displaced equatorward, while cool-summer steppes and semideserts expanded due to the cooling. A slight winter warming in mid latitudes, however, caused warm-temperate evergreen forests and scrub to expand poleward. Such qualitative contrasts in the direction of climate and vegetation change during 5e should be identifiable in the paleorecord.

Atmospheric general circulation models have been used to simulate the sensitivity of regional climates to Late Quaternary changes in insolation, ice sheets and atmospheric CO2. Model results for full-glacial conditions (18 ka) show the Atlantic Westerly jet strengthened and shifted south, a corresponding southward shift of the Icelandic low and a strengthening and northward shift of the subtropical anticyclone (STA). A glacial anticyclone developed over the European ice sheet and cold, dry conditions prevailed over much of Europe. The full-glacial climate anomaly for the North Atlantic sector is explained by a combination of low atmospheric CO2 (reducing global surface temperatures) and circulation changes caused by the ice sheets. By the Early Holocene (9 ka) the ice sheets were much reduced while the orbital anomaly was near its maximum, producing mid-continental summer warming and winter cooling and a reduced latitudinal temperature gradient in the northern hemisphere in both seasons. The jet and the Icelandic low were displaced to north of their present position, producing strong onshore flow and mild, wet winters in northern Europe, while the STA was also shifted northwards, producing offshore flow and dry summers in the same region. During the late-glacial transition the effects of insolation and glaciation were antagonistic for some features of the circulation and synergistic for others. The model results suggest palaeoclimatic hypotheses that could be directly tested by comparison with palaeoclimatic data mapped at a synoptic scale.

A dynamic global vegetation model (DGVM) was used to simulate global terrestrial carbon storage and stable carbon isotope composition changes for the last 21000 years. A paleoclimate scenario was provided by interpolation of coupled AGCM/mixed‐layer ocean model experiments; [CO 2 ] atm data were obtained from the Byrd and Taylor Dome ice core records. According to the model results, terrestrial carbon storage at the Last Glacial Maximum (LGM, 21 ka) was 821 Pg C less than today. The modeled isotopic composition (δ 13 C) of total terrestrial carbon at LGM was enriched by 1.5‰ compared to present. During the deglaciation (17–9 ka), vegetation expanded rapidly into formerly glaciated areas and carbon storage correspondingly increased. Increasing NPP sustained a continuing increase in terrestrial carbon storage through the Holocene. These results do not support the published hypothesis that terrestrial CO 2 outgassing drove the ca. 20 ppm increase in [CO 2 ] atm after 8 ka. They are consistent with an alternative explanation based on the oceanic CaCO 3 compensation response to the extraction of carbon from the atmosphere‐ocean system during the deglaciation.

Increasing greenhouse-gas concentrations in the atmosphere are expected to produce maximum warming in high latitudes, displacing the potential boreal forest zone of the northern hemisphere far to the north. We analyse the implications of this shift for forest composition and biomass dynamics across the present-day boreonemoral zone in Scandinavia, using a forest succession model that includes a generalized disturbance regime and realistic climatic effects on species' regeneration and growth. Temperature increases in the range of 2-4 K in summer and 5-6 K in winter, typical of simulated CO2 doubling effects, force the boreonemoral zone >1000 km northward from central Sweden where dominance passes from Picea (spruce) to Fagus (beech), Quercus (oak) and Pinus (pine) over 150-200 years. Simulated increases of 175-250 mm in annual precipitation are required to counteract drought, which would otherwise reduce forest biomass and increase the representation of the more drought-tolerant species. The simulated forest dynamics are the net result of individualistic responses of species to different aspects of climate, modulated by successional processes that are on the same time scale as human impacts on climate.

Global data sets of the stable carbon isotope composition of plant leaves, of CO 2 in canopy air, and of CO 2 in the background atmosphere were compiled and compared to results of a global vegetation model (BIOME4) that simulated, at these three scales, the magnitude, direction, and timing of fluxes of CO 2 and 13 C between the biosphere and the atmosphere. Carbon isotope data on leaves were classified into 12 Plant Functional Types (PFTs), and measurements from canopy flasks were assigned to 16 biomes, for direct comparison to model results. BIOME4 simulated the observed leaf δ 13 C values to within 1 standard deviation of the measured mean for most PFTs. Modeled δ 13 C for C 3 grasses, tundra shrubs, and herbaceous plants of cold climates deviated only slightly more from measurements, perhaps as a result of the wide geographic range and a limited set of measurements of these PFTs. Modeled ecosystem isotopic discrimination against 13 C (Δ e ) averaged 18.6 globally when simulating potential natural vegetation and 18.1 when an agricultural crop mask was superimposed. The difference was mainly due to the influence of C 4 agriculture in areas that are naturally dominated by C 3 vegetation. Model results show a gradient in Δ e among C 3 ‐dominated biomes as a result of stomatal responses to aridity; this model result is supported by canopy air measurements. At the troposphere scale, BIOME4 was coupled to a matrix representation of an atmospheric tracer transport model to simulate seasonally varying concentrations of CO 2 and 13 C at remote Northern Hemisphere measuring stations. Ocean CO 2 and 13 C flux fields were included, using the HAMOCC3 ocean biogeochemistry model [ Six and Maier‐Reimer , 1996 ]. Model results and observations show similar seasonal cycles, and the model reproduces the inferred latitudinal trend toward smaller isotopic discrimination by the biosphere at lower latitudes. These results indicate that biologically mediated variations in 13 C discrimination by terrestrial ecosystems may be significant for atmospheric inverse modeling of carbon sources and sinks, and that such variations can be simulated using a process‐based model.

Abstract: Question: What are the correlations between the degree of drought stress and temperature, and the adoption of specific adaptive strategies by plants in the Mediterranean region? Location: 602 sites across the Mediterranean region. Method: We considered 12 plant morphological and phenological traits, and measured their abundance at the sites as trait scores obtained from pollen percentages. We conducted stepwise regression analyses of trait scores as a function of plant available moisture (α) and winter temperature (MTCO). Results: Patterns in the abundance for the plant traits we considered are clearly determined by α, MTCO or a combination of both. In addition, trends in leaf size, texture, thickness, pubescence and aromatic leaves and other plant level traits such as thorniness and aphylly, vary according to the life form (tree, shrub, forb), the leaf type (broad, needle) and phenology (evergreen, summer‐green). Conclusions: Despite conducting this study based on pollen data we have identified ecologically plausible trends in the abundance of traits along climatic gradients. Plant traits other than the usual life form, leaf type and leaf phenology carry strong climatic signals. Generally, combinations of plant traits are more climatically diagnostic than individual traits. The qualitative and quantitative relationships between plant traits and climate parameters established here will help to provide an improved basis for modelling the impact of climate changes on vegetation and form a starting point for a global analysis of pollen‐climate relationships.

One of the most exciting discoveries in the earth sciences in recent decades has been the proof that ice ages are governed by deterministic variations in the earth's orbit. These variations modify the latitudinal and seasonal distribution of solar radiation at periods ranging from 103 to 10(5) years, and alternately produce conditions for building and melting continental ice. The same solar radiation variations also govern other aspects of world climate, including the temperatures of the midlatitude continental interiors, the intensity of upwelling in the tropical oceans, and the strength and extent of the monsoons. The interplay of solar radiation, seasonality and ice-sheet changes is responsible for the complex ecological history documented in the fossil record of the past 20 000 years. But the orbital variations have occurred throughout earth's history, and have caused periodic environmental changes in both terrestrial and marine environments even during times when there was no ice. Species have responded to these changes by range migration, an evolved ability that may maintain their genetic coherence in the face of a continually changing environment.

Abstract 1 We model the potential vegetation and annual net primary production (NPP) of China on a 10′ grid under the present climate using the processed‐based equilibrium terrestrial biosphere model BIOME3. The simulated distribution of the vegetation was in general in good agreement with the potential natural vegetation based on a numerical comparison between the two maps using the ΔV statistic (ΔV = 0.23). Predicted and measured NPP were also similar, especially in terms of biome‐averages. 2 A coupled ocean–atmosphere general circulation model including sulphate aerosols was used to drive a double greenhouse gas scenario for 2070–2099. Simulated vegetation maps from two different CO 2 scenarios (340 and 500 p.p.m.v.) were compared to the baseline biome map using ΔV. Climate change alone produced a large reduction in desert, alpine tundra and ice/polar desert, and a general pole‐ward shift of the boreal, temperate deciduous, warm–temperate evergreen and tropical forest belts, a decline in boreal deciduous forest and the appearance of tropical deciduous forest. The inclusion of CO 2 physiological effects led to a marked decrease in moist savannas and desert, a general decrease for grasslands and steppe, and disappearance of xeric woodland/scrub. Temperate deciduous broadleaved forest, however, shifted north to occupy nearly half the area of previously temperate mixed forest. 3 The impact of climate change and increasing CO 2 is not only on biogeography, but also on potential NPP. The NPP values for most of the biomes in the scenarios with CO 2 set at 340 p.p.m.v. and 500 p.p.m.v. are greater than those under the current climate, except for the temperate deciduous forest, temperate evergreen broadleaved forest, tropical rain forest, tropical seasonal forest, and xeric woodland/scrub biomes. Total vegetation and total carbon is simulated to increase significantly in the future climate scenario, both with and without the CO 2 direct physiological effect. 4 Our results show that the global process‐based equilibrium terrestrial biosphere model BIOME3 can be used successfully at a regional scale.

We compared the simulated responses of net primary production, heterotrophic respiration, net ecosystem production and carbon storage in natural terrestrial ecosystems to historical (1765 to 1990) and projected (1990–2300) changes of atmospheric CO 2 concentration of four terrestrial biosphere models: the Bern model, the Frankfurt Biosphere Model (FBM), the High-Resolution Biosphere Model (HRBM) and the Terrestrial EcosystemModel (TEM). The results of the model intercomparison suggest that CO 2 fertilization of natural terrestrial vegetation has the potential to account for a large fraction of the so-called ‘‘missing carbon sink’′ of 2.0 Pg C in 1990. Estimates of this potential are reduced when the models incorporate the concept that CO 2 fertilization can be limited by nutrient availability. Although the model estimates differ on the potential size (126 to 461 Pg C) of the future terrestrial sink caused by CO 2 fertilization, the results of the four models suggest that natural terrestrial ecosystems will have a limited capacity to act as a sink of atmospheric CO 2 in the future as a result of physiological constraints and nutrient constraints on NPP. All the spatially explicit models estimate a carbon sink in both tropical and northern temperate regions, but the strength of these sinks varies over time. Differences in the simulated response of terrestrial ecosystems to CO 2 fertilization among the models in this intercomparison study reflect the fact that the models have highlighted different aspects of the effect of CO 2 fertilization on carbon dynamics of natural terrestrial ecosystems including feedback mechanisms. As interactions with nitrogen fertilization, climate change and forest regrowth may play an important role in simulating the response of terrestrial ecosystems to CO 2 fertilization, these factors should be included in future analyses. Improvements in spatially explicit data sets, whole-ecosystem experiments and the availability of net carbon exchange measurements across the globe will also help to improve future evaluations of the role of CO 2 fertilization on terrestrial carbon storage. DOI: 10.1034/j.1600-0889.1999.00017.x

The magnitude and geographical distribution of natural sources and sinks of atmospheric CH4 in the biosphere are still poorly known. Estimates of the net contribution from northern wetlands have been lowered during recent years. According to current consensus, about 35 Tg CH4/yr originates from northern wetlands and tundra. A process-orientated ecosystem source model for CH4 is used here to obtain an independent estimate for this flux. The model estimates steadystate seasonal cycles of NPP and heterotrophic respiration (HR). It accounts for peatland carbon storage and then obtains CH4 emission as a proportion of HR with the constant of proportionality (as a range) estimated from observations. The model was shown consistent with seasonal data (including winter) on NPP, soil respiration and CH4 emission at sites spanning a range of latitudes and climates. Applied on a 1° grid basis using standard climatological and wetland distribution data sets, this approach yields a total non-forested wetland and tundra emission (> 50°N) of 8.7 ± 5.8 Tg CH4/ yr. After inclusion of forested wetlands, we estimate a total emission from northern wetlands and tundra of 20 ± 13 Tg CH4/yr. This is somewhat lower than current atmospherically based estimates. The difference may be due to localized high emissions, which have been reported, e.g., for West Siberian wetlands but which are not well understood and not included in current models.

1 Uncertainties of Global Biogeochemical Predictions 2 Uncertainties of Global Climate Predictions 3 Uncertainties in the Atmospheric Chemical System 4 Inferring Biogeochemical Sources and Sinks from Atmospheric Concentrations: General Consideration and Applications in Vegetarian Canopies 5 Biogeophysical Feedbacks and the Dynamics of Climate 6 Land-Ocean-Atmosphere Interactions and Monsoon Climate Change: A Paleo-Perspective 7 Paleobiogeochemistry 8 Should Phosphorus Availability Be Constraining Moist Tropical Forest Responses to Increasing CO2 Concentrations? 9 Trees in Grasslands: Biogeochemical Consequences of Woody Plant Expansion 10 Biogeochemistry in the Arctic: Patterns, Processes, and Controls 11 Evaporation in the Boreal Zone During Summer--Physics and Vegetation 12 Past and Future Forest Response to Rapid Climate Change 13 Biogeochemical Models: Implicit vs. Explicit Microbiology 14 The Global Soil Organic Carbon Pool 15 Plant Compounds and Their Turnover and Stability as Soil Organic Matter 16 Input/Output Balances and Nitrogen Limitation in Terrestrial Ecosystems 17 Interactions Between Hillslope Hydrochemistry, Nitrogen Dynamics and Plants in Fennoscandian Boreal Forest 18 The Cycle of Atmospheric Molecular Oxygen and its Isotopes 19 Constraining the Global Carbon Budget from Global to Regional Scales -- the Measurement Challenge 20 Carbon Isotope Discrimination of Terrestrial Ecosystems -- How Well do Observed and Modeled Results Match? 21 Photosynthetic Pathways and Climate 22 Biological Diversity, Evolution and Biogeochemistry 23 Atmospheric Perspectives on the Ocean Carbon Cycle 24 International Instruments for the Protection of the Climate and Their National Implementation 25 A New Tool to Characterizing and Managing Risks 26 Contrasting Approaches: The Ozone Layer, Climate Change and Resolving the Kyoto Dilemma 27 Optimizing Long-Term Climate Management Subject Index

A series of nine simulations has been made with the Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ-DGVM) in order to explore the impacts of climate variability and Holocene changes in variability (as simulated by the Fast Ocean-Atmosphere Model, FOAM) on vegetation in three forest-dominated regions of China and in the semi-arid Sahelian region of northern Africa. The simulations illustrate that changes both in the magnitude of climate variability and in the persistence of above/below average conditions have the potential to modify the vegetation response to changes in mean climate. Simulated changes in moisture availability affect vegetation through drought stress or through changing the fuel availability in semi-arid regions where lack of fuel often limits the incidence of fire. Increasing moisture availability causes trees to replace grasses in China by reducing drought stress; increasing moisture availability in the Sahel increases the available fuel and hence reduces fire return times, favouring grasses. The modelling results imply that climate variability is important to vegetation dynamics; that not only the magnitude, but also the temporal structure of variability is important; and that correctly simulating vegetation changes in response to climate variability requires a realistic "baseline" simulation of plant community composition. They further indicate that the impacts of climate change on ecosystems can sometimes derive as much from changes in variability as from changes in mean climate.

Dynamic global vegetation models (DGVMs) offer explicit representations of the land surface through time and have been used to research large-scale hydrological responses to climate change. These applications are discussed and comparisons of model inputs and formulations are made among and between DGVMs and global hydrological models. It is shown that the configuration of process representations and data inputs are what makes a given DGVM unique within the family of vegetation models. The variety of available climatic forcing datasets introduces uncertainty into simulations of hydrological variables. It is proposed that satellite-derived data, validated thoroughly, could be used to improve the quality of model evaluations and augment ground-based observations, particularly where spatial and temporal gaps are present. This would aid the reduction of model uncertainties and thus potentially enhance our understanding of global hydrological change.

Abstract The objective of the European Terrestrial Ecosystem Modelling Activity (ETEMA) was to address some of the major challenges in developing generalized models to examine responses of natural and seminatural ecosystems to environmental change at the regional to European scale. The approach described herein was to break down the totality of ecosystem functioning into its key components, each with its characteristic spatial and temporal scales. A conceptual framework was developed describing the configuration of these components as modules within a generalized simulation model. The framework describes the key inputs, outputs and state variables, their spatial and temporal contexts, and information flows between modules. The ‘backbone’ of the model is a system of nested timing loops corresponding to the disparate time scales at which different ecosystem processes occur. The framework is a theoretical construct into which ecosystem models at levels of complexity ranging from the very general to the highly detailed can be mapped, and thus provides a guide for development of models for novel, particularly regional‐scale, applications. A number of subsystem studies of the major components of ecosystem functioning, i.e. modules of the conceptual framework, are briefly introduced herein. The general aim of the subsystem studies was to identify the key alternative formulations (as opposed to minor variants) and test these against observational data. The various subsystem studies concern planetary boundary layer–ecosystem interactions, ecosystem CO 2 and H 2 O fluxes, vegetation physiology and phenology, biogeography and vegetation dynamics, detritus and SOM dynamics, soil moisture and human and natural disturbances and, as individual papers, they complete this special ETEMA issue.

The point we wished to make was simply that the more complex geography and topography of eastern Asia compared with eastern North America, in conjunction with climate change and sea-level fluctuations, have provided greater opportunity for allopatric speciation. This explanation of the greater diversity of vascular plants in temperate regions of eastern Asia cannot yet be tested using any particular biome reconstruction, all of which are poorly resolved.

Abstract Small reported growth enhancement factors based on analyses of forest inventory data from the eastern USA ( Caspersen et al . 2000 , Science, 290, 1148–1151) have been interpreted as evidence against CO 2 fertilization in natural forests. We show to the contrary that growth enhancement in response to rising CO 2 , as found in ecosystems with experimental CO 2 enrichment and implemented in terrestrial ecosystem models, is consistent with the data that have been presented within their uncertainties. Comparing forest inventory data with results of an empirical model of age‐dependent biomass accumulation, we find that growth enhancement of plausible magnitude could not be detected in these data, even if it were present. Although forest regrowth due to land‐use change is recognized as an important cause of carbon uptake by eastern US forests, forest inventory data do not provide a basis for eliminating environmentally induced growth enhancement as a substantial contribution to the global terrestrial carbon sink.