The Magnitude and Rate of Past Global Climate Changes

Climate records from Greenland ice cores have shown that climate has changed rapidly and drastically throughout the last glacial period on a millennial time scale. In his Perspective, Shackleton highlights the report by Blunier and Brook, who have correlated the Greenland records with an Antarctic record. There is a systematic difference in the character and timing of temperature changes in the two hemispheres, although the mechanism linking the hemispheres remains uncertain. Absolute dating is essential.

Evidence for climate variability and change during the period of roughly the past millennium is reviewed. Insights are available from both empirical reconstructions based on natural archives (‘proxy’ records) of climate and from the results of simulations using theoretical models of the climate system. Both empirical and model-based approaches provide a fairly consistent picture of past changes. Significant regional climate changes are established as having taken place in past centuries, including periods of substantial cooling and warming in certain regions (e.g., Europe) and notable periods of drought and wetness in other regions (e.g., equatorial Africa and the desert Southwest of the United States). Some of these changes may be associated with the response of the climate to natural changes in radiative forcing of the climate, variations in solar output over time, and the effects of atmospheric aerosols due to past explosive volcanic eruptions. At hemispheric or global scales, late 20th-century warming appears to be without precedent in at least the past 1,000 years. Model simulation studies suggest that this warming is due, in large part, to human or ‘anthropogenic’ climate impacts, particularly the increased concentration of greenhouse gases in the atmosphere due to fossil fuel burning.

Natural climate change has occurred throughout the history of Earth. Human impact on climate change has increased over the last two centuries. During the Quaternary period, cli- mate change on 10 3 -10 5 -year time scales is linked to the so-called Milankovitch cycles that have caused variations in the amount of energy from the sun reaching the Earth. This factor is thought to be a main cause for the shifts between glacial and interglacial periods during the Quaternary. However, superimposed on long-term natural climate change, are events of shorter duration (10-10 2 -year time scales) climatic events. The present paper discusses examples of climate changes on land and in adjoining seas off Norway and Svalbard. The climatic changes from the last glacial maximum to the present, including the abrupt glacial-interglacial transition when the fi rst human settlements occurred in northern Norway, are a particular focus of the paper. Finally, future climate perspectives at the high northern latitudes are discussed.

Initial findings from high-latitude ice-cores implied a relatively unvarying Holocene climate, in contrast to the major climate swings in the preceding late-Pleistocene. However, several climate archives from low latitudes imply a less than equable Holocene climate, as do recent studies on peat bogs in mainland north-west Europe, which indicate an abrupt climate cooling 2800 years ago, with parallels claimed in a range of climate archives elsewhere. A hypothesis that this claimed climate shift was global, and caused by reduced solar activity, has recently been disputed. Until now, no directly comparable data were available from the southern hemisphere to help resolve the dispute. Building on investigations of the vegetation history of an extensive mire in the Valle de Andorra, Tierra del Fuego, we took a further peat core from the bog to generate a high-resolution climate history through the use of determination of peat humification and quantitative leaf-count plant macrofossil analysis. Here, we present the new proxy-climate data from the bog in South America. The data are directly comparable with those in Europe, as they were produced using identical laboratory methods. They show that there was a major climate perturbation at the same time as in northwest European bogs. Its timing, nature and apparent global synchronicity lend support to the notion of solar forcing of past climate change, amplified by oceanic circulation. This finding of a similar response simultaneously in both hemispheres may help validate and improve global climate models. That reduced solar activity might cause a global climatic change suggests that attention be paid also to consideration of any global climate response to increases in solar activity. This has implications for interpreting the relative contribution of climate drivers of recent ‘global warming’.

Climate archives available from deep sea and marine shelf sediments, glaciers, lakes, and ice cores in and around Greenland allow us to place the current trends in regional climate, ice sheet dynamics, and land surface changes in a broader perspective. We show that, during the last decade (2000s), atmospheric and sea surface temperatures are reaching levels last encountered millennia ago, when northern high latitude summer insolation was higher due to a different orbital configuration. Records from lake sediments in southern Greenland document major environmental and climatic conditions during the last 10,000 years, highlighting the role of soil dynamics in past vegetation changes, and stressing the growing anthropogenic impacts on soil erosion during the recent decades. Furthermore, past and present changes in atmospheric and oceanic heat advection appear to strongly influence both regional climate and ice sheet dynamics. Projections from climate models are investigated to quantify the magnitude and rates of future changes in Greenland temperature, which may be faster than past abrupt events occurring under interglacial conditions. Within one century, in response to increasing greenhouse gas emissions, Greenland may reach temperatures last time encountered during the last interglacial period, approximately 125,000 years ago. We review and discuss whether analogies between the last interglacial and future changes are reasonable, because of the different seasonal impacts of orbital and greenhouse gas forcings. Over several decades to centuries, future Greenland melt may act as a negative feedback, limiting regional warming albeit with global sea level and climatic impacts. 2012 John Wiley & Sons, Ltd. How to cite this article:

The present climate, which falls between the last ice age and the next, is relatively stable. New data on past climate change, however, indicates that the climate is capable of undergoing abrupt changes from one mode to another. In his Perspective, Overpeck describes the current research into the history of climate change and how it might influence our thinking about future climate variation.

Glacial–interglacial climate cycles are known to have triggered migrations and reassortments of tropical biota. Although long-term precessionally-driven changes in temperature and precipitation have been demonstrated using tropical sediment records, responses to abrupt climate changes, e.g. the cooling of Heinrich stadials or warmings of the deglaciation, are poorly documented. The best predictions of future forest responses to ongoing warming will rely on evaluating the influences of both abrupt and long-term climate changes on past ecosystems. A sedimentary sequence recovered from Lake Peten-Itza, Guatemalan lowlands, provided a natural archive of environmental history. Pollen and charcoal analyses were used to reconstruct the vegetation and climate history of the area during the last 86,000 years. We found that vegetation composition and air temperature were strongly influenced by millennial-scale changes in the North Atlantic Ocean. Whereas Greenland warm interstadials were associated with warm and relatively wet conditions in the Central American lowlands, cold Greenland stadials, especially those associated with Heinrich events, caused extremely dry and cold conditions. Even though the vegetation seemed to have been highly resilient, plant associations without modern analogs emerged mostly following sharp climate pulses of either warmth or cold, and were paralleled by exceptionally high rates of ecological change. Although pulses of temperature change are evident in this 86,000-year record none matched the rates projected for the 21st Century. According to our findings, the ongoing rapid warming will cause no-modern-analog communities, which given the improbability of returning to lower-than-modern CO2 levels, anthropogenic barriers to migration, and increased anthropogenic fires, will pose immense threats to the biodiversity of the region.

Glaciers around the world retreated as the climate warmed substantially. For the majority of alpine and arctic areas, however, the lack of meteorological data over a long period makes it difficult to build long-term climate and glacial fluctuation relationships, emphasizing the importance of natural proxy archives. Here we use the 230-year record of stem radial growth of birch trees (Betula ermanii) from the treeline forests above the receding glaciers in eastern maritime Kamchatka to analyse temporal variations of climate as well as glacial advance and retreat. Glaciers in Kamchatka Peninsula represent the southern limit of glaciation in far eastern Eurasia, which makes them prone to global warming. Using instrumental climate data (1930-1996) from local meteorological stations, we find that the July temperature had most prominent positive impact on birch growth. On the contrary, smaller ring increments are associated with the positive summer and net annual ice mass balance of Koryto Glacier. The prevailing trend of higher summer temperatures and lower snowfall over the past 70 years has enhanced tree growth while causing the glacier's surface to lower by about 35 m and its front to retreat by about 490 m. Assuming these same relationships between climate, tree growth, and glacier mass balance also existed in the past, we use tree rings as a proxy record of climatically induced temporary halts in the glacier's retreat over the past two centuries, which in total was over 1,000 m. Both direct observations and tree ring proxies indicate several

With the dramatically increasing manifestation of anthropogenic forcing on the Earth's climate, understanding the mechanisms and effects of abrupt climate change is crucial to extend the lead time for mitigation and adaptation. In this context, the climate variability during the Quaternary represents the closest analogy to present-day climate change. Unprecedented insights into both short-term (i.e., decadalto centennial-scale) and long-term (i.e., orbital-scale) climate variability over the last 740 kyr have been derived from ice cores from polar regions (Dansgaard et al., 1993; EPICA community members, 2004). These records show that the higher latitudes repeatedly witnessed temperature changes of more than 10°C within human time scales (Severinghaus et al., 1998). Considerably less information is available on the characteristics of abrupt climate change in the middle and lower latitudes and on their imprint on terrestrial environments. These regions are, however, home to the majority of the Earth’s population, and consequently they will witness the greatest impact of future climate change on people's lives.

This paper reviews developments in our understanding of the state of the Antarctic and Southern Ocean climate, and its relation to the global climate system over the last few millennia. Climate over this and earlier periods has not been stable, as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in Antarctic ice core proxies for past climate. Two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies (also known as the Southern Annular Mode) between ~6000 and 5000 years ago and since 1200-1000 years ago. Following the last of these is a period of major trans-Antarctic reorganization of atmospheric circulation and temperature between AD1700 and 1850. The two earlier Antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the North Atlantic, and the end of the more recent event is coincident with reorganization of atmospheric circulation in the North Pacific. Improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols. Special attention is given to the climate of the past 200 years, which was recorded by a network of recently available shallow firn cores, and to that of the past 50 years, which was monitored by the continuous instrumental record. Significant regional climate changes have taken place in the Antarctic during the past 50 years. Atmospheric temperatures have increased markedly over the Antarctic Peninsula, linked to nearby ocean warming and intensification of the circumpolar westerlies. Glaciers are retreating on the Peninsula, in Patagonia, on the sub-Antarctic islands, and in West Antarctica adjacent to the Peninsula. The penetration of marine air masses has become more pronounced over parts of West Antarctica. Above the surface, the Antarctic troposphere has warmed during winter while the stratosphere has cooled year-round. The upper kilometer of the circumpolar Southern Ocean has warmed, Antarctic Bottom Water across a wide sector off East Antarctica has freshened, and the densest bottom water in the Weddell Sea has warmed. In contrast to these regional climate changes, over most of Antarctica near-surface temperature and snowfall have not increased significantly during at least the past 50 years, and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years. Furthermore, the total sea ice cover around Antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s, despite large but compensating regional changes. The inhomogeneity of Antarctic climate in space and time implies that recent Antarctic climate changes are due on the one hand to a combination of strong multi-decadal variability and anthropogenic effects and, as demonstrated by the paleoclimate record, on the other hand to multi-decadal to millennial scale and longer natural variability forced through changes in orbital insolation, greenhouse gases, solar variability, ice dynamics, and aerosols. Model projections suggest that over the 21st century the Antarctic interior will warm by 3.4° ± 1oC, and sea ice extent will decrease by ~30%. Ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level. Considering the potentially major impacts of a warming climate on Antarctica, vigorous efforts are needed to better understand all aspects of the highly coupled Antarctic climate system as well as its influence on the Earth's climate and oceans.

Key aspects of past variability This and the next two chapters are concerned with Earth-system history over the last ~420 000 years, with special emphasis on the last ~130 000 years, the period since the beginning of the last interglacial. In the present chapter, we are concerned with the changes that are associated with the last four glacial cycles, up to the end of the last glacial maximum (LGM). For much of this period, the mean state of the climate and hence Earth-system components linked to climate, differed greatly from that in the recent past. Our main goal is to elucidate the combination of forcings and feedbacks responsible for the major changes observed, with a view to understanding better the sequences and synergies arising from their interactions, especially during periods of rapid change. The high-latitude records show temperature variability to have been much greater during glacial periods than during interglacials, including the Holocene. By contrast, hydrological variability in lower latitudes has been extreme even during the Holocene. Variability of changing amplitudes, in both temperature and hydrology, is the norm throughout the period. This initial observation is of outstanding importance, for it tells us that even if we discount the likelihood of anthropogenic effects on climate, this does not dispose of future climate change. Scientists and policy makers across the whole spectrum, from the most sceptical about the impact of increasing atmospheric greenhouse-gas concentrations to the most firmly convinced, cannot afford to ignore all the evidence for natural climate variability.

The covariation of carbon dioxide (CO(2)) concentration and temperature in Antarctic ice-core records suggests a close link between CO(2) and climate during the Pleistocene ice ages. The role and relative importance of CO(2) in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO(2) during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO(2) concentrations is an explanation for much of the temperature change at the end of the most recent ice age.

Politicians and the media assume Earth's climate is warming as the result of human activity. Various types of evidence of previous climate changes were investigated as a means of testing the validity of assigning anthropogenic causes to this change. Data with broad geographic coverage indicative of temperature and climate were evaluated. It included records of glacial advance and retreat, sedimentologic evidence of sea level change and glacial activity, palynologic indications of species succession, dendrochronologic evidence of tree-growth response to environment, and continental-ice core parameters indicating accumulation rates as well as other climate surrogates. Also reviewed were historical sources such as explorers' journals, which document significant climate effects over time. Each type of evidence has particular value. Among these are preservation of regional versus local conditions, transport in or out of the system, age–date reliability, correlation between data types, and data (as well as human) bias. All the data indicate that the Holocene has been characterized by ten or more global “little ice ages” irregularly spaced. Each lasted a few centuries separated by sometimes sudden and dramatic global warming events. It is difficult to develop precise paleothermometry. Qualitative evaluations indicate frequent, sudden, and dramatic climate changes. Changes can be rapid, swinging from warmer than today to full glacial conditions within 100 years. The converse can be true. All available data indicate that current climate change is no greater in rate or magnitude, and probably less in both, than many changes that have occurred in the past.

Abstract Existing data indicate that the Earth’s climate is probably warming. Politicians and the media typically assume this warming is the result of human activity. This article summarizes previous climate changes to test the validity of assigning causality to human activity. Records of glacial advances and retreats indicate relative summer temperature. Lacustrine and subaerial sediments afford a record of glacier advances and retreats from the Pleistocene to the present time. Palynology offers a record of species succession in response to climate changes. Dendrochronology is another indicator of summer temperature. Isotope paleontology offers a measurement of temperature at the time of marine sediment deposition, and isotopic evaluation of continental ice is an indicator of temperature at the time of precipitation. Anthropologic sources contain significant climate data such as information about villages overrun by glaciers, open-ocean iceberg density, or harbors filled with ice. Today, scientists are capable of direct measurement of climatic conditions. These sources record continual changes in climate. Broadly, the temperature changed 15 to 20°C from the Paleocene to the Neogene. Perhaps, there was as much as another 10°C change in the Pleistocene. Correlative data from North America, Greenland, and Scandinavia indicate many climate changes were truly global in scope. Although it is difficult to develop precise paleothermometry, qualitative evaluations indicate sudden and dramatic changes in climate. Some are perhaps as great as a change from conditions warmer than today to a full glacial climate in as little as 100 years. The converse can be true. Current data indicate a trend of change that is substantially severe but no greater in rate or magnitude, and probably less in both, than many changes that have occurred in the past.

Global climate has varied since the most primitive atmosphere developed on earth billions of years ago. This variation in climate has occurred on all timescales and has been continuous. The sedimentary rock record reflects numerous sea-level changes, atmospheric compositional changes, and temperature changes, all of which attest to climatic variation. Such evidence, as well as direct historical observations, clearly shows that temperature swings occur in both directions. Past climates have varied from those that create continental glaciers to those that yield global greenhouse conditions. Many people do not comprehend that this means their living climate also varies--it gets warmer or cooler--but typically does not remain the same for extended periods of time. Human history shows us that in general, warmer conditions have been beneficial, and colder conditions have been less kind to society (Lamb, 1995). We currently are living in a not-yet-completed interglacial stage, and it is very likely that warmer conditions lie ahead for humanity, with or without any human interference. Interglacial stages appear to last for about 11,000 years, but with large individual variability. We have been in this interglacial for about 10,000 years.

The climate since 1500 A.D. has been highlighted in the field of global change. Recently, some studies, such as PAGES and WCRP, have opened up broad prospect for the detailed high-resolution climatic records, such as tree-rings, corals and ice-cores etc. More attention is focused on the global climate changes on the basis of paleoclimate information of annual-to-millennial time scales. Especially, the climate of the Little Ice Age(LIA), which may be regarded as a short-scale neoglacial episode or a series similar to the climate change from glacial period, is a very important time stream and have an important role in studying the variations of atmospheric circulation and environment and climate in regional or global extent, and in establishing scientifically contemporary climate matrix. Based on historical documents, dendroclimatic data and ice core records from Europe, Asia, North America, South America and Antarctica since the LIA, a synthetic analysis of the paleoclimatic information is made in this paper. The main results are given as follows: (1) The general tendency of global cold-warm change since the LIA exhibits a consistency in a certain degree, with a high spatial and temporal variation. There were strong cold periods in the 17th and 19th centuries and a strong warm period in the 1920's-1940's. (2) Because of apparent allocation difference between oceans and continents, there is a prominent distinction in the effect of atmospheric circulation patterns, radiation balance of underlying surface and abrupt events on regional and global climate. The strength of circulation systems between Atlantic, Arctic and Pacific, which have different power and duration, has different influence on the global climate change since the LIA from coast to continental interiors of Eurasia. Meanwhile, the uplift of the Tibetan Plateau further increases the regional difference and deeply influences global climate changes. (3) The large-scale effects of solar activity, violent volcanic eruptions and EL Nino events on ocean-atmospheric mechanism play a pivotal role in the global climate system. Especially, solar activity is the main factor affecting the global climate changes since the LIA. Those changes are further influenced by atmospheric circulation system enhanced by violent volcanic eruptions and EL Nino events.erentpowerand

Records of past climates contained in ice cores, ocean sediments, and other archives show that large, abrupt, widespread climate changes have occurred repeatedly in the past. These changes were especially prominent during the cooling into and warming out of the last ice age, but persisted into the modern warm interval. Changes have especially affected water availability in warm regions and temperature in cold regions, but have affected almost all climatic variables across much or all of the Earth. Impacts of climate changes are smaller if the changes are slower or more-expected. The rapidity of abrupt climate changes, together with the difficulty of predicting such changes, means that impacts on the health of humans, economies and ecosystems will be larger if abrupt climate changes occur. Most projections of future climate include only gradual changes, whereas paleoclimatic data plus models indicate that abrupt changes remain possible; thus, policy is being made based on a view of the future that may be optimistic.

Evidence for climate variability and change during the period of roughly the past millennium is reviewed. Insights are available from both empirical reconstructions based on natural archives (‘proxy’ records) of climate and from the results of simulations using theoretical models of the climate system. Both empirical and model-based approaches provide a fairly consistent picture of past changes. Significant regional climate changes are established as having taken place in past centuries, including periods of substantial cooling and warming in certain regions (e.g., Europe) and notable periods of drought and wetness in other regions (e.g., equatorial Africa and the desert Southwest of the United States). Some of these changes may be associated with the response of the climate to natural changes in radiative forcing of the climate, variations in solar output over time, and the effects of atmospheric aerosols due to past explosive volcanic eruptions. At hemispheric or global scales, the late twentieth-century warming appears to be without precedent in at least the past 1000 years. Model simulation studies suggest that this warming is due, in large part, to human or ‘anthropogenic’ climate impacts, particularly the increased concentration of greenhouse gasses in the atmosphere due to fossil fuel burning.

Global warming is real and has been with us for at least two decades. Questions arise regarding the response of the ocean to greenhouse forcing, including expectations for changes in ocean circulation, in uptake of excess carbon dioxide, and in upwelling activity. The large climate variations of the ice ages, within the last million years, offer the opportunity to study responses of the ocean to climate change. A histogram of sealevel positions for the last 700,000 years (based on a new d/sup 18/O stratigraphy here compiled) shows that the present is near the margin of the range of fluctuations, with only 6 percent of positions indicating a warmer climate. Thus, the future will be largely outside of experience with regard to fluctuations of the recent geologic past. The same is true for greenhouse forcing. Our inability to explain sudden climate change in the past, including the rapid rise of carbon dioxide during deglaciation, and differences in ocean productivity between glacial and interglacial conditions, demonstrates a lack of understanding that makes predictions suspect. This is the lesson from ice age studies.

In this paper the current instrumental evidence regarding climate variations and change during the Twentieth Century is reviewed. The questions addressed include: (1) what is the observational evidence for a changing climate, both globally and in the United States, and (2) what are the relevant results from the recently completed U.S. National Assessment examining the potential consequences of climate change in the U.S. Based on global near-surface temperature measurements for the Twentieth Century it is clear that a warming of approximately 0.6 deg C has occurred for the globe and a similar warming has occurred in the U.S. More importantly, however, are the observed asymmetric changes in daily maximum and minimum temperature, with the minimum temperatures increasing at a rate approximately twice that of the maximum temperature. Other temperature sensitive measures, such as glacial and snow cover extent reinforce the observed temperature trends. Examination of the hydrologic cycle indicates that changes also appear to be occurring, although less confidence can be placed on these analyses than those for temperature. Recent studies suggest that precipitation has increased in higher latitudes, particularly in the Northern Hemisphere. The final question regarding climate extremes is much more difficult to assess due to a lack of high temporal resolution climate databases. Of the few studies that have been performed, however, there is evidence that precipitation extremes, particularly heavy rainfall events, are increasing in the U.S., also suggesting an enhanced hydrologic cycle as the planet warms.