Executive Summary

What is the relationship between past greenhouse gas concentrations and climate?

• The sustained rate of increase over the past century in the combined radiative forcing from the three well-mixed greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) is very likely unprecedented in at least the past 16 kyr. Pre-industrial variations of atmospheric greenhouse gas concentrations observed during the last 10 kyr were small compared to industrial era greenhouse gas increases, and were likely mostly due to natural processes.
• It is very likely that the current atmospheric concentrations of CO₂ (379 ppm) and CH₄ (1,774 ppb) exceed by far the natural range of the last 650 kyr. Ice core data indicate that CO₂ varied within a range of 180 to 300 ppm and CH₄ within 320 to 790 ppb over this period. Over the same period, antarctic temperature and CO₂ concentrations co-vary, indicating a close relationship between climate and the carbon cycle.
• It is very likely that glacial-interglacial CO₂ variations have strongly amplified climate variations, but it is unlikely that CO₂ variations have triggered the end of glacial periods. Antarctic temperature started to rise several centuries before atmospheric CO₂ during past glacial terminations.
• It is likely that earlier periods with higher than present atmospheric CO₂ concentrations were warmer than present. This is the case both for climate states over millions of years (e.g., in the Pliocene, about 5 to 3 Ma) and for warm events lasting a few hundred thousand years (i.e., the Palaeocene-Eocene Thermal Maximum, 55 Ma). In each of these two cases, warming was likely strongly amplified at high northern latitudes relative to lower latitudes.

What is the significance of glacial-interglacial climate variability?

• Climate models indicate that the Last Glacial Maximum (about 21 ka) was 3°C to 5°C cooler than the present due to changes in greenhouse gas forcing and ice sheet conditions. Including the effects of atmospheric dust content and vegetation changes gives an additional 1°C to 2°C global cooling, although scientific understanding of these effects is very low. It is very likely that the global warming of 4°C to 7°C since the Last Glacial Maximum occurred at an average rate about 10 times slower than the warming of the 20th century.
• For the Last Glacial Maximum, proxy records for the ocean indicate cooling of tropical sea surface temperatures (average likely between 2°C and 3°C) and much greater cooling and expanded sea ice over the high-latitude oceans. Climate models are able to simulate the magnitude of these latitudinal ocean changes in response to the estimated Earth orbital, greenhouse gas and land surface changes for this period, and thus indicate that they adequately represent many of the major processes that determine this past climate state.
• Last Glacial Maximum land data indicate significant cooling in the tropics (up to 5°C) and greater magnitudes at high latitudes. Climate models vary in their capability to simulate these responses.
• It is virtually certain that global temperatures during coming centuries will not be significantly influenced by a natural orbitally induced cooling. It is very unlikely that the Earth would naturally enter another ice age for at least 30 kyr.
• During the last glacial period, abrupt regional warmings (likely up to 16°C within decades over Greenland) and coolings occurred repeatedly over the North Atlantic region. They likely had global linkages, such as with major shifts in tropical rainfall patterns. It is unlikely that these events were associated with large changes in global mean surface temperature, but instead likely involved a redistribution of heat within the climate system associated with changes in the Atlantic Ocean circulation.
• Global sea level was likely between 4 and 6 m higher during the last interglacial period, about 125 ka, than in the 20th century. In agreement with palaeoclimatic evidence, climate models simulate arctic summer warming of up to 5°C during the last interglacial. The inferred warming was largest over Eurasia and northern Greenland, whereas the summit of Greenland was simulated to be 2°C to 5°C higher than present. This is consistent with ice sheet modelling suggestions that large-scale retreat of the south Greenland Ice Sheet and other arctic ice fields likely contributed a maximum of 2 to 4 m of sea level rise during the last interglacial, with most of any remainder likely coming from the Antarctic Ice Sheet.

What does the study of the current interglacial climate show?

• Centennial-resolution palaeoclimatic records provide evidence for regional and transient pre-industrial warm periods over the last 10 kyr, but it is unlikely that any of these commonly cited periods were globally synchronous. Similarly, although individual decadal-resolution interglacial palaeoclimatic records support the existence of regional quasi-periodic climate variability, it is unlikely that any of these regional signals were coherent at the global scale, or are capable of explaining the majority of global warming of the last 100 years.
• Glaciers in several mountain regions of the Northern Hemisphere retreated in response to orbitally forced regional warmth between 11 and 5 ka, and were smaller (or even absent) at times prior to 5 ka than at the end of the 20th century. The present day near-global retreat of mountain glaciers cannot be attributed to the same natural causes, because the decrease of summer insolation during the past few millennia in the Northern Hemisphere should be favourable to the growth of the glaciers.
• For the mid-Holocene (about 6 ka), GCMs are able to simulate many of the robust qualitative large-scale features of observed climate change, including mid-latitude warming with little change in global mean temperature (<0.4°C), as well as altered monsoons, consistent with the understanding of orbital forcing. For the few well-documented areas, models tend to underestimate hydrological change. Coupled climate models perform generally better than atmosphere-only models, and reveal the amplifying roles of ocean and land surface feedbacks in climate change.
• Climate and vegetation models simulate past northward shifts of the boreal treeline under warming conditions. Palaeoclimatic results also indicated that these treeline shifts likely result in significant positive climate feedback. Such models are also capable of simulating changes in the vegetation structure and terrestrial carbon storage in association with large changes in climate boundary conditions and forcings (i.e., ice sheets, orbital variations).
• Palaeoclimatic observations indicate that abrupt decadal- to centennial-scale changes in the regional frequency of tropical cyclones, floods, decadal droughts and the intensity of the African-Asian summer monsoon very likely occurred during the past 10 kyr. However, the mechanisms behind these abrupt shifts are not well understood, nor have they been thoroughly investigated using current climate models.

How does the 20th-century climate change compare with the climate of the past 2,000 years?

• It is very likely that the average rates of increase in CO₂, as well as in the combined radiative forcing from CO₂, CH₄ and N₂O concentration increases, have been at least five times faster over the period from 1960 to 1999 than over any other 40-year period during the past two millennia prior to the industrial era.
• Ice core data from Greenland and Northern Hemisphere mid-latitudes show a very likely rapid post-industrial era increase in sulphate concentrations above the pre-industrial background.
• Some of the studies conducted since the Third Assessment Report (TAR) indicate greater multi-centennial Northern Hemisphere temperature variability over the last 1 kyr than was shown in the TAR, demonstrating a sensitivity to the particular proxies used, and the specific statistical methods of processing and/or scaling them to represent past temperatures. The additional variability shown in some new studies implies mainly cooler temperatures (predominantly in the 12th to 14th, 17th and 19th centuries), and only one new reconstruction suggests slightly warmer conditions (in the 11th century, but well within the uncertainty range indicated in the TAR).
• The TAR pointed to the ‘exceptional warmth of the late 20th century, relative to the past 1,000 years’. Subsequent evidence has strengthened this conclusion. It is very likely that average Northern Hemisphere temperatures during the second half of the 20th century were higher than for any other 50-year period in the last 500 years. It is also likely that this 50-year period was the warmest Northern Hemisphere period in the last 1.3 kyr, and that this warmth was more widespread than during any other 50-year period in the last 1.3 kyr. These conclusions are most robust for summer in extratropical land areas, and for more recent periods because of poor early data coverage.
• The small variations in pre-industrial CO₂ and CH₄ concentrations over the past millennium are consistent with millennial-length proxy Northern Hemisphere temperature reconstructions; climate variations larger than indicated by the reconstructions would likely yield larger concentration changes. The small pre-industrial greenhouse gas variations also provide indirect evidence for a limited range of decadal- to centennial-scale variations in global temperature.
• Palaeoclimate model simulations are broadly consistent with the reconstructed NH temperatures over the past 1 kyr. The rise in surface temperatures since 1950 very likely cannot be reproduced without including anthropogenic greenhouse gases in the model forcings, and it is very unlikely that this warming was merely a recovery from a pre-20th century cold period.
• Knowledge of climate variability over the last 1 kyr in the Southern Hemisphere and tropics is very limited by the low density of palaeoclimatic records.
• Climate reconstructions over the past millennium indicate with high confidence more varied spatial climate teleconnections related to the El Niño-Southern Oscillation than are represented in the instrumental record of the 20th century.
• The palaeoclimate records of northern and eastern Africa, as well as the Americas, indicate with high confidence that droughts lasting decades or longer were a recurrent feature of climate in these regions over the last 2 kyr.

What does the palaeoclimatic record reveal about feedback, biogeochemical and biogeophysical processes?

• The widely accepted orbital theory suggests that glacial-interglacial cycles occurred in response to orbital forcing. The large response of the climate system implies a strong positive amplification of this forcing. This amplification has very likely been influenced mainly by changes in greenhouse gas concentrations and ice sheet growth and decay, but also by ocean circulation and sea ice changes, biophysical feedbacks and aerosol (dust) loading.
• It is virtually certain that millennial-scale changes in atmospheric CO₂ associated with individual antarctic warm events were less than 25 ppm during the last glacial period. This suggests that the associated changes in North Atlantic Deep Water formation and in the large-scale deposition of wind-borne iron in the Southern Ocean had limited impact on CO₂.
• It is very likely that marine carbon cycle processes were primarily responsible for the glacial-interglacial CO₂ variations. The quantification of individual marine processes remains a difficult problem.
• Palaeoenvironmental data indicate that regional vegetation composition and structure are very likely sensitive to climate change, and in some cases can respond to climate change within decades.