6 Palaeoclimate Coordinating Lead Authors: Eystein Jansen (Norway), Jonathan Overpeck (USA) Lead Authors: Keith R. Briffa (UK), Jean-Claude Duplessy (France), Fortunat Joos (Switzerland), Valérie Masson-Delmotte (France), Daniel Olago (Kenya), Bette Otto-Bliesner (USA), W. Richard Peltier (Canada), Stefan Rahmstorf (Germany), Rengaswamy Ramesh (India), Dominique Raynaud (France), David Rind (USA), Olga Solomina (Russian Federation), Ricardo Villalba (Argentina), De’er Zhang (China) Contributing Authors: J.-M. Barnola (France), E. Bauer (Germany), E. Brady (USA), M. Chandler (USA), J. Cole (USA), E. Cook (USA), E. Cortijo (France), T. Dokken (Norway), D. Fleitmann (Switzerland, Germany), M. Kageyama (France), M. Khodri (France), L. Labeyrie (France), A. Laine (France), A. Levermann (Germany), Ø. Lie (Norway), M.-F. Loutre (Belgium), K. Matsumoto (USA), E. Monnin (Switzerland), E. Mosley-Thompson (USA), D. Muhs (USA), R. Muscheler (USA), T. Osborn (UK), Ø. Paasche (Norway), F. Parrenin (France), G.-K. Plattner (Switzerland), H. Pollack (USA), R. Spahni (Switzerland), L.D. Stott (USA), L. Thompson (USA), C. Waelbroeck (France), G. Wiles (USA), J. Zachos (USA), G. Zhengteng (China) Review Editors: Jean Jouzel (France), John Mitchell (UK) This chapter should be cited as: Jansen, E., J. Overpeck, K.R. Briffa, J.-C. Duplessy, F. Joos, V. Masson-Delmotte, D. Olago, B. Otto-Bliesner, W.R. Peltier, S. Rahmstorf, R. Ramesh, D. Raynaud, D. Rind, O. Solomina, R. Villalba and D. Zhang, 2007: Palaeoclimate. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Palaeoclimate Chapter 6 Table of Contents Executive Summary .................................................... 435 6.6 The Last 2,000 Years .......................................... 466 6.6.1 Northern Hemisphere Temperature Variability .....466 6.1 Introduction ......................................................... 438 Box 6.4: Hemispheric Temperatures in the 6.2 Palaeoclimatic Methods ................................... 438 ‘Medieval Warm Period’ ..................................... 468 6.2.1 Methods – Observations of Forcing 6.6.2 Southern Hemisphere Temperature and Response...................................................... 438 Variability .............................................................474 6.2.2 Methods – Palaeoclimate Modelling ................... 439 6.6.3 Comparisons of Millennial Simulations with Palaeodata ...................................................476 6.3 The Pre-Quaternary Climates ...................... 440 6.6.4 Consistency Between Temperature, Greenhouse 6.3.1 What is the Relationship Between Carbon Gas and Forcing Records; and Compatibility of Dioxide and Temperature in this Time Period? .... 440 Coupled Carbon Cycle-Climate Models with the Proxy Records ...............................................481 6.3.2 What Does the Record of the Mid-Pliocene Show? ...........................................440 6.6.5 Regional Variability in Quantities Other than Temperature ................................................. 481 6.3.3 What Does the Record of the Palaeocene-Eocene Thermal Maximum Show? ...................................442 6.7 Concluding Remarks on Key Uncertainties ............................................... 483 6.4 Glacial-Interglacial Variability and Dynamics ...................................................... 444 Frequently Asked Questions 6.4.1 Climate Forcings and Responses Over FAQ 6.1: What Caused the Ice Ages and Other Important Glacial-Interglacial Cycles ...................................444 Climate Changes Before the Industrial Era? ............ 449 Box 6.1: Orbital Forcing ................................................... 445 FAQ 6.2: Is the Current Climate Change Unusual Compared Box 6.2: What Caused the Low Atmospheric Carbon to Earlier Changes in Earth’s History? ................... 465 Dioxide Concentrations During Glacial Times? ................................................... 446 References ........................................................................ 484 6.4.2 Abrupt Climatic Changes in the Glacial-Interglacial Record ..................................454 6.4.3 Sea Level Variations Over the Last Supplementary Material Glacial-Interglacial Cycle .....................................457 6.5 The Current Interglacial ................................. 459 The following supplementary material is available on CD-ROM and in on-line versions of this report. 6.5.1 Climate Forcing and Response During the Current Interglacial ........................................ 459 Appendix 6.A: Glossary of Terms Specifi c to Chapter 6 Box 6.3: Holocene Glacier Variability ................................. 461 6.5.2 Abrupt Climate Change During the Current Interglacial ..............................................463 6.5.3 How and Why Has the El Niño-Southern Oscillation Changed Over the Present Interglacial? ............................................464 434 Chapter 6 Palaeoclimate Executive Summary • 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 What is the relationship between past greenhouse much greater cooling and expanded sea ice over the high- gas concentrations and climate? latitude oceans. Climate models are able to simulate the magnitude of these latitudinal ocean changes in response • The sustained rate of increase over the past century in to the estimated Earth orbital, greenhouse gas and land the combined radiative forcing from the three well-mixed surface changes for this period, and thus indicate that greenhouse gases carbon dioxide (CO2), methane (CH4), they adequately represent many of the major processes and nitrous oxide (N2O) is very likely unprecedented that determine this past climate state. in at least the past 16 kyr. Pre-industrial variations of atmospheric greenhouse gas concentrations observed • Last Glacial Maximum land data indicate signifi cant during the last 10 kyr were small compared to industrial cooling in the tropics (up to 5°C) and greater magnitudes era greenhouse gas increases, and were likely mostly due at high latitudes. Climate models vary in their capability to natural processes. to simulate these responses. • It is very likely that the current atmospheric concentrations • It is virtually certain that global temperatures during of CO2 (379 ppm) and CH4 (1,774 ppb) exceed by far the coming centuries will not be signifi cantly infl uenced by natural range of the last 650 kyr. Ice core data indicate a natural orbitally induced cooling. It is very unlikely that that CO2 varied within a range of 180 to 300 ppm and CH4 the Earth would naturally enter another ice age for at least within 320 to 790 ppb over this period. Over the same 30 kyr. period, antarctic temperature and CO2 concentrations co- vary, indicating a close relationship between climate and • During the last glacial period, abrupt regional warmings the carbon cycle. (likely up to 16°C within decades over Greenland) and coolings occurred repeatedly over the North Atlantic • It is very likely that glacial-interglacial CO2 variations region. They likely had global linkages, such as with have strongly amplifi ed climate variations, but it is major shifts in tropical rainfall patterns. It is unlikely that unlikely that CO2 variations have triggered the end of these events were associated with large changes in global glacial periods. Antarctic temperature started to rise mean surface temperature, but instead likely involved a several centuries before atmospheric CO2 during past redistribution of heat within the climate system associated glacial terminations. with changes in the Atlantic Ocean circulation. • It is likely that earlier periods with higher than present • Global sea level was likely between 4 and 6 m higher atmospheric CO2 concentrations were warmer than during the last interglacial period, about 125 ka, than in the present. This is the case both for climate states over 20th century. In agreement with palaeoclimatic evidence, millions of years (e.g., in the Pliocene, about 5 to 3 Ma) climate models simulate arctic summer warming of up and for warm events lasting a few hundred thousand to 5°C during the last interglacial. The inferred warming years (i.e., the Palaeocene-Eocene Thermal Maximum, was largest over Eurasia and northern Greenland, 55 Ma). In each of these two cases, warming was likely whereas the summit of Greenland was simulated to be strongly amplifi ed at high northern latitudes relative to 2°C to 5°C higher than present. This is consistent with lower latitudes. ice sheet modelling suggestions that large-scale retreat of the south Greenland Ice Sheet and other arctic ice fi elds What is the signifi cance of glacial-interglacial likely contributed a maximum of 2 to 4 m of sea level rise climate variability? during the last interglacial, with most of any remainder likely coming from the Antarctic Ice Sheet. • Climate models indicate that the Last Glacial Maximum (about 21 ka) was 3°C to 5°C cooler than the present What does the study of the current interglacial due to changes in greenhouse gas forcing and ice sheet climate show? conditions. Including the effects of atmospheric dust content and vegetation