6PalaeoclimateCoordinating 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.434Palaeoclimate Chapter 6Table of ContentsExecutive Summary .................................................... 4356.1 Introduction ......................................................... 4386.2 Palaeoclimatic Methods ................................... 4386.2.1 Methods – Observations of Forcing and Response...................................................... 4386.2.2 Methods – Palaeoclimate Modelling ................... 4396.3 The Pre-Quaternary Climates ...................... 4406.3.1 What is the Relationship Between Carbon Dioxide and Temperature in this Time Period? .... 4406.3.2 What Does the Record of the Mid-Pliocene Show? ........................................... 4406.3.3 What Does the Record of the Palaeocene-Eocene Thermal Maximum Show? ................................... 4426.4 Glacial-Interglacial Variability and Dynamics ...................................................... 4446.4.1 Climate Forcings and Responses Over Glacial-Interglacial Cycles ................................... 444Box 6.1: Orbital Forcing ................................................... 445Box 6.2: What Caused the Low Atmospheric Carbon Dioxide Concentrations During Glacial Times? ................................................... 4466.4.2 Abrupt Climatic Changes in the Glacial-Interglacial Record .................................. 4546.4.3 Sea Level Variations Over the Last Glacial-Interglacial Cycle ..................................... 4576.5 The Current Interglacial ................................. 4596.5.1 Climate Forcing and Response During the Current Interglacial ........................................ 459Box 6.3: Holocene Glacier Variability ................................. 4616.5.2 Abrupt Climate Change During the Current Interglacial .............................................. 4636.5.3 How and Why Has the El Niño-Southern Oscillation Changed Over the Present Interglacial? ............................................ 4646.6 The Last 2,000 Years .......................................... 4666.6.1 Northern Hemisphere Temperature Variability ..... 466Box 6.4: Hemispheric Temperatures in the ‘Medieval Warm Period’ ..................................... 4686.6.2 Southern Hemisphere Temperature Variability ............................................................. 4746.6.3 Comparisons of Millennial Simulations with Palaeodata ................................................... 4766.6.4 Consistency Between Temperature, Greenhouse Gas and Forcing Records; and Compatibility of Coupled Carbon Cycle-Climate Models with the Proxy Records ............................................... 4816.6.5 Regional Variability in Quantities Other than Temperature ................................................. 4816.7 Concluding Remarks on Key Uncertainties ............................................... 483Frequently Asked QuestionsFAQ 6.1: What Caused the Ice Ages and Other Important Climate Changes Before the Industrial Era? ............ 449FAQ 6.2: Is the Current Climate Change Unusual Compared to Earlier Changes in Earth’s History? ................... 465 References ........................................................................ 484Supplementary Material The following supplementary material is available on CD-ROM and in on-line versions of this report.Appendix 6.A: Glossary of Terms Specific to Chapter 6435Chapter 6 PalaeoclimateExecutive SummaryWhat 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