9 Understanding and Attributing Climate Change Coordinating Lead Authors: Gabriele C. Hegerl (USA, Germany), Francis W. Zwiers (Canada) Lead Authors: Pascale Braconnot (France), Nathan P. Gillett (UK), Yong Luo (China), Jose A. Marengo Orsini (Brazil, Peru), Neville Nicholls (Australia), Joyce E. Penner (USA), Peter A. Stott (UK) Contributing Authors: M. Allen (UK), C. Ammann (USA), N. Andronova (USA), R.A. Betts (UK), A. Clement (USA), W.D. Collins (USA), S. Crooks (UK), T.L. Delworth (USA), C. Forest (USA), P. Forster (UK), H. Goosse (Belgium), J.M. Gregory (UK), D. Harvey (Canada), G.S. Jones (UK), F. Joos (Switzerland), J. Kenyon (USA), J. Kettleborough (UK), V. Kharin (Canada), R. Knutti (Switzerland), F.H. Lambert (UK), M. Lavine (USA), T.C.K. Lee (Canada), D. Levinson (USA), V. Masson-Delmotte (France), T. Nozawa (Japan), B. Otto-Bliesner (USA), D. Pierce (USA), S. Power (Australia), D. Rind (USA), L. Rotstayn (Australia), B. D. Santer (USA), C. Senior (UK), D. Sexton (UK), S. Stark (UK), D.A. Stone (UK), S. Tett (UK), P. Thorne (UK), R. van Dorland (The Netherlands), M. Wang (USA), B. Wielicki (USA), T. Wong (USA), L. Xu (USA, China), X. Zhang (Canada), E. Zorita (Germany, Spain) Review Editors: David J. Karoly (USA, Australia), Laban Ogallo (Kenya), Serge Planton (France) This chapter should be cited as: Hegerl, G.C., F. W. Zwiers, P. Braconnot, N.P. Gillett, Y. Luo, J.A. Marengo Orsini, N. Nicholls, J.E. Penner and P.A. Stott, 2007: Under- standing and Attributing Climate Change. 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. Understanding and Attributing Climate Change Chapter 9 Table of Contents Executive Summary .................................................... 665 9.6 Observational Constraints on Climate Sensitivity ............................................. 718 9.1 Introduction ......................................................... 667 9.6.1 Methods to Estimate Climate Sensitivity ............. 718 9.1.1 What are Climate Change and 9.6.2 Estimates of Climate Sensitivity Based on Climate Variability? .............................................. 667 Instrumental Observations .................................. 719 9.1.2 What are Climate Change Detection 9.6.3 Estimates of Climate Sensitivity Based on and Attribution? ................................................... 667 Palaeoclimatic Data ............................................. 724 9.1.3 The Basis from which We Begin .......................... 669 9.6.4 Summary of Observational Constraints for Climate Sensitivity ............................................... 725 9.2 Radiative Forcing and Climate Response ............................................................... 670 9.7 Combining Evidence of Anthropogenic 9.2.1 Radiative Forcing Estimates Used to Climate Change ................................................... 727 Simulate Climate Change .................................... 671 Frequently Asked Questions 9.2.2 Spatial and Temporal Patterns of the Response to Different Forcings and their Uncertainties ....... 674 FAQ 9.1: Can Individual Extreme Events be Explained by Greenhouse Warming? ..................................... 696 9.2.3 Implications for Understanding 20th-Century Climate Change ................................................... 678 FAQ 9.2: Can the Warming of the 20th Century be Explained by Natural Variability? ......................... 702 9.2.4 Summary ............................................................. 678 References ........................................................................ 733 9.3 Understanding Pre-Industrial Climate Change ................................................... 679 Appendix 9.A: Methods Used to Detect 9.3.1 Why Consider Pre-Industrial Climate Externally Forced Signals .......................................... 744 Change? .............................................................. 679 9.3.2 What can be Learned from the Last Glacial Maximum and the Mid-Holocene? ...................... 679 Supplementary Material 9.3.3 What can be Learned from the Past 1,000 Years? ........................................................ 680 The following supplementary material is available on CD-ROM and in on-line versions of this report. 9.3.4 Summary ............................................................. 683 Appendix 9.B: Methods Used to Estimate Climate Sensitivity and Aerosol 9.4 Understanding of Air Temperature Forcing Change During the Industrial Era .............. 683 Appendix 9.C: Notes and technical details on Figures displayed in Chapter 9 9.4.1 Global-Scale Surface Temperature Change ........ 683 Appendix 9.D: Additional Figures and Tables 9.4.2 Continental and Sub-continental Surface References for Appendices 9.B to 9.D Temperature Change ........................................... 693 9.4.3 Surface Temperature Extremes ........................... 698 9.4.4 Free Atmosphere Temperature ............................ 699 9.4.5 Summary ............................................................. 704 9.5 Understanding of Change in Other Variables during the Industrial Era ............. 705 9.5.1 Ocean Climate Change ....................................... 705 9.5.2 Sea Level ............................................................. 707 9.5.3 Atmospheric Circulation Changes ....................... 709 9.5.4 Precipitation ......................................................... 712 9.5.5 Cryosphere Changes ........................................... 716 9.5.6 Summary ............................................................. 717 664 Chapter 9 Understanding and Attributing Climate Change Executive Summary century. Anthropogenic influence has been detected in every continent except Antarctica (which has insufficient observational coverage to make an assessment), and in some Evidence of the effect of external influences on the climate sub-continental land areas. The ability of coupled climate models system has continued to accumulate since the Third Assessment to simulate the temperature evolution on continental scales and Report (TAR). The evidence now available is substantially the detection of anthropogenic effects on each of six continents stronger and is based on analyses of widespread temperature provides stronger evidence of human influence on the global increases throughout the climate system and changes in other climate than was available at the time of the TAR. No climate climate variables. model that has used natural forcing only has reproduced the observed global mean warming trend or the continental mean Human-induced warming of the climate system is warming trends in all individual continents (except Antarctica) widespread. Anthropogenic warming of the climate system can over the second half of the 20th century. be detected in temperature observations taken at the surface, Difficulties remain in attributing temperature changes on in the troposphere and in the oceans. Multi-signal detection smaller than continental scales and over time scales of less than and attribution analyses, which quantify the contributions 50 years. Attribution at these scales, with limited exceptions, of different natural and anthropogenic forcings to observed has not yet been established. Averaging over smaller regions changes, show that greenhouse gas forcing alone during the reduces the natural variability less than does averaging over past half century would likely have resulted in greater than the large regions, making it more difficult to distinguish between observed warming if there had not been an offsetting cooling changes expected from different external forcings, or between effect from aerosol and other forcings. external forcing and variability. In addition, temperature changes It is extremely unlikely (<5%) that the global pattern of associated with some modes of variability are poorly simulated warming during the past half century can be explained without by models in some regions and seasons. Furthermore, the small- external forcing, and very unlikely that it is due to known scale details of external forcing, and the response simulated by natural external causes alone. The warming occurred in both the models are less credible than large-scale features. ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling. Surface temperature extremes have likely been affected Greenhouse gas forcing has very likely caused most of the by anthropogenic forcing. Many indicators of climate extremes observed global warming over the last 50 years. This conclusion and variability, including the annual numbers of frost days, takes into account observational and forcing uncertainty, and warm and cold days, and warm and cold nights, show changes the possibility that the response to solar forcing could be that are consistent with warming. An anthropogenic influence underestimated by climate models. It is also robust to the use has been detected in some of these indices, and there is evidence of different climate models, different methods for estimating that anthropogenic forcing may have substantially increased the the responses to external forcing and variations in the analysis risk of extremely warm summer conditions regionally, such