8 Climate Models and Their Evaluation Coordinating Lead Authors: David A. Randall (USA), Richard A. Wood (UK) Lead Authors: Sandrine Bony (France), Robert Colman (Australia), Thierry Fichefet (Belgium), John Fyfe (Canada), Vladimir Kattsov (Russian Federation), Andrew Pitman (Australia), Jagadish Shukla (USA), Jayaraman Srinivasan (India), Ronald J. Stouffer (USA), Akimasa Sumi (Japan), Karl E. Taylor (USA) Contributing Authors: K. AchutaRao (USA), R. Allan (UK), A. Berger (Belgium), H. Blatter (Switzerland), C. Bonfi ls (USA, France), A. Boone (France, USA), C. Bretherton (USA), A. Broccoli (USA), V. Brovkin (Germany, Russian Federation), W. Cai (Australia), M. Claussen (Germany), P. Dirmeyer (USA), C. Doutriaux (USA, France), H. Drange (Norway), J.-L. Dufresne (France), S. Emori (Japan), P. Forster (UK), A. Frei (USA), A. Ganopolski (Germany), P. Gent (USA), P. Gleckler (USA), H. Goosse (Belgium), R. Graham (UK), J.M. Gregory (UK), R. Gudgel (USA), A. Hall (USA), S. Hallegatte (USA, France), H. Hasumi (Japan), A. Henderson-Sellers (Switzerland), H. Hendon (Australia), K. Hodges (UK), M. Holland (USA), A.A.M. Holtslag (Netherlands), E. Hunke (USA), P. Huybrechts (Belgium), W. Ingram (UK), F. Joos (Switzerland), B. Kirtman (USA), S. Klein (USA), R. Koster (USA), P. Kushner (Canada), J. Lanzante (USA), M. Latif (Germany), N.-C. Lau (USA), M. Meinshausen (Germany), A. Monahan (Canada), J.M. Murphy (UK), T. Osborn (UK), T. Pavlova (Russian Federationi), V. Petoukhov (Germany), T. Phillips (USA), S. Power (Australia), S. Rahmstorf (Germany), S.C.B. Raper (UK), H. Renssen (Netherlands), D. Rind (USA), M. Roberts (UK), A. Rosati (USA), C. Schär (Switzerland), A. Schmittner (USA, Germany), J. Scinocca (Canada), D. Seidov (USA), A.G. Slater (USA, Australia), J. Slingo (UK), D. Smith (UK), B. Soden (USA), W. Stern (USA), D.A. Stone (UK), K.Sudo (Japan), T. Takemura (Japan), G. Tselioudis (USA, Greece), M. Webb (UK), M. Wild (Switzerland) Review Editors: Elisa Manzini (Italy), Taroh Matsuno (Japan), Bryant McAvaney (Australia) This chapter should be cited as: Randall, D.A., R.A. Wood, S. Bony, R. Colman, T. Fichefet, J. Fyfe, V. Kattsov, A. Pitman, J. Shukla, J. Srinivasan, R.J. Stouffer, A. Sumi and K.E. Taylor, 2007: Cilmate Models and Their Evaluation. 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. Climate Models and Their Evaluation Chapter 8 Table of Contents Executive Summary .................................................... 591 8.5 Model Simulations of Extremes ..................... 627 8.5.1 Extreme Temperature ..........................................627 8.1 Introduction and Overview ............................ 594 8.5.2 Extreme Precipitation ..........................................628 8.1.1 What is Meant by Evaluation? .............................594 8.5.3 Tropical Cyclones ................................................628 8.1.2 Methods of Evaluation .........................................594 8.5.4 Summary .............................................................629 8.1.3 How Are Models Constructed? ...........................596 8.6 Climate Sensitivity and Feedbacks ............... 629 8.2 Advances in Modelling .................................... 596 8.6.1 Introduction .........................................................629 8.2.1 Atmospheric Processes ....................................... 602 8.6.2 Interpreting the Range of Climate Sensitivity 8.2.2 Ocean Processes ................................................603 Estimates Among General Circulation Models ....629 8.2.3 Terrestrial Processes ...........................................604 Box 8.1: Upper-Tropospheric Humidity and Water 8.2.4 Cryospheric Processes........................................ 606 Vapour Feedback .............................................. 632 8.2.5 Aerosol Modelling and Atmospheric 8.6.3 Key Physical Processes Involved in Chemistry ............................................................607 Climate Sensitivity ...............................................633 8.2.6 Coupling Advances .............................................607 8.6.4 How to Assess Our Relative Confi dence in Feedbacks Simulated by Different Models?........ 639 8.2.7 Flux Adjustments and Initialisation ......................607 8.3 Evaluation of Contemporary Climate as 8.7 Mechanisms Producing Thresholds and Abrupt Climate Change ................................... 640 Simulated by Coupled Global Models........ 608 8.7.1 Introduction .........................................................640 8.3.1 Atmosphere .........................................................608 8.7.2 Forced Abrupt Climate Change ...........................640 8.3.2 Ocean ..................................................................613 8.7.3 Unforced Abrupt Climate Change .......................643 8.3.3 Sea Ice .................................................................616 8.3.4 Land Surface .......................................................617 8.8 Representing the Global System with 8.3.5 Changes in Model Performance ..........................618 Simpler Models .................................................... 643 8.8.1 Why Lower Complexity? .....................................643 8.4 Evaluation of Large-Scale Climate Variability as Simulated by Coupled 8.8.2 Simple Climate Models....................................... 644 Global Models ..................................................... 620 8.8.3 Earth System Models of Intermediate Complexity........................................................... 644 8.4.1 Northern and Southern Annular Modes .............620 8.4.2 Pacifi c Decadal Variability ..................................621 Frequently Asked Question 8.4.3 Pacifi c-North American Pattern ......................... 622 FAQ 8.1: How Reliable Are the Models Used to Make 8.4.4 Cold Ocean-Warm Land Pattern ........................622 Projections of Future Climate Change? .................. 600 8.4.5 Atmospheric Regimes and Blocking ..................623 References ........................................................................ 648 8.4.6 Atlantic Multi-decadal Variability ........................623 8.4.7 El Niño-Southern Oscillation ..............................623 Supplementary Material 8.4.8 Madden-Julian Oscillation .................................. 625 8.4.9 Quasi-Biennial Oscillation ..................................625 The following supplementary material is available on CD-ROM and in on-line versions of this report. 8.4.10 Monsoon Variability ............................................626 Figures S8.1–S8.15: Model Simulations for Different Climate Variables 8.4.11 Shorter-Term Predictions Using Table S8.1: MAGICC Parameter Values Climate Models ..................................................626 590 Chapter 8 Climate Models and Their Evaluation Executive Summary At the same time, there have been improvements in the simulation of many aspects of present climate. The uncertainty associated with the use of fl ux adjustments This chapter assesses the capacity of the global climate has therefore decreased, although biases and long-term models used elsewhere in this report for projecting future trends remain in AOGCM control simulations. climate change. Confi dence in model estimates of future climate evolution has been enhanced via a range of advances since the • Progress in the simulation of important modes of climate IPCC Third Assessment Report (TAR). variability has increased the overall confi dence in the Climate models are based on well-established physical models’ representation of important climate processes. principles and have been demonstrated to reproduce observed As a result of steady progress, some AOGCMs can now features of recent climate (see Chapters 8 and 9) and past climate simulate important aspects of the El Niño-Southern changes (see Chapter 6). There is considerable confi dence that Oscillation (ENSO). Simulation of the Madden-Julian Atmosphere-Ocean General Circulation Models (AOGCMs) Oscillation (MJO) remains unsatisfactory. provide credible quantitative estimates of future climate change, particularly at continental and larger scales. Confi dence • The ability of AOGCMs to simulate extreme events, in these estimates is higher for some climate variables (e.g., especially hot and cold spells, has improved. The temperature) than for others (e.g., precipitation). This summary frequency and amount of precipitation falling in intense highlights areas of progress since the TAR: events are underestimated. • Enhanced scrutiny of models and expanded diagnostic • Simulation of extratropical cyclones has improved. Some analysis of model behaviour have been increasingly models used for projections of tropical cyclone changes facilitated by internationally coordinated efforts to can simulate successfully the observed frequency and collect and disseminate output from model experiments distribution of tropical cyclones. performed under common conditions. This has encouraged a more comprehensive and open evaluation of models. • Systematic biases have been found in most models’ The expanded evaluation effort, encompassing a diversity simulation of the Southern Ocean. Since the Southern of perspectives, makes it less likely that signifi