Climate Models and Their Evaluation

1 8 Climate Models and Their EvaluationCoordinating 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), 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), Gregory (UK),R. Gudgel (USA), A. Hall (USA), S. Hallegatte (USA, France), H.

2 Hasumi (Japan), A. Henderson-Sellers (Switzerland), H. Hendon (Australia), K. Hodges (UK), M. Holland (USA), 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), Lau (USA), M. Meinshausen (Germany), A. Monahan (Canada), Murphy (UK), T. Osborn (UK), T. Pavlova (Russian Federationi), V. Petoukhov (Germany), T. Phillips (USA), S. Power (Australia), S. Rahmstorf (Germany), Raper (UK), H. Renssen (Netherlands), D. Rind (USA), M. Roberts (UK), A. Rosati (USA), C. Sch r (Switzerland), A. Schmittner (USA, Germany), J. Scinocca (Canada), D. Seidov (USA), Slater (USA, Australia), J. Slingo (UK), D. Smith (UK), B. Soden (USA), W. Stern (USA), Stone (UK), (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, , Wood, S.

3 Bony, R. Colman, T. Fichefet, J. Fyfe, V. Kattsov, A. Pitman, J. Shukla, J. Srinivasan, Stouffer, A. Sumi and 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, Averyt, and Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, Models and Their Evaluation Chapter 8 Table of ContentsExecutive Summary .. Introduction and Overview .. What is Meant by Evaluation ? .. Methods of Evaluation .. How Are Models Constructed? .. Advances in Modelling .. Atmospheric Processes .. Ocean Processes .. Terrestrial Processes .. Cryospheric Aerosol Modelling and Atmospheric Chemistry .. Coupling Advances .. Flux Adjustments and Initialisation.

4 Evaluation of Contemporary Climate as Simulated by Coupled Global Atmosphere .. Ocean .. Sea Ice .. Land Surface .. Changes in model Performance .. Evaluation of Large-Scale Climate Variability as Simulated by Coupled Global Models .. Northern and Southern Annular Modes .. Pacifi c Decadal Variability .. Pacifi c-North American Pattern .. Cold Ocean-Warm Land Pattern .. Atmospheric Regimes and Blocking .. Atlantic Multi-decadal Variability .. El Ni o-Southern Oscillation .. Madden-Julian Oscillation .. Quasi-Biennial Oscillation .. Monsoon Variability .. Shorter-Term Predictions Using Climate Models .. model Simulations of Extremes .. Extreme Temperature .. Extreme Precipitation .. Tropical Cyclones .. Summary .. Climate Sensitivity and Feedbacks .. Introduction .. Interpreting the Range of Climate Sensitivity Estimates Among General Circulation Models .

5 629 Box : Upper-Tropospheric Humidity and Water Vapour Feedback .. Key Physical Processes Involved in Climate Sensitivity .. How to Assess Our Relative Confi dence in Feedbacks Simulated by Different Models ?.. Mechanisms Producing Thresholds and Abrupt Climate change .. Introduction .. Forced Abrupt Climate change .. Unforced Abrupt Climate change .. Representing the Global System with Simpler Models .. Why Lower Complexity? .. Simple Climate Earth System Models of Intermediate 644 Frequently Asked QuestionFAQ : How Reliable Are the Models Used to Make Projections of Future Climate change ? .. 600 References .. 648 Supplementary Material The following supplementary material is available on CD-ROM and in on-line versions of this : model Simulations for Different Climate VariablesTable : MAGICC Parameter Values591 Chapter 8 Climate Models and Their EvaluationExecutive SummaryThis chapter assesses the capacity of the global Climate Models used elsewhere in this report for projecting future Climate change .

6 Confi dence in model estimates of future Climate evolution has been enhanced via a range of advances since the IPCC Third Assessment Report (TAR). Climate Models are based on well-established physical principles and have been demonstrated to reproduce observed features of recent Climate (see Chapters 8 and 9) and past Climate changes (see Chapter 6). There is considerable confi dence that Atmosphere-Ocean General Circulation Models (AOGCMs) provide credible quantitative estimates of future Climate change , particularly at continental and larger scales. Confi dence in these estimates is higher for some Climate variables ( , temperature) than for others ( , precipitation). This summary highlights areas of progress since the TAR: Enhanced scrutiny of Models and expanded diagnostic analysis of model behaviour have been increasingly facilitated by internationally coordinated efforts to collect and disseminate output from model experiments performed under common conditions.

7 This has encouraged a more comprehensive and open Evaluation of Models . The expanded Evaluation effort, encompassing a diversity of perspectives, makes it less likely that signifi cant model errors are being overlooked. Climate Models are being subjected to more comprehensive tests, including, for example, evaluations of forecasts on time scales from days to a year. This more diverse set of tests increases confi dence in the fi delity with which Models represent processes that affect Climate projections. Substantial progress has been made in understanding the inter- model differences in equilibrium Climate sensitivity. Cloud feedbacks have been confi rmed as a primary source of these differences, with low clouds making the largest contribution. New observational and modelling evidence strongly supports a combined water vapour-lapse rate feedback of a strength comparable to that found in General Circulation Models (approximately 1 W m 2 C 1, corresponding to around a 50% amplifi cation of global mean warming).

8 The magnitude of cryospheric feedbacks remains uncertain, contributing to the range of model Climate responses at mid- to high latitudes. There have been ongoing improvements to resolution, computational methods and parametrizations, and additional processes ( , interactive aerosols) have been included in more of the Climate Models . Most AOGCMs no longer use fl ux adjustments, which were previously required to maintain a stable Climate . 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 has therefore decreased, although biases and long-term trends remain in AOGCM control simulations. Progress in the simulation of important modes of Climate variability has increased the overall confi dence in the Models representation of important Climate processes. As a result of steady progress, some AOGCMs can now simulate important aspects of the El Ni o-Southern Oscillation (ENSO).

9 Simulation of the Madden-Julian Oscillation (MJO) remains unsatisfactory. The ability of AOGCMs to simulate extreme events, especially hot and cold spells, has improved. The frequency and amount of precipitation falling in intense events are underestimated. Simulation of extratropical cyclones has improved. Some Models used for projections of tropical cyclone changes can simulate successfully the observed frequency and distribution of tropical cyclones. Systematic biases have been found in most Models simulation of the Southern Ocean. Since the Southern Ocean is important for ocean heat uptake, this results in some uncertainty in transient Climate response. The possibility that metrics based on observations might be used to constrain model projections of Climate change has been explored for the fi rst time, through the analysis of ensembles of model simulations. Nevertheless, a proven set of model metrics that might be used to narrow the range of plausible Climate projections has yet to be developed.

10 To explore the potential importance of carbon cycle feedbacks in the Climate system, explicit treatment of the carbon cycle has been introduced in a few Climate AOGCMs and some Earth System Models of Intermediate Complexity (EMICs). Earth System Models of Intermediate Complexity have been evaluated in greater depth than previously. Coordinated intercomparisons have demonstrated that these Models are useful in addressing questions involving long time scales or requiring a large number of ensemble simulations or sensitivity Models and Their Evaluation Chapter 8 Developments in model formulationImprovements in atmospheric Models include reformulated dynamics and transport schemes, and increased horizontal and vertical resolution. Interactive aerosol modules have been incorporated into some Models , and through these, the direct and the indirect effects of aerosols are now more widely cant developments have occurred in the representation of terrestrial processes.