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Evaluation of climate models, Attribution of climate change IPCC Chpts 7,8 and 12. John F B Mitchell Hadley Centre How well do models simulate present climate? How well do they simulate past climate change? Can natural factors explain the last 100 years? Can human factors explain recent changes? 00/XXXX 1 „Confidence in the ability of models to project future climates has increased.“ [Summary for Policymakers, WG1] 00/XXXX 2 Prediction and Observation of 1997/98 ENSO Prediction May 97 Initialization in December 1996 Prediction for May 1997: Pacific SST good large-scale agreement with observations Observation May 97 (Stockdale et al., 1998, Nature) 00/XXXX 3 Distribution of atmospheric water vapor satellite data – (May, 5) high-resolution atmospheric model – (May, 5) 00/XXXX 4 Stocker et al, IPCC 2001 Global mean temperature from an ensemble of 4 simulations using natural and anthropogenic forcing Stott et al, Science 2000 00/XXXX 5 Arctic Sea Ice Cover: Observation and Simulation onset of decrease in ice extent accelerated decrease (Vinnikov et al., 1999, Science; Chapter 7) 00/XXXX 6 Realism of climate models has improved since SAR: higher grid resolution more physically based parameterizations more complete coupling Yet, models are not perfect! 00/XXXX 7 “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities” SPM 00/XXXX 8 “There is a longer and more closely scutinised temperature record” SPM 00/XXXX 9 Can Natural factors alone explain the recent temperature record? 00/XXXX 10 “The warming over the last hundred years is very unlikely to be due to internal variability alone as estimated from current models” SPM 00/XXXX 11 “Reconstructions of climate data for the last 1000 years also indicate that this warming was unusual and unlikely to be entirely natural in origin” SPM 00/XXXX 12 “Simulations of the response to natural forcings alone … do not explain the warming in the second half of the century” SPM Stott et al, Science 2000 00/XXXX 13 Can anthropogenic factors explain the temperature recent record? 00/XXXX 14 “..model estimates that take into account both greenhouse gases and sulphate aerosols are consistent with observations over this*period” SPM Stott et al, Science 2000 00/XXXX 15 Temperature trends (C), 1949-1997 Anthropogenic forcing improves agreement with observations Knutson et al, 2000 00/XXXX 16 “..attribution studies .. can now take into uncertainty in the magnitude of the modelled response to external forcing…” SPM 00/XXXX 17 Temperature Attributed trends depend on observations not model simulated trends Original model trend Observations Time 00/XXXX 18 Temperature Attributed trends depend on observations not model simulated trends Scaling Original model trend Observations Attributed trend Time 00/XXXX 19 Temperature Attributed trends depend on observations not model simulated trends Original model trend Observations + internal variability Time 00/XXXX 20 Temperature Attributed trends depend on observations not model simulated trends Scaling Original model trend Upper Lower Observations + internal variability Attributed trend + uncertainty estimate Time 00/XXXX 21 Optimal detection 00/XXXX 22 Based on spatial and temporal patterns, not global means Different components can be scaled separately (eg greenhouses gases, aerosols) “..most model estimates that take into account both greenhouse gases and sulphate aerosols are consistent with observations [over the last 50 years]” The observations can be used to “correct” model predictions, with uncertainty limits “The anthropogenic sulphate aerosol forcing, while uncertain, is negative over this period and therefore cannot explain the warming” SPM 00/XXXX 23 Temperature Substantial GHG warming with small sulphate cooling Small sulphate cooling Greenhouse warming Observations Time 00/XXXX 24 Greenhouse warming slightly larger than observed Temperature More substantial GHG warming with large sulphate cooling Large sulphate cooling Greenhouse warming Observations Time 00/XXXX 25 Greenhouse warming much larger than observed Aerosol forcing The response to different forcings may be surprisingly similar Aerosol response 00/XXXX 26 CO2 response Reader and Boer , 1998 “Most studies find that, over the last 50 years, the estimated rate and magnitude of warming due to increasing greenhouse gases is comparable with or larger than the observed warming” SPM Degrees per century 3.0 * range includes sulphate "warming" 2.0 * 1.0 * * 0.0 Full century Last 50 years 00/XXXX 27 Estimated 5-95% range in attributable trends Ha d CM2 Ha d CM2 ECH AM3 Ha d CM3 Ha d CM2 Ha d CM2 Ha d CM2 Ha d CM2 ECH AM3 Ha d CM2 ECH AM3 ECH AM3 Ha d CM3 ECH AM4 Ha d CM3 -1.0 Model uncertainty in predictions 00/XXXX 28 Allen and Raper pers com, 2001 based on TAR results Uncertainties 00/XXXX 29 Difference in recent surface and free atmosphere trends Size of internal variability Natural forcing Anthropogenic forcing , especially aerosols Estimate of response (sensitivity) “In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to be due to the increases in greenhouse gas concentrations” 00/XXXX 30