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?

[Summary for Policymakers, WG1] „Confidence in the ability of models to project future climates has increased.“ [Summary for Policymakers, WG1]

Prediction and Observation of 1997/98 ENSO Prediction May 97 Observation May 97 Initialization in December 1996 Prediction for May 1997: Pacific SST good large-scale agreement with observations (Stockdale et al., 1998, Nature)

Distribution of atmospheric water vapor satellite data (May, 5) high-resolution atmospheric model (May, 5) Stocker et al, IPCC 2001

Global mean temperature from an ensemble of 4 simulations using natural and anthropogenic forcing Stott et al, Science 2000

Arctic Sea Ice Cover: Observation and Simulation onset of decrease in ice extent accelerated decrease agreement with GFDL model depends on sea ice thickness criterion Hadley Centre has systematic bias (Vinnikov et al., 1999, Science; Chapter 7)

Realism of climate models has improved since SAR: higher grid resolution more physically based parameterizations more complete coupling major deficiencies require continued attention. They include: representation of clouds and their interactions with radiation and aerosols natural variability, such as NAO and ENSO, esp. frequency of ENSO oceanic flow systsems in narrow boundary currents and over sills representation of mixing in the ocean simulation of sea ice dynamics Yet, models are not perfect!

“There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities” SPM

“There is a longer and more closely scutinised temperature record” SPM

Can Natural factors alone explain the recent temperature record?

“The warming over the last hundred years is very unlikely to be due to internal variability alone as estimated from current models” SPM

“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

“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

Can anthropogenic factors explain the temperature recent record?

“..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

Temperature trends (C), 1949-1997 Anthropogenic forcing improves agreement with observations Knutson et al, 2000

“..attribution studies .. can now take into uncertainty in the magnitude of the modelled response to external forcing…” SPM

Attributed trends depend on observations not model simulated trends Original model trend Temperature Observations Time

Attributed trends depend on observations not model simulated trends Scaling Original model trend Temperature Observations Attributed trend Time

Attributed trends depend on observations not model simulated trends Original model trend Temperature Observations + internal variability Time

Attributed trends depend on observations not model simulated trends Scaling Original model trend Upper Temperature Lower Observations + internal variability Attributed trend + uncertainty estimate Time

Optimal detection 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

Substantial GHG warming with small sulphate cooling Temperature Small sulphate cooling Greenhouse warming Greenhouse warming slightly larger than observed Observations Time

More substantial GHG warming with large sulphate cooling Temperature Large sulphate cooling Greenhouse warming Greenhouse warming much larger than observed Observations Time

The response to different forcings may be surprisingly similar Aerosol forcing The response to different forcings may be surprisingly similar Aerosol response 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 Estimated 5-95% range in attributable trends

Model uncertainty in predictions Allen and Raper pers com, 2001 based on TAR results

Uncertainties 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”