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Evaluation of climate models,

Attribution of climate change

IPCC Chpts 7,8 and 12.

John F B MitchellHadley 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?

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„Confidence in the ability

of models to project future

climates has increased.“

[Summary for Policymakers, WG1]

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Prediction and Observation of 1997/98 ENSO

Initialization in December 1996 Prediction for May 1997: Pacific SST

(Stockdale et al., 1998, Nature)

Prediction May 97 Observation May 97

good large-scale agreement with observations

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Distribution of atmospheric water vapor

Stocker et al, IPCC 2001

high-resolution atmospheric model

– (May, 5)

satellite data

– (May, 5)

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Global mean temperature from an ensemble of 4 simulations using natural and anthropogenic

forcing

Stott et al, Science 2000

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Arctic Sea Ice Cover: Observation and Simulation

onset of decrease in ice extent

accelerated decrease

(Vinnikov et al., 1999, Science; Chapter 7)

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Realism of climate modelshas improved since SAR:

higher grid resolution

more physically based parameterizations

more complete coupling

Yet, models are not perfect!

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“There is new and stronger evidence that most of the

warming observed over the last 50 years is attributable to human activities” SPM

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“There is a longer and more closely

scutinised temperature record” SPM

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Can Natural factors alone explain the

recent temperature record?

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“The warming over the last hundred years is very unlikely to be due to internal

variability alone as estimated from current models” SPM

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“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

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“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

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Can anthropogenic factors explain the

temperature recent record?

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“..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

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Temperature trends (C), 1949-1997Anthropogenic forcing improves

agreement with observations

Knutson et al, 2000

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“..attribution studies .. can now take into uncertainty in the magnitude of the modelled response to

external forcing…” SPM

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Attributed trends depend on observations not model simulated

trends

Original model trend

Observations

Time

Tem

pera

ture

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Attributed trends depend on observations not model

simulated trends

Original model trend

Observations

Time

Tem

pera

ture

Attributed trend

Scaling

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Attributed trends depend on observations not model

simulated trends

Original model trend

Observations

Time

Tem

pera

ture

+ internal variability

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Attributed trends depend on observations not model

simulated trends

Original model trend

Observations

Scaling

Lower

Upper

Attributed trend

Time

Tem

pera

ture

+ internal variability

+ uncertainty estimate

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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

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“The anthropogenic sulphate aerosol forcing, while

uncertain, is negative over this period and therefore

cannot explain the warming” SPM

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Substantial GHG warming with small sulphate

cooling

Greenhouse warmingslightly larger thanobservedObservations

Time

Tem

pera

ture

Small sulphatecoolingGreenhouse

warming

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More substantial GHG warming with large sulphate

cooling

Greenhouse warmingmuch larger thanobservedObservations

Time

Tem

pera

ture

Large sulphatecoolingGreenhouse

warming

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Reader and Boer , 1998

Aerosol forcing

Aerosol response CO2 response

The response to different forcings may be surprisingly similar

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“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

-1.0

0.0

1.0

2.0

3.0

De

gre

es

pe

r c

en

tury

Full century Last 50 years

**

*

range includes sulphate "warming"*

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Model uncertainty in predictions

Allen and Raper pers com, 2001 based on TAR results

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Uncertainties

Difference in recent surface and free atmosphere trends

Size of internal variability Natural forcing Anthropogenic forcing , especially

aerosols Estimate of response (sensitivity)

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“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”