Transcript model estimates that take into account both greenhouse gases and

Evaluation of climate models,
Attribution of climate change
IPCC Chpts 7,8 and 12.
John F B Mitchell
Hadley Centre
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How well do models simulate present climate?
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How well do they simulate past climate change?
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Can natural factors explain the last 100 years?
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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
Prediction May 97
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Initialization in December 1996
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Prediction for May 1997: Pacific SST
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good large-scale agreement with observations
Observation May 97
(Stockdale et al., 1998, Nature)
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Distribution of atmospheric water vapor
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satellite data
– (May, 5)
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high-resolution atmospheric model
– (May, 5)
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Stocker et al, IPCC 2001
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
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onset of decrease in ice extent
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accelerated decrease
(Vinnikov et al., 1999, Science; Chapter 7)
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Realism of climate models
has improved since SAR:
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higher grid resolution
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more physically based parameterizations
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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), 1949-1997
Anthropogenic 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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Temperature
Attributed trends depend on
observations not model simulated
trends
Original model trend
Observations
Time
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Temperature
Attributed trends depend on
observations not model simulated
trends
Scaling
Original model trend
Observations
Attributed trend
Time
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Temperature
Attributed trends depend on
observations not model simulated
trends
Original model trend
Observations
+ internal variability
Time
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Temperature
Attributed trends depend on
observations not model simulated
trends
Scaling
Original model trend
Upper
Lower
Observations
+ internal variability
Attributed trend
+ uncertainty estimate
Time
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Optimal detection
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Based on spatial and temporal patterns, not global means
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Different components can be scaled separately (eg greenhouses
gases, aerosols)
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“..most model estimates that take into account both greenhouse
gases and sulphate aerosols are consistent with observations [over
the last 50 years]”
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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
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Temperature
Substantial GHG warming
with small sulphate cooling
Small sulphate
cooling
Greenhouse
warming
Observations
Time
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Greenhouse warming
slightly larger than
observed
Temperature
More substantial GHG warming
with large sulphate cooling
Large sulphate
cooling
Greenhouse
warming
Observations
Time
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Greenhouse warming
much larger than
observed
Aerosol forcing
The response to
different forcings
may be
surprisingly similar
Aerosol response
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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
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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
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Allen and Raper pers com, 2001 based on TAR results
Uncertainties
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Difference in recent surface and free atmosphere trends
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Size of internal variability
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Natural forcing
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Anthropogenic forcing , especially aerosols
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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”
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