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Evaluation of climate models,
Attribution of climate change
IPCC Chapters 7,8 and 12.
John F B Mitchell
Hadley Centre
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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?
“Simulations of the response to natural
forcings alone … do not explain the warming in
the second half of the century”
SPM
Global mean temperature from an ensemble of 4
simulations using natural and anthropogenic forcing
Stott et al, Science 2000
“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
Arctic Sea Ice Cover: Observation and
(Vinnikov et al., 1999, Science; Chapter 7)
Simulation
Glacier mass
balance
During the 20th century, glaciers and ice
caps have experienced widespread mass
losses and have contributed to sea level
rise
Decline of mountain glaciers projected
to reduce water availability in many
regions
Cumulative balance
of glacier mass in some regions
“The warming over the last hundred years is very
unlikely to be due to internal variability alone as
estimated from current models” SPM
Optimal detection
• Based on spatial and temporal patterns, not
global means
• Different components can be scaled separately
(e.g. 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
Temperature
Substantial GHG warming with small
sulphate cooling
Small sulphate
cooling
Greenhouse
warming
Greenhouse warming
slightly larger than
observed
Observations
Time
Different Computer Models
Key technologies to reduce emissions
Key mitigation technologies and practices currently commercially available
Energy
Supply
Efficiency; fuel switching; renewable (hydropower, solar,
wind, geothermal and bioenergy); combined heat and
power; nuclear power; early applications of CO2 capture
and storage
Transport
More fuel efficient vehicles; hybrid vehicles; biofuels;
modal shifts from road transport to rail and public
transport systems; cycling, walking; land-use planning
Buildings
Efficient lighting; efficient appliances and aircodition;
improved insulation ; solar heating and cooling;
alternatives for fluorinated gases in insulation and
appliances
Key policies to reduce emissions
Appropriate incentives for
development of technologies
Effective carbon price signal to
create incentives to invest in low-GHG
products, technologies and processes
Appropriate energy infrastructure
investment decisions, which have
long term effects on emissions
Changes in lifestyle and behavior
patterns, especially in building,
transport and industrial sectors
A technological society has two
choices. First it can wait until
catastrophic failures expose
systemic deficiencies, distortion
and self-deceptions…
Secondly, a culture can provide
social checks and balances to
correct for systemic distortion
prior to catastrophic failures.