The Physical Science Basis

20 Years of IPCC Working Group I Assessment

IPCC Working Group I Assessment Reports

1990 - FAR

2001 - TAR

1995 - SAR

2007 – AR4IPCC

1990 Report: 365 pages, 170 lead and contributing authors

from 25 countries and 200 reviewers 35 countries at final plenary

2007 Report: 987 pages, 152 lead authors and 400 contributing authors

from 40 countries and 600 reviewers 113 countries at final plenary

Contributions from Scientists and Governments have Increased Over Time

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20 YEARS OF THE IPCCProgression in

• how much climate change has been observed

• how much is due to human activities

• understanding of climate change and uncertainties

• projections of future climate change

• acceptance of IPCC results

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TAR

SARFAR AR4

Atmospheric Temperatures: On The Rise

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Rising atmospheric temperature

Rising sea level

Reductions in NH snow cover

And……

Atmospheric water vapor increasing

Glaciers retreating

Arctic sea ice extent decreasing

Extreme temperatures increasing

……..

Warming is Unequivocal

Many Changes Signal A Warming World

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TARSAR

FAR AR4

A Progression of Understanding: Greater and Greater Certainty in Attribution

FAR (1990): “unequivocal detection not likely for a decade”

SAR (1995): “balance of evidence suggests discernible human influence”

TAR (2001): “most of the warming of the past 50 years is likely (odds 2 out of 3) due to human activities”

AR4 (2007): “most of the warming is very likely (odds 9 out of 10) due to greenhouse gases”

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Test of IPCC’s Near-Term Projections

IPCC projections in FAR in 1990 & SAR in 1995, can now be compared to observations.

What about near-term projections for the next decades?

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Increases in hot extremes and heat waves (very likely)

How Much More Warming Can We Expect?(2090s: medium emissions scenario)

Global average warming: 2.8°C

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A World of Change: More Rain for Some, Less for OthersRegional changes (+/-) of up to 20% in average rainfall

And also…..

•Increases in heavy rainfall (very likely) •Increases in drought (likely)

Northern hemisphere summer precipitation(2090s: medium emissions scenario; high confidence

in stippled areas)

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Presentation of Uncertainties in IPCC 2007 WGI Report (1)

Likelihood Terminology

• Virtually certain >99% probability• Very likely >90% probability• Likely >66% probability• Unlikely <33% probability• Very unlikely <10% probability

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Presentation of Uncertainties in IPCC 2007 WGI Report (2)

Confidence Terminology Chance of being correct

• Very high confidence >9 out of 10• High confidence ~8 out of 10• Medium confidence ~5 out of 10• Low confidence ~2 out of 10• Very low confidence <1 out of 10

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from Joint Science Academies’ Statement (by G8 plus China, India & Brazil, June 2005)

Global Response to Climate Change• We call on world leaders to acknowledge that

the threat of climate change is clear and increasing

• We recognize the international scientific consensus of the IPCC

… an unequivocal endorsement from the world’s top scientists

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Scientific Priorities for AR5For adaptation/mitigation studies, improved understanding of

• key aspects of climate system down to regional scale, -e.g. precipitation, circulation, sea level, extreme events etc

To inform policy choices through improved climate projections, better understanding of:• Climate feedbacks related to clouds, carbon cycle & biosphere,

mass balance of ice sheets, ocean circulation• Climate forcings (e.g. from all aerosol sources, land use change)

& couplings of forcings to changes other than global mean temperature (e.g. circulation changes)

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