Human Induced Climate Change: The IPCC Fourth Assessment

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Human Induced Climate Change The IPCC Fourth
Assessment

• AKE-Programme
• Annual Conference the German Physical Society
  (DPG)
• Regensberg, March 26-27 2007

Bill Hare, Visiting Scientist, PIK
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IPCC AR4 - Physical Science report in context

• Climate Change 2007 The Physical Science Basis
• Accepted Paris, Feb 2007
• Climate Change 2007 Impacts, Adaptation and
  Vulnerability
• Under final government review, to be adopted
  Brussels, April 2-5. 2007
• Climate Change 2007 Mitigation of Climate Change
• Under final government review, to be adopted
  Bangkok, April 30-3 May 2007
• AR4 Synthesis Report 
• Under preparation, final government review begins
  May 15, 2007 and report to be adopted in
  Valencia, November 2007

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Overview

• Context of IPCC AR4
• Unless otherwise stated figures are from IPCC
  Summary for Policy Makers and/or Chapters wuth
  references in notes to slides.
• SPM refers to Summary for Policy Makers
• TS Technical Summary and numbers eg Fig X.Y
  refer to Chapters and figure number
• Projections
• Emissions to climate response
• Uncertainty in climate sensitivity
• Global and regional projections
• Coupled carbon cycle
• Perspective on AR4 sea level rise projections
• Beyond 21st century climate change

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Emissions to climate response
Improved understanding global and regional
climate system response to forcing
Improved understanding of coupled carbon cycle
climate interactions
Improved understanding of gas cycles
Improved understanding radiative forcing due to
gases, aerosols, land use change, volcanic
activities and solar cycles
Figure 10.1.
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Radiative forcing - response

• Energy balance of the perturbed climate system

Radiative forcing at the top of troposphere
Surface temperature change
Total global mean feedback parameter
Total heat content perturbation of the
ocean

• Climate sensitivity defined at equilibrium for a
  radiative forcing equivalent to a doubling of CO2
  concentrations

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Improved understanding of radiative forcing

• The understanding of anthropogenic warming and
  cooling influences on climate has improved since
  the Third Assessment Report (TAR)
• Very high confidence that the globally averaged
  net effect of human activities since 1750 has
  been one of warming, with a radiative forcing of
  1.6 0.6 to2.4 W m-2.

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Radiative forcing of the climate system
FIGURE SPM-2. Global-average radiative forcing
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Climate sensitivity

• Global mean warming at equilibrium for a
  radiative forcing equivalent to a doubling of CO2
  concentration

• Likely 2 to 4.5C
• Likelihood range for first time
• Best estimate 3C
• Best estimate in 1990 and 1995 was 2.5C
• Very unlikely to be less than 1.5C.
• Values higher than 4.5C cannot be excluded
• Cloud feedbacks remain the largest source of
  uncertainty.

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Uncertainty in climate sensitivity
Box 10.2, Figure 1
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Improved climate change projections

• Large number of simulations available from a
  broader range of models.
• Quantitative basis for estimating likelihoods for
  many aspects of future climate change.
• Comparison with projections since 1990
  strengthens confidence in near-term projections.
• Next two decades warming of about 0.2C per
  decade is projected for range of SRES emission
  scenarios.
• Even if the concentrations of all greenhouse
  gases and aerosols had been kept constant at year
  2000 levels, a further warming of about 0.1C per
  decade would be expected.
• Comment Caveat on sea level rise projections

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IPCC Projections vs observations
Figure TS-26. Model projections of global mean
warming
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Ranges of surface warming to 2100
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AOGCM projections of surface temperature
FIGURE SPM-6. Projected surface temperature
changes for
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Sea Ice Changes
Figure 10.13. Multi model simulated anomalies in
sea ice extent
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Precipitation projections
FIGURE SPM-7.
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North America snow
Figure 11.13. Percent snow depth changes in March
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Changes in extremes
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Projections of extreme weather events
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Projected changes in extremes
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Frosts, heatwaves, growing season
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Regional projections

• There is now higher confidence in projected
  patterns of warming and other regional-scale
  features, including changes in wind patterns,
  precipitation, and some aspects of extremes and
  of ice.

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Regional changes
Temperature anomalies with respect to 19011950
for 19062005 (black line) and as simulated (red
envelope) and as projected for 20012100
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Carbon cycle climate coupling

• TAR and AR4 Warming tends to reduce land and
  ocean uptake of atmospheric carbon dioxide,
  increasing the fraction of anthropogenic
  emissions that remains in the atmosphere.
• Climate carbon cycle coupling is expected to add
  carbon dioxide to the atmosphere as the climate
  system warms, but the magnitude of this feedback
  is uncertain

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Carbon cycle climate coupling

• Stronger assessed climate-carbon cycle feedbacks
  increases upper range of temperatures
• eg IPCC SRES A2 scenario global average warming
  at 2100 by more than 1C.
• Decreases CO2 emissions required to achieve a
  particular CO2 stabilisation level
• For 450 ppm CO2 stabilization stronger
  climate-carbon cycle feedbacks reduce cumulative
  allowed emissions over 21st century be from
  approximately 670 GtC to approximately 490 GtC

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Oceanic acidification
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IPCC sea level projections vs observations
Sea level rise trend is at top of IPCC TAR range
IPCC TAR SRES range Land ice (ice sheet)
uncertainty range
Observations solid red (tide gauge) and blue
(satellite) with non linear trend
IPCC TAR SRES range
Rahmstorf et al 2007 Science
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Sea level rise projections
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Non uniform sea level rise
Figure 10.32. Local sea level change (m) due to
ocean density and circulation change relative to
the global average
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Sea level rise Observations vs Models
Opposite sign to observations ice dynamics
Within error estimates of sum of contributions
35 of observed SLR unexplained?
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Total sea level projections
1993-2003 mean trend extrapolated to 2100
Antarctic ice sheet reduces sea level over 21s
century
Ad Hoc adjustment for ice sheet dynamics not
included in models
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Uncertainty in AR4 sea level rise projections

• Models substantially underestimate past SLR
• Uncertain implications for future projections but
  of concern for risk assessment
• For both ice sheets recent accelerations in ice
  flow have contributed significantly to recent SLR
  but this is not included in the models.
• Full AOGCM temperature range with carbon cycle
  feedback not included
• warming up to ca 5oC considered but not 6.4oC
• Ice sheet dynamics unable to be modeled at
  present
• Ad Hoc adjustment is just that - Ad Hoc.
• VIEWPOINT Likely that IPCC AR4 sea level rise
  projections are biased low due to these factors.

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Beyond the 21st century

• Anthropogenic warming and sea level rise would
  continue for centuries due to the timescales even
  if greenhouse gas concentrations were to be
  stabilized.
• Inertia in climate system
• Past and future carbon dioxide emissions will
  continue to contribute to warming and sea level
  rise for more than a millennium.
• Further increase in global average temperature of
  about 0.5C
• (Radiative forcing stabilized in 2100)
• Sea level rise from thermal expansion alone would
  lead to 0.3 to 0.8 m of sea level rise by 2300
  and continue for many centuries
• (Radiative forcing stabilized in 2100)

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WWW.IPCC.CH Not the best web site in the
business, but it is all there...
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The End
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Additional slides if needed for questions
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Inertia in the climate system
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Energy content changes
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Projected global mean temperature
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Surface mean temperature change for 2100
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Probabilistic assessments
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Uncertainty in climate sensitivity
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IPCC Assessments 1990-2006

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Regional changes
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African changes
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African precipitation
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Carbon cycle
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Thermal SLR commitment
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Greenland ice sheet