Transcript Overview - uni

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
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 25. 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
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
Emissions to climate response
Improved understanding of coupled carbon cycle climate
interactions
Improved understanding
of gas cycles
Figure 10.1.
Improved understanding global and regional climate system
response to forcing
Improved understanding radiative forcing due
to gases, aerosols, land use change, volcanic
activities and solar cycles
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
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
to+2.4] W m-2.
Radiative forcing of the climate
system
FIGURE SPM-2. Global-average radiative forcing
Climate sensitivity
• Global
mean warming at equilibrium for a
radiative forcing equivalent to a doubling
of CO2 concentration
•Likely: 2 to 4.5°C
• Likelihood range for first time
• Best estimate: 3°C
•Best estimate in 1990 and 1995 was 2.5°C
•Very unlikely to be less than 1.5°C.
•Values higher than 4.5°C cannot be excluded
•Cloud feedbacks remain the largest source of uncertainty.
Uncertainty in climate sensitivity
Box 10.2, Figure 1
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 0.2°C 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 0.1°C per decade would be
expected.
• Comment: Caveat on sea level rise projections
IPCC Projections vs observations
Figure TS-26. Model projections of global mean warming
Ranges of surface warming to 2100
AOGCM projections of surface
temperature
FIGURE SPM-6. Projected surface temperature changes for
Sea Ice Changes
Figure 10.13. Multi model simulated anomalies in sea ice
Precipitation projections
FIGURE SPM-7.
North America snow
Figure 11.13. Percent snow depth changes in March
Changes in extremes
Projections of extreme weather events
Projected changes in extremes
Frosts, heatwaves, growing
season
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.
Regional changes
Temperature anomalies with respect to 1901–1950 for 1906–2005 (black line)
and as simulated (red envelope) and as projected for 2001–2100
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
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 1°C.
• 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
Oceanic acidification
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
Rahmstorf et al 2007 Science
IPCC TAR
SRES
range
Sea level rise projections
Sea Level Rise
(m at 2090-2099 relative to 19801999)
Case
Model-based range
excluding future rapid dynamical
changes in ice flow
B1 scenario
A1T scenario
B2 scenario
0.18 – 0.38
0.20 – 0.45
0.20 – 0.43
A1B scenario
A2 scenario
A1FI scenario
0.21 – 0.48
0.23 – 0.51
0.26 – 0.59
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
Sea level rise: Observations vs
Models
Source of sea level rise
1993-2003
Observed
mm/yr
Modeled
mm/yr
Thermal expansion
1.6
1.5
Glaciers and ice caps
0.8
0.6
Greenland ice sheet
0.2
0.1
Antarctic ice sheet
0.2
-0.2
Sum of contributions
2.8
2.0
Observed total SLR
3.1
Difference
0.3
Within error estimates
of sum of
contributions
1.1
35% of observed SLR
unexplained?
Opposite
sign to
observations
: ice
dynamics
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
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.
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 0.5°C
• (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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The End
Additional slides if needed for
questions
Inertia in the climate system
Energy content changes
Projected global mean
temperature
Surface mean temperature change
for 2100
Probabilistic assessments
Uncertainty in climate sensitivity
IPCC Assessments 1990-2006
Year
1990
1995
2001
2006
Observed global
mean change
0.6ºC (1880s
to 1990)
0.3-0.6ºC
(1880s to
1990)
0.4-0.8ºC
0.8°C (0.6-1.0°C)
(1880s to mid (1880s to 2001–
1990s
2005)
Human influence
due to
anthropogenic
increase in
greenhouse gas
concentrations
Unequivocal
detection
“not likely
for a decade
or more.”
“Balance of
evidence
suggests a
discernable
human
influence.”
“likely” that
“most of the
observed
warming
over the last
50 years”
“very likely”
“Most of the
observed increase
in globally averaged
temperatures since
the mid-20th
century”
Projected
warming to 2100
2-5ºC
1-3.5ºC
1.4-5.8ºC
(SRES
range)
1.1-6.4ºC
(SRES range)
Projected sea
level rise to 2100
30-100cm
13-94cm
9-88cm
(SRES
range)
18-59 cm
(SRES range)
Regional changes
African changes
African precipitation
Carbon cycle
Thermal SLR commitment
Greenland ice sheet