Transcript Slide 1

Reducing Uncertainty in Predictions of Climate Change
University of Washington, Program on Climate Change – February 11, 2010
The perception of a scientific controversy
is often exaggerated in the media
Hero of the movie is a Climatologist!
Intergovernmental Panel on Climate Change
2007 IPCC Report
 Started 2004
 Completed February 2007
 152 Authors
 ~450 contributors
 ~600 expert reviewers
 30,000+ review comments
Contents
 Summary for Policymakers
 Technical Summary
 11 Chapters
 Frequently Asked Questions
 ~5000 literature references
 ~1000 pages
The IPCC Sequence of Findings
IPCC (1990) “The unequivocal detection of the enhanced
greenhouse effect from observations is not
likely for a decade or more.”
IPCC (1995) “The balance of evidence (>50%) suggests a
discernible human influence on global climate”
IPCC (2001) “Most of the warming of the past 50 years is
likely (>66%) to be attributable to human
activities.”
IPCC (2007)
Actual 1995-2000 observations
“One of the most important outcomes of your study
could be a clear statement of our present ignorance”
-Climate Change Panel Respondent (1978)
Over 97% of climate scientists believe humans are
causing the planet to warm.
-EOS, American Geophysical Union (2009)
The World Has Warmed
NASA/GISS
• Globally averaged, the planet is about ~1.5°F warmer over the past century.
Consistent Patterns of Warming
• Mountain glaciers are retreating
• Arctic sea ice is decreasing
• Greenland is melting
• Snow/permafrost decreasing
• Sea level is rising
• Ocean heat content is increasing
• More intense droughts
• Atmospheric moisture increasing
• Heavier rainfall events
• Increased heat waves
• Decreased cold spells, …
WARMING IS UNEQUIVOCAL
The IPCC Sequence of Findings
IPCC (1990) “The unequivocal detection of the enhanced
greenhouse effect from observations is not
likely for a decade or more.”
IPCC (1995) “The balance of evidence (>50%) suggests a
discernible human influence on global climate”
IPCC (2001) “Most of the warming of the past 50 years is
likely (>66%) to be attributable to human
activities.”
IPCC (2007) “Warming is unequivocal, and most of the
warming of the past 50 years is very likely
(>90%) due to increases in greenhouse gases.”
Where Do We Go From Here?
Climate Projections for the 21st Century
+11 F
You are here
+3.5 F
Sources of Uncertainty in Future Projections
1) How much will CO2 (and other man-made GHGs)
increase from the burning of fossil fuels?
2) How much will the climate warm in response to a
given increase in CO2?
Climate Sensitivity
•
The equilibrium change in global mean surface
temperature that results from a doubling of CO2.
“Charney Report” (1979)
IPCC
1990
1995
2001
2007
“We estimate the most probable global
warming for a doubling of CO2 to be near
3 C with a probable error of +/- 1.5 C.”
(In other words 1.5 to 4.5 C)
The likely range of climate sensitivity is:
1.5 to 4.5 C
1.5 to 4.5 C
1.5 to 4.5 C
2.0 to 4.5 C
The Greenhouse Effect
Without
Greenhouse Effect:
Global-mean
Temperature = 0 F
With
With Greenhouse
Doubling of
Effect:
CO2:
Global-mean
Temperature = 62
60 F
• The Greenhouse Effect is natural.
• Most important greenhouse gases: Water Vapor (60%) and CO2 (25%).
• Global warming results from an anthropogenic enhancement of the GHE.
Climate Feedback
•
A sequence of interactions that may amplify
(positive) or dampen (negative) the response of
the climate to an initial perturbation.
Example: Snow/Ice Positive Feedback Loop
Warmer
Surface T
More Absorbed
Sunlight
Less
Ice/Snow Cover
Climate Sensitivity Depends On Feedbacks:
Water Vapor
Surface T
+
+
Greenhouse
Effect
+
H2O Vapor
All models predict a strong positive feedback
from water vapor.
IPCC Assessments: Water Vapor Feedback
1990:
“The best understood feedback mechanism is water vapor feedback,
and this is intuitively easy to understand”
Water Vapor Feedback
Atmospheric Water Vapor (kg/m2)
Satellite observations illustrate how
water vapor enhances regional
differences in ocean temperature.
1.
Ocean Surface Temperature (K)
2.
Greenhouse Effect (W/m2)
3.
1. Warmer oceans  more water vapor.
2. More water vapor  larger Greenhouse Effect.
3. Larger GHE  warmer oceans.
IPCC Assessments: Water Vapor Feedback
1990:
“The best understood feedback mechanism is water vapor feedback,
and this is intuitively easy to understand”
IPCC Assessments: Water Vapor Feedback
1990:
“The best understood feedback mechanism is water vapor feedback,
and this is intuitively easy to understand”
1992:
“There is no compelling evidence that water vapor feedback is
anything other than positive—although there may be difficulties with
upper tropospheric water vapor”
IPCC Assessments: Water Vapor Feedback
1990:
“The best understood feedback mechanism is water vapor feedback,
and this is intuitively easy to understand”
1992:
“There is no compelling evidence that water vapor feedback is
anything other than positive—although there may be difficulties with
upper tropospheric water vapor”
1995:
“Feedback from the redistribution of water vapor remains a substantial
source of uncertainty in climate models”
IPCC Assessments: Water Vapor Feedback
1990:
“The best understood feedback mechanism is water vapor feedback,
and this is intuitively easy to understand”
1992:
“There is no compelling evidence that water vapor feedback is
anything other than positive—although there may be difficulties with
upper tropospheric water vapor”
1995:
“Feedback from the redistribution of water vapor remains a substantial
source of uncertainty in climate models”
2001:
“The balance of evidence favours a positive clear-sky water vapour
feedback of magnitude comparable to that found in (model) simulations“
IPCC Assessments: Water Vapor Feedback
1990:
“The best understood feedback mechanism is water vapor feedback,
and this is intuitively easy to understand”
1992:
“There is no compelling evidence that water vapor feedback is
anything other than positive—although there may be difficulties with
upper tropospheric water vapor”
1995:
“Feedback from the redistribution of water vapor remains a substantial
source of uncertainty in climate models”
2001:
“The balance of evidence favours a positive clear-sky water vapour
feedback of magnitude comparable to that found in (model) simulations“
2007:
“Observational and modelling evidence provide strong support for a
combined water vapour/lapse rate feedback of around the strength found
in GCMs”
Testing Model Predictions of Water Vapor
Models capture:
Moistening of tropical
atmosphere during
warm (El Nino) events.
Drying of tropical
atmosphere during
cold (La Nina) events.
La Nina
El Nino
La Nina
(cold)
El Nino
(warm)
Pinatubo
Eruption of
Mt. Pinatubo
June 1991
Water Vapor (mm)
Temperature (C)
Global Cooling and Drying after Mt. Pinatubo
• Atmosphere cools and dries following eruption.
• Climate models successfully reproduce observed
cooling and drying.
Testing Water Vapor Feedback
Observed
• Model without water vapor feedback significantly underestimates cooling.
• Water vapor amplifies pre-existing temperature change (either warming or cooling).
Climate Sensitivity Depends On Feedbacks:
Clouds
Reflected
Sunlight
Surface T
+
Greenhouse
Effect
?
+
Cloud Cover
+
Cloud feedback is uncertain in both magnitude and sign.
Cloud Feedback in Models: A Case Study
GFDL
IPCC TAR
Models
(2001)
NCAR
2xCO2 Climate Sensitivity (K)
Summer 2002
Change in Low Cloud Amount (%/K)
GFDL
Fall 2003
NCAR
2xCO2 Climate Sensitivity (K)
Cloud Feedback in Models: A Case Study
Change in Low Cloud Amount (%/K)
The Problem Clouds
Regional contribution to intermodel spread in cloud feedback
Subtropical marine stratocumulus clouds are responsible for most
(~2/3) of the uncertainty in cloud feedback in current models.
http://www.atmos.washington.edu/2008Q2/101/student_cloud_photos/SC_j_reuer.jpg
Resolving Uncertainties

Improved observations and more sophisticated
models are important tools to resolving the
uncertainties in cloud feedback, but ...
GFDL Model
Cloud Radar
Cloud Feedback Puzzle
Model Predicted Change
in Low Cloud from 2xCO2
Change in Low Cloud Amount (%/K)
Model Simulated Change in Low Cloud
From “Observable” (ENSO) Variability
?
Key Climate Feedbacks
Current model estimates of climate feedbacks
Adapted from Gregory et al. (2009)
Negative Positive
Snow/Ice
Water Vapor
Negative
Positive
Climate Feedback Strength (W/m2/K)
• Water vapor provides a strong positive feedback in all models.
• Clouds and the carbon cycle are potentially strong, but very uncertain.
Climate Feedbacks Interact
Model Range w/out
Water Vapor Feedback
Current Model Range
• The presence of a strong positive feedback from water vapor amplifies both the
magnitude of climate change and the impact of uncertainty in other feedbacks.
• This makes it hard to reduce uncertainty at high sensitivities (Roe and Baker 2007)
Global Warming is NOT Fair

The poorest countries do not contribute
significantly to the problem - but they will
pay the greatest cost in adapting to it.
Thank you