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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