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UW ATMS 211 - Lecture Notes

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1TitleAerosol Forcing and Causal Attribution of 20th Century WarmingTad AndersonAtmospheric Sciences, University of WashingtonMarch 2, 2004Collaborators*Bob Charlson, University of WashingtonSteve Schwartz, Brookhaven National LabReto Knutti, University of BernOlivier Boucher, University of LilleHenning Rodhe, Stockholm UniversityJost Heintzenberg, IfT, Leipzig* aerosol researchclimate modelingmathematics/inverse modelingDemoDemonstration•generation of particles from gases• scattering of visible light by particlesEnergy Balance Theory of Climate ChangeEquilibrium (stable global-mean temperature)…EOUTlongwave, infrared energy emitted by EarthEINshortwave, solar energyabsorbed by Earth=Forced change ...∆Ts= λ∆F∆F: forcing, change in energy balance (W/m2)∆Ts: response, change in surface temperature (K)λ: feedbacks, climate sensitivity {K/(W/m2)}2Aerosol vs GHG forcingForcing bargraphKnowledge of climate forcings...Aerosol forcings are extremely uncertain, but . . .A-Train configurationA-Train satellite constellation (2005-2008). . . dramatic progress is coming over the next few years.Current paradigm –detection and attribution summaryWarming of the Earth’s surface…•Is predicted by climate models forced with GHG’s• Has in fact been detected• Match between prediction and observation is sufficiently good that attribution has been claimedCurrent global-warming paradigmConceptual framework for this paradigm:∆T = λ∆F (Eq. 6.1 of IPCC, 2001)∆F: externally imposed change in TOA energy balance (W/m2)∆T: resulting change in surface temperature (K)λ: climate sensitivity3Recast in premisesPremise 1: Positive Forcing∆F is positive and of substantial magnitudePremise 2: "Detection"∆T is outside the range of natural variability.Premise 3: "Attribution" to human causes∆T is consistent with current knowledge of ∆F and of the forcing/response relationship as embodied in climate models.Current GW paradigm rests on three, interconnectedpremises:1a. Unequivocal, positive forcing from greenhouse gases.1b. Substantial enough to cause significant warming (according to climate models)Premise 1: Initially self-evidentGlobalCooling?Premise 2: DetectionGlobalWarming?Premise 2: Detection4Premise 3: Attribution (to human causes)proxyinstrumentaltheory/predictionGlobal warming forecast in context of temperature recordPremise 1: Re-examinedPremise 1: Climate forcing over the industrial era, ∆F, has been positive and substantial in magnitude.∆Ts= λ ∆FPremise 1: Re-examinedKnowledge of forcings as of 2001. What is missing from this graph?5statistical sum of IPCC bar graphreference: Boucher and Haywood, 2001, Clim. Dyn. 18, 297.Statistical summation of IPCC bar graphEasy question: What causes this breadth???B&H: aerosol dominates uncertaintyAerosol forcing dominates the uncertainty in total forcing.Boucher and Haywood (cont)To evaluate Premise 1, we need to grapple with aerosol forcing.Two inde pendent methodsWe propose a new method of examining Premise 1:• more sensitive and robust than simply summing the forcings• based on a comparison of the two, independent approaches to calculating aerosol forcings1. Forward calculations (study the aerosol)• aerosol mass from chemical transport model• mass-dependent optical and cloud-nucleating propertiesfrom measurements (* our group *)• rapidly advancing satellite data as constraint2. Inverse calculations (fit the T-record)Forward calcula tion – CTM resultAnthro DustFF OC & BCSulfateBB OC & BCModel-estimated direct aerosol forcingsHaywood, J and O Boucher, Revs. Geophys., 38, 513-543, 2000Models: Penner et al., 1998; Grant et al., 1999; Tegen et al., 1996Forward calculation6Photo credit: NASA Earth Sciences & Image Analysis Laboratory, Johnson Space Centershuttle photo: hi resAerosol forcing is visible from spaceTwo inde pendent methods1. Forward calculations (study the aerosol)2. Inverse calculations (fit the T-record)• assume Premise 1 (i.e. observed warming is the response toa substantial, positive forcing) • multiple runs of simple climate models with varying valuesfor the poorly known input parameters (aerosol forcing,climate sensitivity, ocean heat diffusivity)• assign probabilities based on fit to simplified versions ofthe temperature recordTesting Premise 1compare two, independent ways of calculating aerosol forcingAerosol forcing estimates - refsAerosol forcing estimatesA. Boucher and Haywood, Clim. Dyn. 18, 297 (2001)B. Kiehl et al., JGR, 105, 1441 (2000) (sulfate only)C. Hansen and Sato, Proc. NAS 98, 14778 (2001)D. Hansen et al., Proc. NAS 95, 12753 (1998)E. Schwartz and Andreae, Science 296, 2139 (1996)F. Menon et al., J Atmos Sci 59, 692 (1998) ; Lohmannand Feichter, JGR 102, 13685 (1997) (indirect only)Forward calculations (aerosol-based):Inverse calculations (T-record and climate-model-based):G. Wigley and Raper, Science 293, 451 (2001)H. Harvey and Kaufmann, J. Clim. 15, 2837 (2002)I. Gregory et al., J. Clim. 15, 3117 (2002)J. Andronova and Schlesinger, JGR 106, 22605 (2001)K. Knutti et al., Nature 416, 719 (2002)L. Forest et al., Science 295, 113 (2002)Aerosol forcing estimates - resultsThe comparison is robust method of testing Premise 1** Premise 1: Industrial-era forcing is positive and substantial77%19%forward calcsapplicationsinverse calcsCase A:Boucher andHaywood7Applications -referencesAerosol forcing applicationsM. - fits paleo temperature record with solar and volcanic forcings- shows recent warming is anomalous and is explained by theknown anthropogenic forcingsN. - review of detection/attribution studies for industrial era- shows that pattern of 100-year warming is very well explainedby the known anthropogenic and natural forcingsO. - IPCC climate change projections for various emission scenariosP. - new detection/attribution study for industrial eraQ. - climate diagnostic study; exploring natural variability, climatesensitivity, etc.Applications in climate studies:M. Crowley, Science 289, 270 (2000) (attribution)N. Mitchell et al., 2001 (IPCC Chap 12) (attribution)O. Cubasch et al., 2001 (IPCC Chap 9) (projection)P. Tett et al., 2002, J. Geophys. Res. (attribution)Q. Meehl et al., 2003, J. Climate (diagnosis)IPCC attributionIPCC (2001) Attribution summaryCurrent paradigm –side note on attribution Hansen 1981Source:Hansen et al.1981, Science,213, 957-966By the way… fitting the T-record is not a new resultNote: very differentset of


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