Unformatted text preview:

1Climate Feedbacks - A Study Guide to Chapter 13Richard Somerville SIO 217A 21 Nov. 2006This topic is extremely important and is central tomodern research on climate, especially anthropogenicclimate change. It is no exaggeration to say thatunderstanding feedbacks in the climate system is amajor challenge to science, and indeed to humanity.However, the Chapter 13 formalism (from controltheory) can obscure rather than illuminate key issues.The chapter emphasizes 3 especially importantfeedbacks (but there are many others).Section 13.3 Water Vapor FeedbackSection 13.4 Cloud-radiation FeedbackSection 13.5 Snow/Ice-albedo FeedbackOne complexity is that these feedbacks (and others)can and do interact with one another. The partialderivatives in the control theory formalism are anidealization. Rarely does only one variable vary.Feedbacks also often involve the following complexities:The climate system is not linear. Important “tippingpoints” or thresholds or instabilities are known to exist.The base state can influence the feedbacks. Example:Snow-albedo feedback depends on snow being present.A global average can be misleading. Feedbacks may belocally important but less important in the global mean.Climate sensitivity depends strongly on feedbacks, andthe usual way of defining it, as a surface temperaturechange in response to doubling CO2, is not always best.Sea level riseSteady rise in sea level overrecent decades.Causes particular problems whenstorms come during high tide2Greenland: up to 7 meters ofsea level“Ice sheets have contributed metersabove modern sea level in responseto modest warming [~3oC, or5oF]…a threshold triggering manymeters of sea-level rise could becrossed well before the end of thecentury.”Overpeck et al., Science, 2006The Enhanced Greenhouse EffectSolar (S) and longwave (L) radiation in Wm−2 at the top of the atmosphereS L236 236T = −18°CS L236 232CO2 x 2S L236 236CO2 x 2S L236 236CO2 x 2+ FeedbacksH2O (+60%)Ice/Albedo (+20%)Cloud?Ocean?TS = 15°C TS = 15°CΔTS ~ 1.2K ΔTS ~ 2.5KThe Development of Climate models, Past,Present and FutureAtmosphere Atmosphere Atmosphere Atmosphere Atmosphere AtmosphereLand surfaceLand surfaceLand surfaceLand surfaceLand surfaceOcean & sea-ice Ocean & sea-ice Ocean & sea-ice Ocean & sea-iceSulphateaerosolSulphateaerosolSulphateaerosolNon-sulphateaerosolNon-sulphateaerosolCarbon cycle Carbon cycleAtmosphericchemistryOcean & sea-icemodelSulphurcycle modelNon-sulphateaerosolsCarboncycle modelLand carboncycle modelOcean carboncycle modelAtmosphericchemistryAtmosphericchemistryOff-linemodeldevelopmentStrengthening coloursdenote improvementsin modelsMid 1970sMid 1980s Early 1990s Late 1990s Present day Early 2000s?3Section 13.3 Water Vapor FeedbackKey points:This is a strongly positive feedback, nearly doublingthe effect of carbon dioxide alone.Relative humidity seems to be approximately constantunder climate change (p. 359).Relatively dry regions, such as the upper troposphereand polar regions, are especially sensitive (p. 362).Water VaporFeedback LoopIncrease CO2Increase TemperatureIncrease Water VaporWater Vapor Feedback Approximately Doublesthe Sensitivity of Climate ~1 to 2 ˚C warming +3C ~ +20% H2O Natural Feedbacks within theClimate System:Water Vapor FeedbackChange in Global Water Vapor at 2xCO2All models predict total column water vapor to increase at 7% /KChange in Upper Tropospheric WaterVapor at 2xCO2All models predict upper trop water vapor to increase at ~15% /KSection 13.4 Cloud-radiation FeedbackKey points:Clouds affect both shortwave (low clouds) andlongwave (high clouds). Present climate has cloudcooling dominating cloud warming (pp. 368-369).Many different mechanisms, including those involvingaerosol-cloud interactions, may be important, but thesign and magnitude of cloud feedbacks is still largelyunknown (p. 374). Clouds have big effects in models.4Model Estimates of Cloud Radiative Forcing withCO2 DoublingEffect of cloud feedback formulation onclimate prediction• Feedback scheme Global Av Tempchange,C for doubledCO2–RH 5.3–CW 2.8–CWRP 1.9– after Senior & Mitchell, Hadley CentreSenior, C. A., and J. F. B. Mitchell, 1993: CarbonDioxide and Climate: The Impact of CloudParameterization. Journal of Climate, 6, 393-418.This classic paper showed that the same climatemodel can produce different sensitivites to carbondioxide, varying by a factor of three in terms ofglobal average warming, depending on the treatmentof clouds and cloud-radiation interactions.Evolution of Climate Sensitivity inAM20123452xCO2 Sensitivity (K)4.53.94.34.62.62.9p5p7p9p10p12ap12bNew BL SchemeGlobal Mean ΔTs (K)4.51.5Large change in sensitivity occurred between AM2p10 and AM2p12adue to the introduction of a new boundary layer parameterization, whichmodifies the response of low-level clouds to warming5GFDL AM2p5 vs NCAR CAM2B. SodenGFDL AM2p12a vs NCAR CAM2xB. SodenSection 13.5 Snow/Ice-albedo FeedbackKey points:This feedback is large and positive in high northernlatitudes.Observations show that this effect is occurring now.Melting ice on land has another large effect,unrelated to albedo: it causes sea level to rise.Land areas are projected to warm more than theoceans with the greatest warming at high latitudesAnnual mean temperature change, 2071 to 2100relative to 1990: Global Average in 2085 = 3.1oC6Global mean surface air temperature anomalies from 1,000 year control simulations with threedifferent climate models, - Hadley, GFDL and Hamburg, compared to the recent instrumentalrecord. No model control simulation shows a trend in surface air temperature as large as theobserved trend. If internal variability is correct in these models, the recent warming is likely notdue to variability produced within the climate system alone.IPCC Third Assessment ReportSimulated annual global mean surface temperaturesNaturalforcingAnthropogenic forcingSimulated annual global mean surface temperaturesIPCC statements on Detection• “The balance of evidence suggests adiscernible human influence on globalclimate” 1995 Report• “There is new and stronger evidence thatmost of the warming observed over thelast 50 years is attributable to humanactivities” 2001 ReportJohn Tyndall (1820-1893)Investigated radiant(infrared) heat in 1859.Was first to measureabsorptive powers of gasessuch as H2O, CO2, O3 andhydrocarbons (allgreenhouse gases).Demonstrated that H2O,CO2, O3 are some of thebest absorbers of


View Full Document

UCSD SIO 217A - Lecture

Download Lecture
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Lecture and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Lecture 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?