Stefan-Boltzmann Feedback

Climate Forcing, Sensitivity and Feedback ProcessesAT606 Fall 20067/16/12CSU ATS Scott Denning1Earths Climate SystemWhat have we learned?Earth is a planetPlanetary temperature is determined by Brightness of our starEarth-sun distanceAlbedo of the planetComposition of Earths atmosphereSo how can climate ever change?

Energy In = Energy OutEarths Climate SystemWhat have we learned?Earths overall temperature is determined by sunshine and albedo (-18 C)Temperature varies dramatically with height because of greenhouse effect!Surface temperature is much warmer (+15 C) than planetary radiation temp Molecules that have many ways to wiggle are called Greenhouse moleculesOOCHHOEarths Energy Budget

What have we learned?

Surface climate depends on heating 51 units of absorbed solar96 units of downward infrared (almost 2x sunshine)!Surface climate depends on cooling117 units of upward infrared 23 units of evaporation, 7 units of rising thermalsEarths Climate as a Black BoxSunshineInSurface Temperature OutClimate System1367W m-215 CS0TSForcing(change in sunshine)Response:(Change in Surface Temperature)Climate System+ 1W m-2? CDS0DTSClimate Forcing, Response, and SensitivityForcing(change in sunshine)Response:(Change in Surface Temperature)Climate Forcing, Response, and Sensitivity

Lets do the math

A 1 W m-2 change in sunshine would produce about a 0.26 C change in planets temperature Climate Feedback ProcessesPositive Feedbacks (amplify changes)Water vaporIce-albedoHigh cloudsDSDTSDvaporD albedoD LWD hi cloudD lo cloudNegative feedbacks (damp changes)Longwave coolingLow clouds

Our Variable StarChanges of ~ 0.2% (= 2.7 W m-2) reflect 11-year sunspot cycle

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Cycle of Solar Variability11 Year Cycle of Magnetic DisturbancesActive sun is ~ 0.1 % brighter than quiet sunBOOM!Volcanos release huge amounts of SO2 gas and heatSO2 oxidizes to SO4 aerosol and penetrates to stratosphereSO4 aerosol interacts with solar radiation

Mt. Pinatubo, 199111Stratospheric Aerosol Forcing

12Aerosol

13Aerosol-Cloud Albedo FeedbackShip tracks off west coastAerosol serves as CCNMakes more/smaller cloud dropsHigher albedo

14Learning from the Past

Estimating Total Climate SensitivityAt the Last Glacial Maximum (~ 18k years ago) surface temp ~ 5 K colderCO2 was ~ 180 ppm (weaker greenhouse, 3.7 W m-2 more OLR)Brighter surface due to snow and ice, estimate 3.4 W m-2 more reflected solar Almost 3x as sensitive as suggested by Stefan-Boltzmann alone Other feedbacks must be going on as well

Note different scalesModern changes comparable to postglacial, but much faster!Greehouse Radiative Forcing

17Reconstructed Radiative Forcings

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http://commons.wikimedia.org/wiki/File:2000_Year_Temperature_Comparison.pngThe Past 2000 Years

Historical Thermometer Recordhttp://commons.wikimedia.org/wiki/File:Instrumental_Temperature_Record.pngComparison of Radiative Forcings

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