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UT GEO 387H - Chapter 9: Climate Sensitivity and Feedback Mechanisms

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PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Chapter 9: Climate Sensitivity and Feedback Chapter 9: Climate Sensitivity and Feedback Mechanisms Mechanisms This chapter discusses:This chapter discusses:1.1.Climate feedback processesClimate feedback processes2.2.Climate sensitivity and climate feedback Climate sensitivity and climate feedback parameterparameter3.3.ExamplesExamples((Materials are drawn heavily from D. Hartmann’s textbook and online materials Materials are drawn heavily from D. Hartmann’s textbook and online materials by J.-Y. Yu of UCI. Guo-Yue Niu contributed significantly to the by J.-Y. Yu of UCI. Guo-Yue Niu contributed significantly to the preparation of this lecture.)preparation of this lecture.)Climate Feedback and SensitivityFeedback is a circular causal process whereby some proportion of a system's output is returned (fed back) to the input. ΔTfinal = ΔT + ΔTsensitivityClimate SystemΔTΔQΔTfinalΔQfeedback can be either negative or positive inputoutputΔQfinal = ΔQ + ΔQfeedbackΔQfinalAn objective measure of climate feedback and sensitivityThe strength of a feedback depends on how sensitive the change in input (Q) responds to the change in output (T) : Feedback strength: λ = ΔQ / ΔT Climate sensitivity: λ-1 = ΔT / ΔQ1. Positive values  negative feedbacks, stable Negative values  positive feedbacks, unstable λBB = 4σT3 = 3.75Wm-2K-12. The larger λ, the stronger feedback.Stefan-Boltzmann feedbackOutgoing longwave radiation: F = σT4 σ = 5.67x10-8 The strength of the feedback:1. A negative feedback, stable2. 1K increase in T would increase F by 3.75 Wm-2 (see Fig. 9.1) λBB = ∂F / ∂T = 4σ T3 = 3.75 Wm-2K-1Water vapor feedbackClausius-Clapeyron relationship: es = f(T) 1% increase in T would increase 20% in esWater vapor is the principal greenhouse gases.The feedback strength: λv= – 1.7 Wm-2K-11. A positive feedback, unstable 2. Weaker than λBB3. λBB + λv = 2.05 Wm-2K-1 (see Fig. 9.1)Ice (snow) albedo feedbackStriking contrast between ice-covered and ice-free surfacesIn ice-covered regions, more solar energy reflected back to space:Feedback strength: λice= –0.6 Wm-2K-1 1. Positive feedback, unstable2. λBB + λv + λice=1.45 Wm-2K-1An example of climate feedbackGlobal Temperature AnomaliesNorthern Hemisphere Snow Cover AnomaliesΔTΔQSnow (ice)-albedo climate feedbackChapin et al. (2005), Science1. Decrease in snow-cover and snow season 2. Tundra  treesSnow cover change  Temperature changeTotal feedbackλtotal =1.45 Wm-2K-13.75 Wm-2K-1Positive feedback negative feedbackλtotalDoubling of atmospheric CO2  2.9 KWithout ice-albedo feedback  2.0 K−31−48−17+15%Cloud feedbackCloud feedback1. It is unclear what is the strength and even directions (negative or positive). From GCM simulations, λcloud = 0 ─ −0.8.2. Could effects can be either “umbrella” or “blanket”. umbrella blanket Low cumulus cloudsNegative feedbackHigh cirrus cloudsPositive feedbackCloud feedback (con.)Cloud feedback (con.)3. It is uncertain whether an increased temperature will lead to increased or decreased cloud cover. 4. It is generally agreed that increased temperatures will cause higher rates of evaporation and hence make more water vapor available for cloud formation, the form (e.g., type, height, and size of droplets) which these additional clouds will take is much less certain.Energy-balance climate modelsEnergy-balance climate models1. Zero-dimensional EBMs (1-α) S0 /4 = σTe4 shortwave in = Longwave outThe surface T: Ts = Te + ΔT (greenhouse effects)The Erath: S0 = 1376 Wm-2, α = 0.3, Te= 255 K, Ts =288 KVenus: S0 = 2619 Wm-2, α = 0.7, Te = 242 K, greenhouse gases Ts = 730 KEnergy balance climate models (con.)2. One-dimensional EBMs (Sellers and Budyko in 1969) Shortwave in = Transport out + Longwave out S(x) [1 - α(x) ] = C [ T(x) - Tm ] + [ A + B T(x) ]S(x) = the mean annual radiation incident at latitude (x) = S0/4 *s(x)α(x) = the albedo at latitude (x) for ice-free (Ts > −10°C) : 0.3 for ice (Ts < −10°C) : 0.62C = the transport coefficient (3.81 W m-2 °C-1)T(x) = the surface temperature at latitude (x)Tm = the mean global surface temperatureA and B are constants A = 204.0 W m-2 and B = 2.17 W m-2 K-1 This B is equivalent to λBB (3.75) or λBB + λv = 2.05 (see Fig. 9.1)Energy balance climate models (con.)Changeable parameters: S0 α(x) (0.62) C (3.81 W m-2 °C-1) A and B are (B = 2.17 W m-2 K-1) The model contains four kinds of climate feedbacks:1) Ice-albedo feedback (Ts> − 5°C ; 0.8) (see Fig. 9.5)2) Stefan-Boltzmann feedback: B (λBB) = 3.753) water-vapor feedback: B (λBB + λv) = 2.05 ; 1.45 (Budyco, 1969); 1.6 (Cess, 1974)4) dynamical feedbacks and zonal energy transport: C=0 means no such a feedbackYou may also add cloud feedbacks by changing: B smaller (positive feedbacks) B larger (negative feedbacks)Try Toy Model 4 at the course websiteBiogeochemical feedbacks – A Daisyworld modelBiogeochemical feedbacks – A Daisyworld model)()(xAdtd AxAdtd AbbbwwwBiogeochemical feedbacks – A Daisyworld modelGrowth Factorwhite = 1 - 0.003265*(295.5K -Twhite)2Global mean temperature: σTe4 = S0 (1 – αp) /4 αp=Agαg + Awαw+ AbαbLocal temperature: σTi4 = S0 (1 – αi) /4 Ti4 = η(αp – αi) + Te4 where 0<η < S0/(4σ) represents the allowable range between the two extremes in which horizontal transport of energy is perfectly efficient (0) and least efficient [S0/(4σ)].A Daisyworld modelGlobal mean emission temperature is remarkably stable for a wide range of solar constant values. (see Fig. 9.9d); Run Toy Model 1 at the course website.Climate Trend 1976 to 2000Increase in T  melting of snow and frozen soil  larger area of wetlandsmore soil carbon released as CH4  increase in TTogether with ice-albedo feedback, the warming trend will be acceleratedOther feedbacks at regional scalesAlbedoIncrease in albedoSW radiation absorbed decreasesRn decreasesH, LE decreasesReduction in:CloudnessPrecipitationconvergenceIncrease in insolationIncrease in Rn Increase in albedoReduction in:Soil


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UT GEO 387H - Chapter 9: Climate Sensitivity and Feedback Mechanisms

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