Climate Models & Climate Sensitivity: A Review

Transcription

1 Climate Models & Climate Sensitivity: A Review Stroeve et al. 2007, BBC Paul Kushner Department of Physics, University of Toronto

2 Recent Carbon Dioxide Emissions x x Raupach et al & NOAA CDIAC

3 Recent Carbon Dioxide Concentrations

4 Observed Temperatures, Past and Present (NASA GISS)

5 The Spread in Climate Change Predictions Sources of spread: IPCC 2007 Uncertainty in forcings (anthropogenic emissions). Uncertainty in timing of response (oceans). Uncertainty in climate sensitivity equilibrium climate warming (atmosphere/cryosphere).

6 Band Saturation The increase in greenhouse trapping is logarithmic in CO2 concentration once you get to current concentrations. Archer (2007)

7 Climate Sensitivity Parameter Change in Global Mean Surface Temperature Per Doubling of CO2 Loading For Fixed Net Radiation Absorbed. At equilibrium, R = 0.

8 Blackbody Emission Spectra Shortwave Longwave Yochanan Kushnir

9 Earth s Radiation, February 2009 Shortwave Absorbed Longwave Emitted (NOAA)

10 Emitted Longwave Radiation High Clouds (8-12 km) Low Clouds (0-3 km) Bony et al Clear sky

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12 0-D Radiative Model for Temperature Te Ze Ts L, λ~4 μm EARTH SUN S, λ~600 nm Radiative eq., blackbody Net shortwave flux Emission Temperature

13 Emitted Longwave Radiation High Clouds (8-12 km) Low Clouds (0-3 km) Bony et al Clear sky

14 0-D Radiative Model for Temperature Te Ze Ts L, λ~4 µm EARTH SUN S, λ~600 nm Observed emission height Observed surface temperature Greenhouse effect (Fourier et al.) from radiatively active gases (CO2, H20, CFCs) accounts for the extra 33K.

15 One-D Radiative Model for T(Z) Z L Lapse rate Γ=!dT/dZ Ze T=TS-ΓZ Greenhouse gas optical thickness τ Te Ts T Surface T jump (small τ) Radiative lapse rate Convectively unstable Convectively unstable if Γrad > Γad = g/cp To understand response of Ts and Γ to radiative destabilization, we briefly consider tropical and extratropical general circulation.

16 Extratropical Macroturbulence: Rotational, shallow, horizontal transports, continental to planetary scale, moisture is more passive. Tropical Macroturbulence: Divergent, deep, vertical transports, multiscale, driven by moist heating. Mike Wallace

17 One-D Radiative-Dynamical Model for T(Z) Z Tobs L Ze Trad Te Ts T Observed lapse rates are neutral (adiabatic) or subcritical. In the tropics, radiative destabilization relieved by vertical convective motions, involving moisture: radiative-convective equilibrium. In the extratropics, both vertical convection and large-scale horizontal motions are active.

18 Z Greenhouse Warming Tobs Ze Te Ts Suppose we add more CO2. Let s keep Γ and S fixed. What would happen to the emission temperature Te? T

19 Z Greenhouse Warming Tobs Ze Te Ts Suppose we add more CO2. Let s keep Γ and S fixed. What would happen to the emission temperature Te? Nothing! But the troposphere would become more optically thick, and Ze would increase. T

20 Z Ze + δze Ze Greenhouse Warming δze~100 m for 2XCO2 Te Ts Suppose we add more CO2. Let s keep Γ and S fixed. What would happen to the emission temperature Te? Nothing! But the troposphere would become more optically thick, and Ze would increase. T

21 Z Greenhouse Warming Ze + δze Ze Te Ts Suppose we add more CO2. Let s keep Γ and S fixed. What would happen to the emission temperature Te? Nothing! But the troposphere would become more optically thick, and Ze would increase. T TS + δts Then, given the assumptions, so would Ts. Let s calculate δts

22 Reference Climate Sensitivity L and S depend on atmospheric structure and composition. Define radiative imbalance: Under radiative equilibrium How do variations in CO2 affect surface temperature if everything else is fixed?

23 Reference Climate Sensitivity Under radiative equilibrium, A gedanken experiment: realizable in principle. Thus, if the linearization is valid: (by the chain rule for partial derivatives.)

24 Reference Climate Sensitivity From Stefan-Boltzmann Greenhouse, from radiative transfer Reference sensitivity The reference sensitivity is small: 1K per doubling of CO2 But other changes will occur that affect temperature and radiation: feedbacks.

25 Climate Feedbacks Feedback involves any quantity that is affected by Ts and affects R. E.g. allow water vapor, a powerful greenhouse gas, to vary: H20 = H20(Ts)

26 Climate Feedbacks Using the same variational method, the sensitivity with water vapor feedback is Sensitivity: Gain factor: Radiative transfer Thermo, circulation Stefan-Boltzmann ve feedback No feedback +ve feedback This feedback increases climate sensitivity by 2/3: Estimated Runaway

27 Climate Feedbacks We can incorporate additional feedbacks: The gains are additive, and many of them are understood to be positive. Indirect feedbacks on CO2, e.g. from the biosphere, can be included formally. Current generation climate models provide quantitative estimates of the factors.

28 We can evaluate how models capture feedback related processes. But climate sensitivity is difficult to infer from observations, so we lean heavily on the models for this. We will now highlight recent advances in calculations of climate sensitivity in climate models. Model vs. Observed Water Vapor Greenhouse Trapping: El Niño Soden and Held

29 Using Models to Build a Theory of Climate Sensitivity Radiative Kernel Our simple ideas can lead to insightful quantitative calculations. The sensitive regions for water vapor are in some of the dry regions of the atmosphere. Circulation and clouds have an important influence. Total Sky Clear Sky Soden et al. 2008