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Global Warming & Climate Sensitivity: Climate Feedbacks in the Tropics Professor Dennis L. Hartmann Department of Atmospheric Sciences University of Washington.

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Presentation on theme: "Global Warming & Climate Sensitivity: Climate Feedbacks in the Tropics Professor Dennis L. Hartmann Department of Atmospheric Sciences University of Washington."— Presentation transcript:


2 Global Warming & Climate Sensitivity: Climate Feedbacks in the Tropics Professor Dennis L. Hartmann Department of Atmospheric Sciences University of Washington Seattle, Washington Berkeley Atmospheric Sciences Symposium November 8, 2002 U.C. Berkeley

3 Two approaches to understanding climate change. Top Down Approach - Take observed climate record and attempt to extrapolate intelligently into the future. Bottom Up Approach - Attempt to understand and model the critical climate processes, then use the resulting detailed model to predict how future climates might respond to specified forcing like CO 2 increase.

4 Greenhouse gas trends are large and can be associated directly with human actions. Carbon dioxide trends Can be uniquely associated with fossil fuel burning through isotopes of carbon like 14 C and 13 C.

5 The Instrumental Record of Global Temperature Anomalies.

6 IPCC - 2001

7 Model of Global Temperature Anomalies through time. Energy Equation: Climate = Heat + Heat Forcing Storage Loss In Equilibrium, temperature is constant with time and so, is a measure of climate sensitivity; ˚K per Wm -2 of climate forcing

8 To Project future climates by using the observed record of climate over the past century, we need to know three things to interpret the temperature time series: Climate Forcing =  Q (Wm -2 ) Heat capacity = C (J o K -1 m -2 ) Climate sensitivity = ( o K per Wm -2 )


10 Heat Storage: Mostly the Oceans 1955-1996; Levitus et al. 2001: Science World Ocean = 18.2 x10 22 Joules Atmosphere = 0.7 x10 22 Joules Land Ice = 0.8 x10 22 Joules observed Modeled Model includes forcing from Greenhouse Gases, Sulfate Aerosols Solar irradiance changes, and volcanic aerosols. Model minus solar irradiance changes and volcanic aerosols. Model -

11 Top-Down Approach: Determine sensitivity of climate from observed record over past 130 years. Use simple model to extrapolate into future. Problems: Need to know: No two of these are known with enough precision to usefully constrain uncertainty in the third, with the data available, although it is possible to fit the observations with fair precision using even a simple model. Climate forcing - uncertain, especially solar and aerosol forcing. Heat storage - somewhat uncertain. Climate sensitivity - also uncertain.

12 IPCC 2001 1850-2000 ~0.6 o C Warming; 0.4 o C per century *mostly warming from CO 2 already in atmosphere 2000-2030 ~0.6 o C Warming; 2.0 o C per century*

13 IPCC - 2001 Predictions for the year 2100 1.4 o C <  T < 5.8 o C Between 1990 and 2100 global mean surface temperature will increase by This large range of uncertainty arises in equal measure from two principle sources: Uncertainty about how much climate forcing humans will do, principally through fossil fuel consumption. (Depends on political decisions, economic events, technical innovation and diffusion.) Uncertainty about how the climate system will respond to climate forcing by humans - Climate Sensitivity. (Depends on natural processes.)

14 Bottom-up approach Understand and model key physical processes that affect climate sensitivity. i.e. Feedback Processes Water vapor feedback Cloud feedback Ice-albedo feedback Many more

15 Water Vapor Feedback: Water vapor is the most important greenhouse gas controlling the relationship between surface temperature and infrared energy emitted from Earth. Saturation vapor pressure increases about 20% for each 1% change in temperature (3 o C). Therefore, assuming that the relative humidity remains about constant, the strength of the greenhouse effect will increase with surface temperature.


17 Infrared Greenhouse Effect: The amount by which the atmospheric reduces the longwave emission from Earth. Greenhouse effect = Surface infrared emission - Earth infrared emission =390 Wm -2 235 Wm -2 - 155 Wm -2

18 Greenhouse effect = Surface longwave emission - Earth emission

19 To a first approximation, the clear-sky greenhouse effect is proportional to the surface temperature. Sea Surface Temperature

20 Upper Troposphere Water Vapor And the Greenhouse Effect is related to the amount of water vapor.

21 Mount Pinatubo Eruption As a test of Water Vapor Feedback Soden, et al., Science, 26 April 2002 Philippines June 1991

22 Year Water Vapor Observed and Simulated Water Vapor Observed and Simulated Temperature Soden, et al., Science, 2002 Testing Water Vapor Feedback

23 Why is fixed relative humidity a good approximation during climate change? The relative humidity RH is required to be between 0 and 1. The mass of air moving upward (RH ~1), must be equal to the mass of air moving downward (RH~0+) The RH in the free atmosphere should be about 0.5 The inadequacies in this theory for relative humidity do not change that rapidly with climate, compared to the saturation vapor pressure dependence on temperature.

24 Water Vapor Feedback is a measure of climate sensitivity; o K per Wm -2 of climate forcing o = for fixed absolute humidity = 0.25 o K/(Wm -2 ) Effect on long-term response to doubled CO 2 RH = for fixed relative humidity = 0.50 o K/(Wm -2 ) (NRC, 1979, still good?)

25 Ice-Albedo Feedback As the Earth warms, ice melts in high latitudes and altitudes This lowers the albedo of Earth and leads to further warming. Ice reflects more solar radiation than other surfaces

26 Add Ice-Albedo Feedback to Water Vapor Feedback (NRC, 1979 still good) Add these changes to the basic relative humidity feedback and get as the uncertainty range for the long-term response to CO 2 doubling. IPCC - 2001gives NRC - 1979 gave

27 Conclusions: Uncertainties in projections of global warming are closely related to uncertainties in climate sensitivity to external forcing. Official scientific estimates of climate sensitivity have remained constant for 20 years, but so have the uncertainties in sensitivity, which are large. Increased efforts to understand the underlying physical processes behind the key climate feedback processes are needed, and many are underway. For the time being, however, policymaking on climate will need to be conducted in the presence of large uncertainty about the exact consequences of greenhouse gas emissions.

28 Part II: The Tropics and Climate Sensitivity 1.The net radiative effect of tropical convective clouds. 2.The Fixed Anvil Temperature (FAT) Hypothesis.

29 GMS-5 IR image Cloud Feedback IR Emission Temperature (˚K)

30 The Greenhouse Effect of Clouds: Clouds absorb all IR radiation and emit like black bodies at their temperature. Usually they are cold and emit less energy than clear skies would in their absence.


32 Cloud Radiative Effect Amount by which clouds affect the energy balance at the top-of-atmosphere



35 High Cloud (p<440mb) in the tropics is most common over warmest SST, or over land.

36 22% 9% 10%  > 1

37 Tropical clouds in the convective regions can have strongly positive or negative effects on the top-of- atmosphere energy balance, depending on their top altitude and albedo (optical depth). But, their populations tend to arrange themselves so that their net effect on the radiation balance is small  R i = R i cloudy - R i clear  i   R i

38 Conceptual Model: A convective box with upper- level clouds, and a clear box. Connect the two with a large-scale mass flux, M. Assume that the albedo of the convective cloud is proportional to the strength of the mass flux, and keep the emission temperature of the convective clouds fixed. Make the mass flux proportional to the difference in SST between the two boxes  T = T 1 - T 2.

39 Solve for the equilibrium using column energy balance. Hartmann, Moy and Fu, 2001, J. Climate Ocean Atmosphere Cloud

40 Net Radiation in convective and nonconvective regions of the tropics must be the same if: Albedo of convective cloud responds sensitively to circulation or SST gradients. Column energy export is uniform in domain of interest. Equilibrium conditions hold. Circulation responds sensitively to SST gradients. Then if cloud radiative forcing in non-convective regions is small, it must be small for convective clouds also.

41 Within the warm SST region, the correlation between high cloud amount and net radiation is small.

42 FAT Hypothesis Finally, the FAT Hypothesis, Fixed Anvil Temperatures for All Climates. This was assumed in previous ‘zero net radiative effect of convective cloud theory’. Now, I want to argue that tropical anvil clouds appear at a fixed temperature given by fundamental considerations of: Clausius-Clapeyron definition of saturation vapor pressure dependence on temperature. Dependence of emissivity of rotational lines of water vapor on vapor pressure.

43 Clear-sky Radiative Cooling and Relaxation: In the tropical atmosphere, and the in the global atmosphere, radiative cooling approximately balances heating by latent heat release in convection. The global mean precipitation rate is about 1 meter per year, which equals an energy input of about 80 Watts/sq. meter, Requiring a compensating atmospheric radiative cooling of about 0.7 ˚K/day, averaged over atmosphere. for tropical climatological conditions

44 Rotational Lines of Water Vapor and Upper- TroposphericCooling Total Beyond 18.5  m -->


46 Hypothesis: 200 hPa Convective outflow and associated large-scale divergence near 200 hPa are both associated by radiatively-driven divergence in clear skies. Fact: The radiatively-driven divergence in the clear regions is related to the decrease of water vapor with temperature following the Clausius-Clapeyron relation and the consequent low emissivity of water vapor at those low temperatures. Further Conjecture: The temperature at which the radiatively-driven divergence occurs will always remain the same, and so will the temperature of the cloud anvil tops.

47 Larson and Hartmann (2002a,b) Model Study: MM5 in doubly periodic domain a) 16x16 box with uniform SST (297, 299, 301, 303K) b) 16x160 box with sinusoidal SST Clouds and circulation are predicted Cloud interact with radiation Basically, a radiative-convective model in which the large-scale circulation is allowed to play a role by dividing the domain into cloudy (rising) and clear (sinking) regions. Testing the FAT Hypothesis in a model.

48 Radiative Cooling in non-convective region for SST’s ranging from 297K to 303K. From Larson & Hartmann (2002a). Cooling profile moves up in upper troposphere with increasing SST. But what happens to the temperature at the top of the cooling profile?

49 The temperature at which the radiative cooling reaches -0.5 K/day remains constant at about 212K. The temperature at which the visible optical depth of upper cloud reaches 0.1 remains constant at about 200K. The temperature of the 200 hPa surface increases about 13K, while the surface temperature rises 6K.

50 Conclusions: Net radiation in convective regions should be the same as that in adjacent non-convective regions of the tropics. Hartmann, Moy and Fu, J. Climate, 2001. This would mean that the net radiative effect of tropical convective clouds must remain small. The favored temperature for tropical anvil cloud tops should remain approximately constant during climate changes of reasonable magnitude. FAT Hypothesis. Hartmann and Larson, GRL, 2002. The emission temperature of the rotational lines of water vapor should also remain approximately constant during climate change. Hartmann and Larson, GRL, 2002. These assertions imply relatively strong positive water vapor and IR cloud feedback, but small net convective cloud radiative feedback.

51 Remaining Questions: Will the area occupied by tropical convection change with climate? If so, how? IRIS? Will the area, or optical properties of boundary layer clouds change with climate? Feedback looks negative. To what extent is what I just said correct? What will happen at the tropical tropopause? Will it get warmer or colder and what will this mean for climate? Fin

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