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

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 CO2 increase.

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 14C and 13C.

The Instrumental Record of Global Temperature Anomalies.

IPCC - 2001

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

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 = DQ (Wm-2) Heat capacity = C (J oK-1 m -2) Climate sensitivity = l (oK per Wm-2)

Heat Storage: Mostly the Oceans 1955-1996; Levitus et al. 2001: Science World Ocean = 18.2 x1022 Joules Atmosphere = 0.7 x1022 Joules Land Ice = 0.8 x1022 Joules Model - observed Modeled Model includes forcing from Greenhouse Gases, Sulfate Aerosols Solar irradiance changes, and volcanic aerosols. Model minus solar irradiance changes and volcanic aerosols.

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: • Climate forcing - uncertain, especially solar and aerosol forcing. • Heat storage - somewhat uncertain. • Climate sensitivity - also uncertain. 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.

1850-2000 ~0.6oC Warming; 0.4oC per century IPCC 2001 1850-2000 ~0.6oC Warming; 0.4oC per century 2000-2030 ~0.6oC Warming; 2.0oC per century* *mostly warming from CO2 already in atmosphere

Predictions for the year 2100 IPCC - 2001 Predictions for the year 2100 Between 1990 and 2100 global mean surface temperature will increase by 1.4oC < DT < 5.8oC 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.)

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

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 oC). • Therefore, assuming that the relative humidity remains about constant, the strength of the greenhouse effect will increase with surface temperature.

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

Greenhouse effect = Surface longwave emission - Earth emission

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

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

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

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

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.

Effect on long-term response to doubled CO2 Water Vapor Feedback Effect on long-term response to doubled CO2 l is a measure of climate sensitivity; oK per Wm-2 of climate forcing lo = for fixed absolute humidity = 0.25 oK/(Wm-2) lRH = for fixed relative humidity = 0.50 oK/(Wm-2) (NRC, 1979, still good?)

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

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 CO2 doubling. IPCC - 2001gives NRC - 1979 gave

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.

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

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

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.

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

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

t > 1 9% 22% 10%

DRi = Ricloudy - Riclear 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). Ai •DRi But, their populations tend to arrange themselves so that their net effect on the radiation balance is small

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 DT = T1 - T2.

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

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. Circulation responds sensitively to SST gradients. Column energy export is uniform in domain of interest. •Equilibrium conditions hold. •Then if cloud radiative forcing in non-convective regions is small, it must be small for convective clouds also.

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

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.

Clear-sky Radiative Cooling and Relaxation: for tropical climatological conditions 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.

Rotational Lines of Water Vapor and Upper- Tropospheric Cooling Total Beyond 18.5mm -->

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. Hypothesis: 200 hPa Convective outflow and associated large-scale divergence near 200 hPa are both associated by radiatively-driven divergence in clear skies. 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.

Testing the FAT Hypothesis in a model. 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.

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?

The temperature of the 200 hPa surface increases about 13K, while the surface temperature rises 6K. 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.

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.

Remaining Questions: To what extent is what I just said correct? 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. What will happen at the tropical tropopause? Will it get warmer or colder and what will this mean for climate? Fin