Global Warming and Climate Sensitivity

1. Global Warming and Climate Sensitivity Professor Dennis L. Hartmann Department of Atmospheric Sciences University of Washington Seattle, Washington

2. 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.

3. The Instrumental Record of Global Temperature Anomalies.

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 14C and 13C.

5. IPCC - 2001

6. 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

7. 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)

9. 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.

10. 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.

11. IPCC -2001

12. 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

13. 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.)

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 oC). • 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 - 155 Wm-2 = 390 Wm-2 235 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. Sea Surface Temperature And the Greenhouse Effect is related to the amount of water vapor. Upper Troposphere Water Vapor

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

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

23. 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?)

24. 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.

25. 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

26. 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.

27. Estimated Strength of Water Vapor Feedback Earliest studies suggest that if the absolute humidity increases in proportion to the saturation vapor pressure (constant relative humidity), this will give rise to a water vapor feedback that will double the sensitivity of climate compared to an assumption of fixed absolute humidity. Most observational and modeling studies have supported this conclusion.