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Karen Shell - Understanding Climate Feedbacks in Computer Models of the Earth's ClimateKaren Shell

Many different feedbacks influence how the earth's temperature, precipitation, and winds respond to changes in the energy budget of the planet (caused, for example, by increases in carbon dioxide or other greenhouse gases).  One example of a climate feedback is the water vapor feedback.  As the temperature of the planet warms, the air is able to "hold" more evaporated water.  Thus, we expect the amount of water vapor, in general, to increase in a warmer climate.  However, water vapor traps some of the infrared radiation (heat) that the earth emits, further warming the planet.  Thus, the water vapor feedback is a positive feedback.

Because there is only one realization of the actual climate (i.e. there is only one earth), climate models are useful tools for studying different scenarios and climate configurations in a very controlled setting.  They also allow us to isolate particular climate feedbacks to determine how much they amplify or damp climate change. However, different climate models do not always produce the same results for a given climate feedback.

To study climate feedbacks, Jeff Kiehl (NCAR, CGD division) and I have been splitting the different feedbacks into two parts: the change in the particular climate component between a doubled CO2 case and a present-day CO2 case, and the effect that a standard change in that component has on the climate (measured by the effect on the earth's radiation balance).  We can then combine these two values to determine the feedback for a particular climate change experiment.  This technique allows us to compare similar results from different models to determine if differences in feedbacks are due to differences in how the components respond to climate change or due to differences in how the changed components affect the climate.

The figures show an example of this technique.  The first figure shows the modeled effect that changes in water vapor at different latitudes and heights have on the earth's outgoing longwave radiation (heat) when no clouds are present.  Negative values correspond to a warming effect, since less radiation leaves the earth's atmosphere.  This plot shows us that water vapor changes higher in the atmosphere have a larger effect.

The next plot shows how water vapor changes from the present-day for a doubled CO2 model simulation.  Water vapor increases everywhere, but the increase is largest at lower altitudes and near the equator.

We can combine the data in these two plots to determine the total effect of the water vapor changes, as shown in the third plot.  This plot shows the effect as a function of latitude and longitude.  Thus, the effect of water vapor varies from region to region.  It is small near the poles and largest in the tropics.  Again, negative values correspond to a warming of the climate.  (For comparison, the total amount of solar radiation the planet absorbs is about 240 W/m2.)

We are continuing this work by looking at other climate feedbacks and comparing results between different climate models.  By gaining an understanding of how these feedbacks operate in climate models, we will be able to improve the models themselves, leading to better predictions of future climate change.

ASP Spotlight June 2006
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