Radiative Feedbacks

The climate's greenhouse effect comes about because of the presence of well-mixed gases that absorb efficiently in the thermal infrared while the same atmosphere is largely transparent at visible wavelengths where most of the solar forcing occurs. The shortwave radiative forcing occurs largely at the Earth's surface, and the surface cannot easily cool itself by radiating to space in the thermal infrared because of the greenhouse gases present. As a consequence, the surface has to warm more to maintain a radiative balance than it would have without the greenhouse gases present.

It is possible for some elements of the atmosphere to respond to surface temperature change in such a way that radiation from the troposphere is either enhanced or suppressed. When temperature increases and thermal infrared (longwave) radiation from the troposphere is partially suppressed, the action is considered a positive radiative feedback; when temperature increases and longwave radiation from the troposphere is enhanced, the action is considered a negative radiative feedback.

An injection of anthropogenic greenhouse gases, before anything else happens, has the immediate effect of blocking photons from escaping the troposphere. The amount of radiation flux blocked is called a radiative forcing A Frad. To first order, the surface responds by increasing its temperature by an amount AT(1), thus increasing the flux through the tropopause and restoring radiative balance. The statement is

r in which r « 4ea T3 is the gray-body radiation term for the surface, e a combination of surface emissivity and the fraction of radiation from the surface that escapes to space, and a the Stefan-Boltzman constant. The climate system responds dynamically and thermodynamically to such a surface temperature change, and some of those reactions act to enhance radiation to space and some to suppress it. A continuum of such feedbacks exists, a geometric series for surface temperature change results, and the final surface temperature change is

where the longwave feedback factors y1LW and shortwave feedback factors y1sW are defined by y LW _ ( dFW \ dXi

where FLW is the net downward longwave flux at the tropopause, FSW is the net downward shortwave flux at the tropopause, and x' can be any one of a long list of meteorological, thermodynamic, or constituent properties that can affect longwave or shortwave radiation. A positive feedback has y > 0, and a negative feedback has y < 0. The largest feedbacks are thought to be the water vapor-longwave feedback, the cloud-shortwave feedback, the upper tropospheric temperature-longwave ("lapse rate") feedback, and the hypothesized aerosol indirect effects in the shortwave. The most uncertain feedbacks are thought to be the cloud-shortwave feedback and the aerosol indirect effect. This calculus of feedbacks has been presented elsewhere (Cess 1976; Wetherald and Manabe 1988) as have reviews about feedbacks implicit in climate models (Held and Soden 2000; Colman 2003; Bony et al. 2006; Soden and Held 2006).

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