Thermostats, Radiator Fins, and the Local Runaway Greenhouse

The author has reconsidered the question of the regulation of tropical sea surface temperature. This has been done in general terms through consideration of the tropical beat budget and in specific terms through consideration of an idealized radiative-dynamic model of the tropical general circulation. It is argued that evaporation on its own cannot provide an effective regulating mechanism. Clouds cannot serve as regulators unless there are substantial departures from the observed cancellation between cloud greenhouse and cloud albedo effects. In particular, it is shown that the prediction by Ramanathan and Collins of highly stable tropical climates is based on an inconsistent set of assumptions about the behavior of the atmospheric heat transports. When the heat transports are treated in a consistent manner, clouds are found to have little impact, and the tropical climate can he quite sensitive to radiative perturbations. It is found that the main determinant of tropical climate is the clear-sky water vapor greenhouse effect averaged over the entire Tropics. In the absence of dry “radiator fins” maintained by subsidence, the tropical temperature would tend to fall into a runaway greenhouse state that could be stabilized only by heat export to the extratropics. Some speculative results on sensitivity of the climate to perturbations are presented. Determination of the relative area of dry and subsiding versus moist and convecting regions of the Tropics, and of the degree of dryness of the subsiding regions, are identified as key unsolved problems concerning the tropical climate.