Does the earth have an adaptive infrared iris?

Observations and analyses of water vapor and clouds in the Tropics over the past decade show that the boundary between regions of high and low free-tr opospheric relative humidity is sharp, and that upper-level cirrus and high free-tropospheric relative humidity tend to coincide. Most current studies of atmospheric climate feedbacks have focused on such quantities as clear sky humidity, average humidity, or differences between regions of high and low humidity, but the data suggest that another possible feedback might con sist of changes in the relative areas of high and low humidity and cloudine ss. Motivated by the observed relation between cloudiness (above the trade wind boundary layer) and high humidity, cloud data for the eastern part of the western Pacific from the Japanese Geostationary Meteorological Satellit e-5 (which provides high spatial and temporal resolution) have been analyze d, and it has been found that the area of cirrus cloud coverage normalized by a measure of the area of cumulus coverage decreases about 22% per degree Celsius increase in the surface temperature of the cloudy region. A number of possible interpretations of this result are examined and a plausible on e is found to be that cirrus detrainment from cumulus convection diminishes with increasing temperature. The implications of such an effect for climat e are examined using a simple two-dimensional radiative-convective model. T he calculations show that such a change in the Tropics could lead to a nega tive feedback in the global climate, with a feedback factor of about -1.1, which if correct, would more than cancel all the positive feedbacks in the more sensitive current climate models. Even if regions of high humidity wer e not coupled to cloudiness, the feedback factor due to the clouds alone wo uld still amount to about -0.45, which would cancel model water vapor feedb ack in almost all models. This new mechanism would, in effect, constitute a n adaptive infrared iris that opens and closes in order to control the Outg oing Longwave Radiation in response to changes in surface temperature in a manner similar to the way in which an eye's iris opens and closes in respon se to changing light levels. Not surprisingly, for upper-level clouds, thei r infrared effect dominates their shortwave effect. Preliminary attempts to replicate observations with GCMs suggest that models lack such a negative cloud/moist areal feedback.