Is there a cosmic ray signal in recent variations in global cloudiness and cloud radiative forcing?

Abstract

In order to evaluate a recent hypothesis of a coupling between galactic cosmic rays, clouds, and climate we have investigated temporal variations in global cloudiness and radiative fluxes at the top of the atmosphere. For this purpose we have used the best available global data sets, i.e., those of the International Satellite Cloud Climatology Project (ISCCP) and the Earth Radiation Budget Experiment (ERBE), respectively. Both globally and over midlatitude oceans only, we find a decrease in total cloud coverage between 1986 and 1990 of 2%, while between 1990 and 1993 there is a slightly smaller decrease. When the results are related to temporal variations in cosmic ray activity, we do not find support for a coupling between cosmic rays, total cloudiness, and radiative forcing of climate. A possible exception is low marine clouds at midlatitudes, characterized by few cloud condensation nuclei and a large net cooling effect, but no physical mechanism is known which might explain a connection between cosmic rays and low clouds. The net radiative effect of clouds during the period 1985–1989 shows an enhanced cooling effect despite a reduction in both total and low cloud cover. This contradicts the simple relationship between cloud cover and radiation assumed in the cosmic‐ray‐cloud‐climate hypothesis. The interpretation of the results is rendered difficult by the short time series of ISCCP and ERBE data and by uncertainties concerning instrument calibrations and changes of satellites. When a 43‐year time series of synoptic observations over sea is related to cosmic rays over the same period, a weak, negative correlation is found.