Trends in Global Marine Cloudiness and Anthropogenic Sulfur

Abstract

A statistical analysis using published data on the global distribution of total cloud cover and cloud type amounts over the ocean, reduced from the Comprehensive Ocean?Atmosphere Data Set (COADS), shows a significant positive trend (4.2% increase from the 1930 baseline) in total oceanic cloud amount in the period between 1930 and 1981. The increase of total cloud amount for the Northern Hemisphere (5.8% ) was twice that for the Southern Hemisphere (2.9% ), The more consistent 30-yr ( 1952?1981 ) data show that the change in cloud amount ( 1952 base) was 1.5% for the globe, 2.3% for the Northern Hemisphere, and 1.2% for the Southern Hemisphere. The analysis also shows that the greatest cloud amount increase was for altocumulus and altostratus clouds and that this increase was most pronounced at midlatitudes (30°?50°N). The trend and the pattern of cloud amount variations appear to be in accord with the temporal trend and geographic distribution of S02 emissions. It is hypothesized that sulfate particles converted from S02, may modify cloud droplet spectra, causing affected clouds to be more colloidally stable than unaffected clouds. The longer residence times of affected clouds could cause increases of cloud frequency and cloud amount.