Background
Based on both satellite and surface measurements, Satheesh and Ramanathan (2000) determined that polluted air from south and southeast Asia absorbed enough solar radiation over the northern Indian Ocean during the dry monsoon season to heat the atmosphere there by 1-3°C/day at solar noon.  Based on a model of trade cumulus boundary-layer development, Ackerman et al. (2000) calculated that this atmospheric heating would decrease cloud-layer relative humidity, reduce boundary-layer mixing, and lead to a 25-50% drop in daytime cloud cover, relative to an aerosol-free atmosphere.  Based on these observations, the author of the current study hypothesized that the great increase in anthropogenic aerosol emissions from south and southeast Asia over the last half-century should have significantly decreased low-level cloud cover over the northern Indian Ocean over this period.

What was done
To evaluate his hypothesis, Norris used data from the Extended Edited Cloud Report Archive to calculate and plot the temporal progression of decadal averages of Northern Hemispheric and Southern Hemispheric percent sky cover by low-level clouds from the 1950s to the 1990s.

What was learned
It was determined that daytime low-level ocean cloud cover not only did not decrease over this time period, it increased.  Furthermore, it increased in both the Northern and Southern Hemispheric regions of the study area and at essentially all hours of the day.

What it means
In the words of the author, "the observed all-hours increase in low-level cloud cover over the time period when soot aerosol has presumably greatly increased argues against a dominant effect of soot solar absorption contributing to cloud 'burn-off'."  Hence, he says, "other processes must be compensating."  Those other processes, however, remain hidden.  One of the important take-home messages of this humbling exercise, therefore, is that earth's climate system is amazingly complex, and that it has the ability to readily thwart, or significantly blunt, the seemingly straightforward climatic implications of a major atmospheric radiative perturbation.  And if it can do so for soot, it can probably do so for CO2 as well.

Satheesh, S.K. and Ramanathan, V.  2000.  Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface.  Nature 405: 60-63.