Upper-level tropical cloudiness plays a key role in the climate system through its strong impact on the radiative energy budget of the Earth, but the processes controlling cloud properties are poorly understood and simulated in climate models. This lack of understanding is a major contributor to our uncertainty of the sensitivity of the climate system to increasing greenhouse gases and the future magnitude of global warming. Therefore is important to determine past changes in tropical cloudiness and their impact on the radiation budget.

Analysis of surface synoptic cloud reports made by volunteer observing ships provides a consistent record of upper-level cloud cover extending back to 1952. Interannual anomalies of surface-observed upper-level cloud cover are found to be locally well-correlated with anomalies in satellite-measured reflected shortwave and outgoing longwave radiation over most of the Tropics during 1985-1997. The time series of tropical mean upper-level cloud also matches the time series of tropical mean shortwave and longwave radiation. Satellite-measured radiation is regressed on surface-observed cloud cover to estimate changes in radiation prior to 1985. Tropical mean net outgoing radiation is estimated to have increased by about 0.25 W/m*m from 1952 to 1997 due to changes in upper-level cloud cover.