On Trends and Possible Artifacts in Global Ocean Cloud Cover between 1952 and 1995

Abstract

Synoptic surface cloud observations are used to examine interdecadal variability in global ocean cloud cover between 1952 and 1995. Global mean total cloud cover over the ocean is observed to increase by 1.9% (sky cover) between 1952 and 1995. Global mean low cloud cover over the ocean is observed to increase by 3.6% between 1952 and 1995. Trends in zonal mean total and low cloud cover in 10°-latitude bands between 40°S and 60°N are all positive, and trends in the Southern Hemisphere and Tropics are generally as large or larger than trends in the midlatitude Northern Hemisphere. This argues against attribution of increased cloud cover to increased anthropogenic aerosol. Although it is possible that global cloud cover is responding to some other global parameter, perhaps global temperature, it is not clear what underlying physical mechanism would cause substantially different processes in the Tropics, subtropics, and midlatitudes to all produce increasing cloudiness. On the other hand, the fact that ships with a common observing practice travel over most of the global ocean suggests a possible observational artifact may be largely responsible for the upward trends observed at all latitudes. Potential causes of artifacts are examined but do not provide likely explanations for the observed interdecadal variability. Thus, it remains uncertain whether the observed increases in global mean ocean total and low cloud cover between 1952 and 1995 are spurious. Corroboration by related meteorological parameters and satellite-based cloud datasets should be required before the trends are accepted as real.