Observed Interdecadal Changes in Cloudiness: Real or Spurious?

Abstract

This study documents multidecadal variations in low-level, upper-level, and total cloud cover over land and ocean independently obtained from surface synoptic observations and from satellite data produced by the International Satellite Cloud Climatology Project. Substantial agreement exists between global mean time series of surface- and satellite-observed upper-level cloud cover, indicating that the reported variations in this cloud type are likely to be real. Upper-level cloud cover has decreased over almost all land regions since 1971 and has decreased over most ocean regions since 1952. Global mean time series of surface- and satellite-observed low-level and total cloud cover exhibit very large discrepancies, however, implying that artifacts exist in one or both data sets. The global mean satellite total cloud cover time series appears spurious because the spatial pattern of correlations between grid box time series and the global mean time series closely resembles the fields of view of geostationary satellites rather than geophysical phenomena. The surface-observed low-level cloud cover time series averaged over the global ocean appears suspicious because it reports a very large 5%-sky-cover increase between 1952 and 1997. Unless low-level cloud albedo substantially decreased during this time period, the reduced solar absorption caused by the reported enhancement of cloud cover would have resulted in cooling of the climate system that is inconsistent with the observed temperature record.