Spurious changes in the ISCCP dataset

The International Satellite Cloud Climatology Project (ISCCP) data set for July 1983-December 1990 exhibits a long term decrease in mean cloud optical depth and mean cloudtop temperature which is large compared to the mean for this period. It is here suggested that this decrease is an artifact of incomplete normalization of the visible channel on successive polar orbiters employed as the ISCCP's calibration standard; more accurate calibration techniques are required for the establishment of long-term climate trends.