What Do We Really Know About Cloud Changes over the Past Decades?

Clouds are a critical component of Earth's climate system. Although satellite-based irradiance measurements are available over approximately the past 30 years, difficulties in measuring clouds means it is unclear how global cloud properties have changed over this period. From the International Satellite Cloud Climatology Project (ISCCP) and Moderate Resolution Imaging Spectroradiometer (MODIS) datasets we have examined the validity of long-term cloud changes. We find that for both datasets, low-level (>680mb) cloud changes are largely a reflection of higher-level (<= 680mb) variations. Linear trends from ISCCP also suggest that the dataset contains considerable features of an artificial origin. Despite this, an examination of ISCCP in relation to the MODIS dataset shows that over the past ten years of overlapping measurements between 60 degrees N-60 degrees S both datasets have been in close agreement (r = 0.63, p = 7x10(-4)). Over this time total cloud cover has been relatively stable. Both ISCCP and MODIS datasets show a close correspondence to Sea Surface Temperatures (SST) over the Pacific region, providing a further independent validation of the datasets.