What was done
Based on approximately 185 million synoptic weather observations obtained from some 5400 stations worldwide, covering all continents and many islands, the authors developed separate day and night histories of cloud amount for nine different cloud types for the 26-year period 1971-1996.

What was learned
Warren et al. report "there are large regional changes in cloud-type amounts, and significant changes in the global averages of some cloud types." More specifically, they say that "the time series of total-cloud-cover anomalies for individual continents show a large decrease for South America, small decreases for Eurasia and Africa, and no trend for North America." They also state that "the zonal average trends of total cloud cover are positive in the Arctic winter and spring, 60°-80°N, but negative in all seasons at most other latitudes." In addition, they state that "night trends agree with day trends for total cloud cover and for all cloud types except cumulus," and that "cirrus trends are generally negative over all continents." However, they find that all of these changes "compensate each other to result in only a small trend of global average land cloud cover, -0.7% decade^-1." What is more, they note that "this small negative trend is further compensated by a small positive trend over the ocean of +0.4% decade^-1 (Norris, 1999), resulting in almost no trend for global average cloud cover over the past few decades."

What it means
The researchers readily acknowledge the great significance of changes in cloud type and amount for global and regional climate change and vice versa; but they do not speculate on the climatic implications of their specific findings, noting that "it will be important to prepare cloud datasets for the more recent years [post 1996], when changes may become more noticeable with increased global warming." Interestingly, these statements are rather telling in and of themselves, for they suggest that over the studied period, when climate alarmists claim the temperature of the earth warmed at an unprecedented rate, there were no "noticeable" changes that could readily be attributed to "increased global warming."

Reference
Norris, J.R. 1999. On trends and possible artifacts in global ocean cloud cover between 1952 and 1995. Journal of Climate 12: 1864-1870.