3.4.3.1 Surface Cloud Observations
As noted in the TAR and extended with more recent studies, surface observations suggest increased total cloud cover since the middle of the last century over many continental regions including the USA (Sun, 2003; Groisman et al., 2004; Dai et al., 2006), the former USSR (Sun and Groisman, 2000; Sun et al., 2001), Western Europe, mid-latitude Canada, and Australia (Henderson-Sellers, 1992). This increasing cloudiness since 1950 is consistent with an increase in precipitation and a reduction in DTR (Dai et al., 2006). However, decreasing cloudiness over this period has been reported over China (Kaiser, 1998), Italy (Maugeri et al., 2001) and over central Europe (Auer et al.,2007). If the analyses are restricted to after about 1971, changes in continental cloud cover become less coherent. For example, using a worldwide analysis of cloud data (Hahn and Warren, 2003; Minnis et al., 2004) regional reductions were found since the early 1970s over western Asia and Europe but increases over the USA.
Changes in total cloud cover along with an estimate of precipitation over global and hemispheric land (excluding North America) from 1976 to 2003 are shown in Figure 3.22. During this period, secular trends over land are small. The small variability evident in land cloudiness appears to be correlated with precipitation changes, particularly in the SH (Figure 3.22). Note that surface observations from North America are excluded from this figure due to the declining number of human cloud observations since the early 1990s over the USA and Canada, as human observers have been replaced with Automated Surface Observation Systems (ASOS) from which cloud amounts are less reliable and incompatible with previous records (Dai et al., 2006). However, independent human observations from military stations suggest an increasing trend (~1.4% of sky per decade) in total cloud cover over the USA.
The TAR also noted multi-decadal trends in cloud cover over the ocean. An updated analysis of this information (Norris, 2005a) documented substantial decadal variability and decreasing trends in upper-level cloud cover over mid- and low-latitude oceans since 1952. However, there are no direct observations of upper-level clouds from the surface and instead Norris (2005a) infers them from reported total and low cloud cover assuming a random overlap. These results partially reverse the finding of increasing trends in mid-level cloud amount in the northern mid-latitude oceans that was reported in the TAR, although the new study does not distinguish between high and middle clouds. Norris (2005b) found that upper-level cloud cover had increased over the equatorial South Pacific between 1952 and 1997 and decreased over the adjacent subtropical regions, the tropical Western Pacific, and the equatorial Indian Ocean. This pattern is consistent with decadal changes in precipitation and atmospheric circulation over these regions noted in the TAR, which further supports their validity. Deser et al. (2004) found similar spatial patterns in inter-decadal variations in total cloud cover, SST and precipitation over the tropical Pacific and Indian Oceans during 1900 to 1995. In contrast, low-cloud cover increased over almost all of the tropical Indian and Pacific Oceans, but this increase bears little resemblance to changes in atmospheric circulation over this period, suggesting that it may be spurious (Norris, 2005b). When averaged globally, oceanic cloud cover appears to have increased over the last 30 years or more (e.g., Ishii et al., 2005).
During El Niño events, cloud cover generally decreases over land throughout much of the tropics and subtropics, but increases over the ocean in association with precipitation changes (Curtis and Adler, 2003). Multi-decadal variations are affected by the 1976–1977 climate shift (Deser et al., 2004), and these dominate the low-latitude trends from 1971 to 1996 found in Hahn and Warren (2003).