AR4 reported that surface-observed total cloud cover may have increased over many land areas since the middle of the 20th century, including the USA, the former USSR, Western Europe, mid-latitude Canada and Australia. A few regions exhibited decreases, including China and central Europe. Trends were less globally consistent since the early 1970s, with regional reductions in cloud cover reported for western Asia and Europe but increases over the USA.
Analyses since AR4 have indicated decreases in cloud occurrence/cover in recent decades over Poland (Wibig, 2008), China and the Tibetan Plateau (Duan and Wu, 2006; Endo and Yasunari, 2006; Xia, 2010b), in particular for upper level clouds (Warren et al., 2007) and also over Africa, Eurasia and in particular South America (Warren et al., 2007). Increased frequency of overcast conditions has been reported for some regions, such as Canada, from 1953 to 2002 (Milewska, 2004), with no statistically significant trends evident over Australia (Jovanovic et al., 2011) and North America (Warren et al., 2007). A global analysis of surface observations spanning the period 1971–2009 (Eastman and Warren, 2012) indicates a small decline in total cloud cover of about 0.4% per decade which is largely attributed to declining mid- and high-level cloud cover and is most prominent in the middle latitudes.
Regional variability in surface-observed cloudiness over the ocean appeared more credible than zonal and global mean variations in AR4. Multidecadal changes in upper-level cloud cover and total cloud cover over particular areas of the tropical Indo-Pacific Ocean were consistent with island precipitation records and SST variability. This has been extended more recently by Deser et al. (2010a), who found that an eastward shift in tropical convection and total cloud cover from the western to central equatorial Pacific occurred over the 20th century and attributed it to a long-term weakening of the Walker circulation (Section 2.7.5). Eastman et al. (2011) report that, after the removal of apparently spurious globally coherent variability, cloud cover decreased in all subtropical stratocumulus regions from 1954 to 2008.