Cloud Trends and their Influence on Eg

Changes in cloudiness during parts of the dimming and brightening periods were studied by Joel Norris [52]. The data was from both surface data sets and satellite observations. The surface set, which was divided into 10° x 10° cells, was from the Extended Edited Cloud Report Archive (EECRA), and included ground based cloud observations from land stations (1971 1996) and ship reports (1952 1997). These showed that zonal mean upper-level cloud cover at low and middle latitudes decreased by 1.5%-sky-cover between 1971 and 1996 over land and by about 1%-sky-cover between 1951 and 1997 over ocean. The upper level data were closely related to satellite (ISCCP) estimates for an overlapping period. Estimates of the cloud cover influence on solar radiation showed that between 1952 and 1997 over mid-latitude oceans cloud changes decreased Eg# by about 1 Wm2, and over northern mid-latitude land areas cloud changes increased Eg# slightly. For low-latitude land and ocean regions cloud changes increased Eg# from the 1980s to the mid-1990s. These changes in cloudiness are relatively small, and although they probably played a significant part in global dimming and brightening, they could not be considered to be major players. Similar conclusions, that is, that cloud trend influences on short wave radiative forcing could not account for most of the global dimming and brightening, were made by Norris and Wild [53], who subtracted the estimated cloud cover influence on solar radiation from surface Eg# data in the GEBA archive and found that dimming and brightening trends in the residual Eg# were unchanged.

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