In spite of increasing surface temperatures, a falling trend in surface insolation has been observed over the past 50 years at stations worldwide, and a link to anthropogenic aerosols has been suggested. In the present thesis, reports on this "global dimming" are studied and briefly compared to other climate changes such as global warming and atmospheric circulation changes. To get a better understanding of the connection between global dimming and changes in cloud cover, monthly records of surface insolation and cloud amount of varying lengths are investigated for six stations in north-western Europe.
The results indicate that global dimming has been present in north-western Europe up to the late 1980s/early 1990s, after which the dimming has turned to a brightening. Although the representativity of merely six stations to the tendency of entire north-western Europe may be debatable, the results correspond well to observed changes in the regional pollution level: There was heavy industrial build-up (particularly in eastern Europe) up to the late 1980s, after which the collapse of the East Bloc and political interventions to limit pollution lead to re-improved air quality. Additionally, some stations experienced an increase in cloud amounts prior to about 1990, succeeded by a decreasing trend, which will have contributed to the observed variations in global radiation.
The correlation between surface insolation and cloud cover was found to be high and significant at all stations, and did at two of the stations seem to be the dominating cause of the radiation trend. %At one station, the variations in insolation seemed primarily to be governed by aerosol loads, while at the remaining three stations, the cause was probably a combination of the effects of clouds and aerosols. It appears that in some cases, the effect of aerosol loads and cloud cover collaborate, while in other cases they compete. As the present study shows, dimming may occur in spite of improved aerosol levels, and brightening may occur in spite of increasing cloud covers. The fact that the surface insolation trend is equally dependent on natural variations in clouds as on human-induced variations in aerosol loads, is an important point as debates on global dimming in general tend to be focused on the effect of anthropogenic pollution.