Global Dimming Reports in the Twentieth Century

Suraqui et al. [29] reported 'severe changes over the years in solar radiation' and issued a call for 'a careful study of incoming radiation at different places throughout the world ... to determine the exact kind, order of magnitude and their causes ...'. The 'severe changes' referred to emerged from the measurements at the site of the Smithsonian Institution's former solar radiation monitoring station on Mt. St. Katherine in the southern Sinai peninsula (28°31'N, 33°56'E, 2643 m altitude). Measurements using modern radiometers as well as some of the original instruments employed between 1933 and 1937 showed a 12% loss in global radiation during the intervening four decade interval.

Atsumu Ohmura, whose background was in glaciology, and who headed the GEBA archive [24], reported at a conference that solar radiation was decreasing at many sites where it was being measured. His colleagues, who were highly sceptical of his findings, discouraged him from pursuing this, and the report was published (or temporarily buried) in a little known conference proceedings [30]. Russak [31] reported decreasing trends of 0.2 0.6 Wm 2a 2 for a few stations in northern Europe. Gerald Stanhill, who used solar radiation measurements for determining evaporation and crop water use in arid environments, was intrigued by the decreasing trends in solar radiation that he found in radiation records. Stanhill and Moreshet [32] analyzed data from 45 stations for the years 1958, 1965, 1975 and 1985, and found a statistically significant average worldwide decrease of Eg# totalling 5.3% (or 0.34 W-m 2-a ) from 1958 to 1985. Decreasing trends of the same order of magnitude were found for sites in Australia [33], Japan [34], the arctic [35], Antarctica [36], Israel [37] and Ireland [38]. The largest decrease, found in Hong Kong, was 1.8 Wm 2a 2, that is, a decrease in excess of 1% per year [39]. Other groups reported dimming for China [40], the former Soviet Union [41] and Germany [42,43]. Reductions in solar radiation were larger for urban industrial sites, but even at sites remote from pollution Eg# was usually decreasing at a rapid rate.

Gilgen et al. [44] reviewed trends found in the GEBA archive. Their paper, entitled 'Means and trends of short wave irradiance at the surface estimated from GEBA Data', included analyses of accuracy and biases, and trends in Eg# for different regions of the world. The final sentence of the abstract noted that 'on most continents, shortwave irradiance decreases significantly in large regions, and significant positive trends are observed only in four small regions'.

Stanhill and Cohen [23] tabulated the negative trends for different sites around the world. Of the 30 stations where detailed analyses of trends had been published, at 28 Eg# had decreased and only at two, Dublin, Ireland and Griffith, Australia, had Eg# increased (by 0.56 and 0.76 Wm 2 a 2, respectively). They also analysed solar radiation records from the geophysical year, 1958, and the years 1965, 1975, 1985 and 1992. These records were from between 145 (1958) and 303 (1992) stations whose measurements conformed to WMO standards. Average transmittance of a unit atmosphere for the northern hemisphere was 0.52 in 1957 and declined steadily to 0.44 in 1992 while that for the southern hemisphere averaged 0.57 until 1985 and declined between 1985 and 1992 to 0.52. A spline fit to the latitudinal distribution of Eg# showed that the decrease during the 34 a period had been especially large in the industrialized region of the northern hemisphere with a centre at ^35°N and a width of ^20°. This feature and an analysis of the various possible reasons for the dimming phenomenon, led to the conclusion that particulate aerosols, and especially those from anthropogenic sources, were the cause of the changes. Similar conclusions were drawn at about the same time by Liepert and Lohmann [45].

Many subsequent studies have highlighted similar trends based on data collected from the mid twentieth century and onwards. Trends for individual sites are highly variable, and for some places and some parts of the world no change or increases in solar radiation have been found.

0 0

Post a comment