Chapter 29 - Widespread Surface Solar Radiation Changes and Their Effects: Dimming and Brightening

Abstract

Global radiation (Eg↓) is the total solar radiation reaching a horizontal surface at the Earth's surface, i.e. at the bottom of the atmosphere (BOA). Precise widespread measurements of Eg↓ began in the early twentieth century and although it was first assumed that no multi-annual trends in this quantity occurred, by the 1970s there was evidence of significant decreases at some sites. As the evidence for large multi-decadal trends in Eg↓ grew, the relationship between decreasing solar radiation or global dimming in the late twentieth century and widespread decreasing pan evaporation during that period was noticed. The energetic similarity of these changes led to scientific recognition that changes in Eg↓ were playing a significant role in climate change. Previous assumptions that other parts of the Earth's radiation balance were unchanging have subsequently come under scrutiny. This chapter provides background material on solar radiation and reviews the recent changes in Eg↓ and their influences on Earth's climate. The values of Eg↓ decreased significantly, by several percent, in much of the world from the 1950s until the 1980s or early 1990s. This was followed by a partial recovery until the early 2000s and later by mixed trends. The two most likely, but not necessarily independent, causes for these changes were the continuous increases in anthropogenic aerosol emissions until the 1990s followed by reductions in the developed countries due to legislation in the early 1990s. These changes in aerosol load coincided with widespread changes in cloud cover. Dimming has continued in some developing countries, notably India. It has been tied to widespread decreases in evaporation and possible spinning-down of the hydrological cycle, as well as cooling trends that may have offset global warming before the 1990s.