Introduction

The flux density and wavelength of electro-magnetic radiation emitted from a body depend on its temperature. On the earth's surface the wavebands that contain the most energy, and are therefore of prime interest in the context of climate influences, are those emitted by the sun and the earth. The calculation of spectral distributions from Planck's law using their approximate temperatures of 5800 and 300 K, for sun and earth, shows that 97% of the energy of solar and >99% of that of terrestrial radiation fall within the wavebands of 0.29 3 and 3 100 mm, respectively. Those wavebands are referred to as short wave (or solar) and long wave (or terrestrial) radiation [1]. The problem with the current ubiquitous, steady increase in atmospheric carbon dioxide concentration stems not from its direct influence on climate, but rather from its absorption of radiation in the long wave band, which decreases long wave radiative losses from the earth. Since its absorption in the solar spectrum is small, CO2 has a negligible influence on the earth's solar radiation balance.

Global radiation (Eg#) is the total solar radiation falling on a horizontal surface at the earth's surface, that is, at the bottom of the atmosphere (BOA). Precise wide-spread 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) and wide spread decreasing pan evaporation 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 paper provides some background material on solar radiation and reviews some of the work done on the changing Eg# and its influences on earth's climate.

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