Decadal changes in surface radiative fluxes and their role in global climate change

Abstract

The major anthropogenic impact on climate over the 20th century occurred through a modification of the earth radiation balance by changing the amount of greenhouse gases and aerosol in the atmosphere. Radiative energy reaching the ground is particularly important for mankind as it is a key determinant of the climate of our environments and strongly influences the thermal and hydrological conditions at the Earth surface. Recent evidence suggests that significant anthropogenic-induced variations occurred in both surface solar and thermal radiation over the past decades, related to anthropogenic air pollution and greenhouse gas emissions, respectively. Observed solar radiation incident at the surface showed a continuous decrease (“global dimming” or “surface solar dimming”) since the beginning of worldwide measurements in the mid-20th century up to the 1980s, when a widespread trend reversal towards an increase (“global brightening” or “surface solar brightening”) occurred. This trend reversal was favoured by an increasing transparency of the cloud-free atmosphere, due to air pollution regulations and the breakdown of the economy in former communist countries. In the thermal spectrum of radiation, which is directly modified by changes in atmospheric greenhouse gas concentrations, a gradual increase in surface downwelling thermal radiation over recent years can be seen, in line with our expectations from an increasing greenhouse effect. This increasing greenhouse effect has become only fully apparent after the decline of solar dimming, which effectively masked greenhouse warming prior to the 1980s. The present article discusses the variations in surface radiation and their impact on various aspects of the climate system over the past decades.