Reflecting on Earth's Climate

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Science  28 May 2004:
Vol. 304, Issue 5675, pp. 1207
DOI: 10.1126/science.304.5675.1207l

Earth's albedo—the fraction of sunlight that is reflected back to space as shortwave radiation—is one of the parameters that control the temperature of the atmosphere. One way to estimate Earth's albedo is to analyze global cloud coverage and surface characteristics, such as vegetation cover and mineralogy, and attempt to calculate how much sunlight is reflected. However, this process is complex, and different attempts have produced different results. Another way is to measure “earthshine,” the amount of light reflected by Earth and re-reflected by the dark side of the moon. Pallé et al. (p. 1299) performed both analyses to cross-validate the techniques and to estimate Earth's albedo for periods when satellite analyses of cloud coverage are not available. They find that there was a steady decrease in Earth's reflectance from 1984 to 2000, followed by a complete reversal of the decline after 2000. The radiative forcing implied by these decadal changes may have had an impact on climate.

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