Changes in the Earths's Albedo Over the Past Two Decades
Enric Palle [epb@bbso.njit.edu], Philip R. Goode, and Pilar Montanes-Rodriguez, Big Bear Solar Observatory (New Jersey Institute of Technology); Steven E. Koonin and P. R. Goode, W. K. Kellogs Radiation Laboratory, California Institute of Technology, Pasadena, CA

The net sunslight reaching the Earth is the main driver of the climate system. This net radiation depends both on the solar constant and the earth's albedo. Several mechanisms have been proposed in the literature to affect the earth's clouds/albedo perhaps providing an indirect amplification mechanism of the solar signal. Since 1998, earthshine measurements of the Earth's reflectance have been routinely carried out at Big Bear Solar Observatory. We correlate the overlapping period (1999 through mid-2001) of these observations with satellite observations of global cloud properties to construct from the latter a proxy measure of the Earth's global shortwave reflectance. This proxy shows a steady decrease in the earth's reflectance from 1984 to 2000, with a strong drop during the 1990's. During 2001-2003 only earthshine data are available, and they indicate a complete reversal of the decline. The radiative forcing implied by either of these decadal changes in reflectance is climatologically significant. Understanding how these changes are apportioned between natural variability, direct forcing, and feedbacks, is fundamental to confidently assessing and predicting climate change.