Interannual variations in Earth's reflectance 1999–2007

Abstract

The overall reflectance of sunlight from Earth is a fundamental parameter for climate studies. Recently, measurements of earthshine were used to find large decadal variability in Earth's reflectance of sunlight. However, the results did not seem consistent with contemporaneous independent albedo measurements from the low Earth orbit satellite, Clouds and the Earth's Radiant Energy System (CERES), which showed a weak, opposing trend. Now more data for both are available, all sets have been either reanalyzed (earthshine) or recalibrated (CERES), and they present consistent results. Albedo data are also available from the recently released International Satellite Cloud Climatology Project flux data (FD) product. Earthshine and FD analyses show contemporaneous and climatologically significant increases in the Earth's reflectance from the outset of our earthshine measurements beginning in late 1998 roughly until mid‐2000. After that and to date, all three show a roughly constant terrestrial albedo, except for the FD data in the most recent years. Using satellite cloud data and Earth reflectance models, we also show that the decadal‐scale changes in Earth's reflectance measured by earthshine are reliable and are caused by changes in the properties of clouds rather than any spurious signal, such as changes in the Sun‐Earth‐Moon geometry.