[Previous] | [Session 2] | [Next]
P.R. Goode, J. Hickey, J. Qiu, V. Yurchyshyn (BBSO), S.E. Koonin, T. Brown (Caltech), E. Kolbe (Universitat Basel), M. Chu (Chinese University of Hong Kong)
The earth's climate is driven by the net sunlight reaching the earth, which depends on the solar irradiance and the earth's reflectivity. Changes in the the solar irradiance have been well-studied for twenty years, but the degree of variation in the earth's albedo is not so clear. We have been measuring the earthshine from Big Bear Solar Observatory (BBSO) since December 1998. Earthshine, or "ashen light", is sunlight reaching the eye of a nighttime observer after being successively reflected from the day side of the earth, and the dark face of the moon. The ratio of reflected light from the dark part of the moon to that from the bright part provides an absolutely calibrated, large scale measure of the earth's albedo. We have solved the long-standing problem of determining the scattering of sunlight from the moon as a function of lunar phase. Earthshine measurements of the earth's albedo are complementary to those from satellites.
We find the earth's albedo varies by 20% with season, and by as much at 5% from night to night.
We put contemporaneous cloud cover data into a scene model and calculate a good agreement with our observations, but we also find some interesting differences.
Using the scene model and monthly averaged cloud cover data from ISCCP, we find between 1986 and 1990 (solar minimum to near solar maximum) that the change in the net irradiance into the climate system is several times larger from the varying albedo than from the varying solar irradiance. The two changes are in phase.