P. R. Goode1, J. Hickey1, J. Qiu1,
V. B. Yurchyshyn1, S. E. Koonin2,
T. Brown2, E. Kolbe3, M.-C. Chu4
(1) Big Bear Solar Observatory/NewJersey Institute of Technology
(2) California Institute of Technology (3) Universität Basel,
Switzerland (4) Chinese University, 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.
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