Geoscience Reference
In-Depth Information
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Figure 4.2 Daily (gray) and annually averaged values of the solar constant composited
from radiometers on various satellites. See Fröhlich (2006) for the composite
methodology. Courtesy of the SOHO consortium, a project of the ESA and NASA.
ago, with a luminosity of about 70% of today's value. According to Eq. 4.10,
the earth's radiative equilibrium temperature varies with the one-fourth power
of solar luminosity, so the surface temperature of the early earth would have
been about
0.70
4
of today's value, or about 23 K cooler, assuming no
other changes in the system. Climate theory predicts global-scale glaciation
for such a decrease in temperature, if all other factors are the same, but there
is ample evidence of widespread liquid water (e.g., sedimentary rocks) on the
early earth and no evidence of extensive glaciation. This apparent contradic-
tion is known as the
faint young sun paradox
.
Several theories have been advanced to resolve the faint young sun para-
dox. One is that there were high levels of greenhouse gases in the atmosphere
(section 4.5). Another is that less extensive continental surfaces on the earth
2 billion years ago were associated with a lower surface albedo and reduced
cloud cover, leading to a significantly lower planetary albedo. Another idea
stems from the possibility that Venus was not in orbit about the sun 2 billion
years ago, leading to a shorter distance between the earth and the sun and an
increase in the solar constant.
91%
4.4 SOLAR AND TERRESTRIAL SPECTRA
The Planck function (Eq. 4.3) with
T
5500 K, an emission temperature
representative of the sun's photosphere, is drawn in
Figure 4.3a.
This Planck
curve, also known as a
blackbody spectrum
, shows how the emitted energy
is distributed across wavelengths for a perfect blackbody with temperature
