The Atmosphere and Climate Modeling - Introduction to Carbon Capture and Sequestration - page 57

surface of the earth, and the corresponding blackbody radiation spec-
trum will effectively shift to correspond to a higher temperature. The net
result is that more energy will be emitted at shorter wavelengths and less
at the longer wavelengths. These shifts would theoretically continue until
the total emitted energy equals the energy received from the sun.
Water as a greenhouse gas
Water vapor is the most abundant and important greenhouse gas in the
atmosphere. Why are we not more worried about the water content of the
atmosphere? As most of the surface of the earth is ocean, human activity
has very little, if any, direct infl uence on the amount of water vapor in the
atmosphere. That said, there is, however, an important amplifi cation effect.
For example, if the temperature of the surface of the earth decreases, the
vapor pressure of water in the atmosphere decreases, its absorption of the
earth's blackbody radiation decreases, the greenhouse effect decreases,
and the earth gets cooler. Similarly, if the surface temperature of the earth
increases, the increased water vapor concentration will amplify the green-
house effect. In Chapter 3 we will discuss in detail how the earth naturally
regulates the surface temperature by controlling CO 2 levels, but in pro-
cesses with time scales on the order of thousands of years.
Other sources contributing to greenhouse effects
In the previous section, we focused on greenhouse gasses as the main
cause of the increasing temperature of the surface of the earth. We have
already seen that CO 2 is not the only greenhouse gas that plays a role;
other gasses also contribute.
In addition, there are other factors that are not directly related to
greenhouse gasses that also have an infl uence on the energy balance of
the planet. For example, decreasing the amount of snow on the ground
decreases the amount of energy refl ected from the earth. Energy not
refl ected is absorbed, and thus decreasing ground snow has a net effect
similar to a greenhouse gas. The concept of “ radiative forcing ” is intro-
duced to quantitate these different effects. For example, how might one
compare the effects of different forms of land use on CO 2 levels? Radiative
forcing is a measure of how the energy balance of the earth-atmosphere
system is infl uenced when factors that affect climate are altered. It is given