Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 17

presence of clouds tends to reduce differences in net albedo above land and oceans.
Albedo is very important in determining the Earth's heat balance. The global
average albedo depends on the state of the Earth. During an ice age, it is relatively
higher than during an interglacial, due to the reflectivity of ice sheets, and the
spreading of sea ice, the extending of mountain glaciers, as well as the fact that
the smaller oceans are replaced to some extent by land, and, there are changes in
vegetation as well. Averaged over the whole Earth, the present albedo is currently a
little over 0.3, so the net solar input to Earth, independent of the greenhouse effect, is
about 0.7 342 239W/m 2 . This net irradiance warms the Earth and, if there were
no greenhouse effect, the temperature of the Earth would increase to the point where
it would radiate enough energy to just balance this solar input. The average tem-
perature of the Earth would be something of the order of -18 C. All the water on the
Earth would freeze, and the resultant high albedo of the ice-covered Earth would
further reduce temperatures until the entire Earth became a veritable snowball. In
fact, it seems likely that the Earth may have passed through such a snowball state in
its distant past.
The presence of greenhouse gases in the atmosphere acts as a barrier to escape
of radiant energy emitted by the Earth. Greenhouse gases absorb some of the
radiant energy emitted by the Earth, and then reradiate this energy. The transfer
of this radiant energy through the atmosphere is highly complex but, ultimately,
some of the reradiated energy finds its way into space while the remainder heads
downward to Earth. Thus, greenhouse gases act as filters for some parts of the
infrared spectrum emitted by Earth, reducing the net flux of radiant energy
emitted by the Earth to space. The Earth must then warm until it radiates enough
energy to achieve equilibrium.
Water vapor is the most important greenhouse gas. After water vapor, carbon
dioxide and methane are the next most important greenhouse gases. A sharp
decrease in the concentrations of these gases in the atmosphere could trigger a
cooling trend that would be amplified by lowered water vapor pressure and
increased albedo as snow and ice spread. Alternatively, an increase in the concen-
tration of greenhouse gases will tend to increase the heat retained by the Earth,
leading to global warming. The temperature increase due to an increase in a green-
house gas concentration depends upon (a) the absorption characteristics of the
greenhouse gas (absorption bandwidth and degree of saturation of absorption
bands) and (b) concentration of the greenhouse gas. As the concentration of any
greenhouse gas increases, the additional warming produced gradually diminishes
as the absorption bands become saturated. At present concentrations, the main
absorption band of CO 2 is quite saturated, and further increases in concentration
produce diminishing increases in global temperature due to increased absorption
at the edges (''wings'') of the absorption band. However, water vapor and
methane are not as saturated, and increases in these concentrations produce sig-
nificant heating. When greenhouse gases produce a warming, secondary effects can
act as feedbacks to either further increase the warming or oppose it. Climate
models have been used to estimate the warming produced by increasing CO 2 con-
centrations. As the Earth warms, climate models assume that the average humidity