Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 16

2

Variability of the Earth's climate

2.1 FACTORS THAT INFLUENCE GLOBAL CLIMATE

The geothermal gradient delivers about 61.5 mW/m 2 from the interior of the
Earth to the surface. Since the surface area of the Earth ¼ 5.1 10 8 km 2 , the rate
at which energy is released from the interior of the Earth is 3.1 10 10 kW. The
solar power that impinges on the Earth is 342W/m 2 . If all of this were absorbed,
the Earth would receive 1.7 10 14 kW. Actually, only about 70% of the incident
solar energy is absorbed by the Earth; nevertheless, the solar input far exceeds that
which derives from the interior. Therefore geothermal energy may be ignored in
dealing with the heat balance of the Earth.
The Earth is suspended in a vacuum. Hence the only way that it can lose
energy is via radiation. The Earth's surface climate derives from a tenuous
balance between the rate of solar energy input and the rate at which the Earth
loses energy by radiation to space. Since the Earth has a significant heat capacity,
it does not respond immediately to changes in solar input or radiant output.
The overall heat balance of the surface of the Earth is dictated by a number
of factors. Three important elements are:

. Rate at which solar energy impinges on the Earth.
.

Fraction of solar energy reflected by the Earth into space (albedo).

.

Effect of greenhouse gases (particularly, water vapor, CO 2 , and CH 4 ) in the
atmosphere in preventing escape of radiation emitted by the Earth.

The rate at which solar energy impinges on the Earth is 342W/m 2 averaged
over the whole Earth. A fraction of this (called the albedo) is reflected back into
space. Ice and snow have high albedos (0.5 to 0.9) while land ( 0.3) and oceans
( 0.1) have lower albedos. Clouds have an important effect on albedo and the