The Earth's temperature is maintained at a level where it can sustain life by a balance
between heat from the sun, and cooling from reflecting some of the heat by the
Earth's warm surface and atmosphere back to space (Agarwal, 2001). Greenhouse
gases effectively absorb thermal infrared radiation, emitted by the Earth's surface, by
the atmosphere itself due to the same gases, and by clouds. Atmospheric radiation is
emitted to all sides, including downward to the Earth's surface. Thus, greenhouse
gases trap heat within the surface-troposphere system. This is called the greenhouse
effect. Thermal infrared radiation in the troposphere is strongly coupled to the
temperature of the atmosphere at the altitude at which it is emitted. In the
troposphere, the temperature generally decreases with height. Effectively, infrared
radiation emitted to space originates from an altitude with a temperature of, on
average, -19°C, in balance with the net incoming solar radiation, whereas the Earth's
surface is kept at a much higher temperature of, on average, +14°C. An increase in
the concentration of greenhouse gases leads to an increased infrared opacity of the
atmosphere, and therefore to an effective radiation into space from a higher altitude
at a lower temperature. This causes a radiative forcing that leads to an enhancement
of the greenhouse effect, the so-called enhanced greenhouse effect (Solomon et al.,
2007).
