Greenhouse Effect

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).