Glossary

Refers to the effect of greenhouse gases in the atmosphere on temperatures at a planet’s surface, e.g. the surface of the Earth, where temperatures rise. The effect is produced by the fact that the atmosphere is transparent for incoming shortwave solar radiation but opaque for the longwave infrared radiation emitted by the Earth’s warmer surface and the warmer air.

Greenhouse gases absorb thermal infrared radiation emitted by the Earth’s surface, by the atmosphere itself through the same gases, and by clouds. Atmospheric radiation is emitted in all directions, including toward the Earth’s surface. In this way, greenhouse gases trap heat in the surface tropospheric system. This is referred to as the “natural greenhouse effect”. The thermal infrared radiation in the troposphere largely depends on the temperature of the atmosphere at the altitude in which it is emitted. In the troposphere, temperatures generally decline with increasing altitude. In fact, the infrared radiation emitted into space stems from an altitude with a mean temperature of -19°C, in equilibrium with the net incoming solar irradiance, whereas the Earth’s surface is kept at the substantially higher mean temperature of 14°C. Rising greenhouse-gas concentrations make the atmosphere increasingly impenetrable for infrared and yield emissions into space at higher altitudes and therefore lower temperatures. This produces radiative forcing that further intensifies the greenhouse effect (“amplified greenhouse effect”).