1.1 Climate Change

Physical principles of climate change

All life on earth depends on radiation from the sun. When solar radiation reaches the earth's surface, one fraction of the incoming radiation is directly reflected. However, the greater part of the radiation is absorbed by the earth's surface and later re-emitted in the form of long-wave thermal radiation. If the earth had no atmosphere, this long-wave heat radiation would be completely emitted into space and the earth’s surface temperature would not exceed -18 °C.

Thankfully, the earth does have an atmosphere: some of its contained gases, the so-called greenhouse gases (GHG), absorb parts of the outgoing long-wave radiation emitted from the earth’s surface. When the radiation absorbed by greenhouse gases such as water vapour, carbon dioxide and methane is set free again, it is either emitted into space or else returned to the earth’s surface. As a result of the back radiation from greenhouse gases, the earth's surface is additionally heated. Thus, the temperature at the earth is raised by 33 °C to an average temperature of 15 °C.

Greenhouse effect (Sunrays impact on atmosphere and earth surface) — Greenhouse gas effect on earth von US EPA (CC BY-ND)

This natural greenhouse effect ensures the possibility of life on earth. However, since the industrial revolution, humans influenced the composition of the atmosphere by emitting more greenhouse gases than are extracted from the atmosphere by natural processes, for example, photosynthesis. The accumulation of greenhouse gases in the atmosphere leads to a greater proportion of long-wave thermal radiation which is re-emitted to the earth's surface. Thus, less heat leaves the atmosphere resulting in a temperature increase on the earth. The by human influence increased natural greenhouse effect is referred to as the anthropogenic greenhouse effect.