A3.7.1 The Greenhouse Effect
The Sun is the Earth’s only external form of heat. It emits solar radiation mainly in the form of shortwave visible and ultraviolet (UV) energy. As this radiation travels toward the Earth, 25% of it is absorbed by the atmosphere and 25% is reflected by the clouds back into space. The remaining radiation travels unimpeded to the Earth and heats its surface.
Greenhouse gases like water vapour, carbon dioxide, methane and nitrous oxide trap the infrared radiation released by the Earth’s surface. The atmosphere acts like the glass in a greenhouse, allowing much of the shortwave solar radiation to travel through unimpeded, but trapping a lot of the longwave heat energy trying to escape back to space.
This process makes the temperature rise in the atmosphere just as it does in the greenhouse. This is demonstrated in the diagram below:
Source: IPCC (Intergovernmental Panel on Climate Change); 2007, Cambridge University Press “Climate Change: The Physical Sciences Bases” available via http:www.ipcc.ch
During the last 200 years, man has been releasing substantial quantities of extra greenhouse gases to the atmosphere, through the burning of fossil fuels and deforestation. These extra gases are trapping more heat in the atmosphere, and it is now suspected that the observed warming of the Earth by about 0.6°C since the late 19th century is due to this man-made enhancement of the natural greenhouse effect.
The BBC provides an animated guide to the Greenhouse Effect, which is available via this following web link:
http://news.bbc.co.uk/1/shared/spl/hi/sci_nat/04/climate_change/html/greenhouse.stm