Greenhouse Effect

     The Greenhouse effect is raised temperature at the planet’s surface as result of heat energy being trapped by gases in the atmosphere. Certain gases cause the atmosphere to act like the glass in a greenhouse. As a result, the temperature of a planet’s surface may be higher than it otherwise would be–on Earth about 33º C (59ºF) higher. The main gases that produce the greenhouse effect on Earth are water vapour and carbon dioxide. Scientists suspect that an increased discharge of carbon dioxide from human activity (notably motor transport and industry) is contributing to global warming.

     Greenhouse gases are fairly transparent to the short-wavelength, visible and ultraviolet light that brings most of the Sun’s energy, though about 25% is reflected by the atmosphere, and 25% is absorbed by it. About 5% is reflected from the Earth, which absorbs the rest. Some of this absorbed energy rises again in thermals or in the heat of evaporated moisture. The rest is reradiated as long–wavelength infrared rays. The infrared radiation emitted by the Earth is partially transmitted straight back into space. A much greater amount is absorbed by the greenhouse gases, which are very efficient absorbers of the long infrared wavelengths. Some of the absorbed heat is reradiated into space, but a lot is radiated downwards to fuel global warming. Half the world’s population lives on low–lying coastal areas, such as the Bay of Bengal which is particularly vulnerable to flooding. Global warming could cause the sea level to rise. A 2m (6ft) rise in sea level would inundate close to 20% of Bangladesh and require tens of millions to be evacuated. A larger rise of 5m (16ft) would drown close to half the country.

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