greenhouse effect

The warming effect of the Earth's atmosphere is called the greenhouse effect. Radiation from the Sun enters the atmosphere but is prevented from escaping back into space by gases such as carbon dioxide (produced for example, by the burning of fossil fuels), nitrogen oxides (from car exhausts), and CFCs (from aerosols and refrigerators). As these gases build up in the atmosphere, the Earth's average temperature is expected to rise.

Phenomenon of the Earth's atmosphere by which solar radiation, trapped by the Earth and re-emitted from the surface as long-wave infrared radiation, is prevented from escaping by various gases (the ‘greenhouse gases’) in the air. These gases trap heat because they readily absorb infrared radiation. As the energy cannot escape, it warms up the Earth, causing an increase in the Earth's temperature (global warming). The main greenhouse gases are carbon dioxide, methane, and chlorofluorocarbons (CFCs) as well as water vapour. Fossil-fuel consumption and forest fires are the principal causes of carbon dioxide build-up; methane is a by-product of agriculture (rice, cattle, sheep).

The United Nations Environment Programme estimates that by 2025, average world temperatures will have risen by 1.5°C/2.7°F with a consequent rise of 20 cm/7.9 in in sea level. Low-lying areas and entire countries would be threatened by flooding and crops would be affected by the change in climate. However, predictions about global warming and its possible climatic effects are tentative and often conflict with each other.

At the 1992 Earth Summit it was agreed that by 2000 countries would stabilize carbon dioxide emissions at 1990 levels, but to halt the acceleration of global warming, emissions would probably need to be cut by 60%. Any increases in carbon dioxide emissions are expected to come from transport. The Berlin Mandate, agreed unanimously at the climate conference in Berlin in 1995, committed industrial nations to the continuing reduction of greenhouse gas emissions after 2000, when the existing pact to stabilize emissions runs out. The stabilization of carbon dioxide emissions at 1990 levels by 2000 would not be achieved by a number of developed countries, including Spain, Australia, and the USA, according to 1997 estimates. Australia is in favour of different targets for different nations, and refused to sign a communiqué at the South Pacific Forum meeting in the Cook Islands in 1997 which insisted on legally-binding reductions in greenhouse gas emissions. The United Nations Framework Convention on Climate Change (UNFCCC) adopted the Kyoto Protocol in 1997, committing the world's industrialized countries to cutting their annual emissions of harmful gases. By July 2001 the Protocol had been signed by 84 parties and ratified by 37; the USA announced its refusal to ratify the Protocol in June 2001.

Dubbed the ‘greenhouse effect’ by Swedish scientist Svante Arrhenius, it was first predicted in 1827 by French mathematician Joseph Fourier.

The concentration of carbon dioxide in the atmosphere is estimated to have risen by 25% since the Industrial Revolution, and 10% since 1950; the rate of increase is now 0.5% a year. Chlorofluorocarbon levels are rising by 5% a year, and nitrous oxide levels by 0.4% a year, resulting in a global warming effect of 0.5% since 1900, and a rise of about 0.1°C/0.18°F a year in the temperature of the world's oceans during the 1980s. Arctic ice was 6-7 m/20-23 ft thick in 1976 and had reduced to 4-5 m/13-17 ft by 1987.

A medium-sized power station, generating around 500 megawatts, produces 500 tonnes of carbon in the form of carbon dioxide, every hour. It is possible to remove the gas from flue emissions and store it under pressure, though the energy needed to do so would amount to a fifth of the plant's electricity. In addition, the waste is bulky and must be disposed of safely in solid or liquid form (carbon sequestration).

It is also thought that aircraft vapour trails could be contributing to global warming. Aircraft release exhaust gases into the cold upper air, which freeze and form cirrus clouds. These clouds trap heat, increasing the warming of the atmosphere beneath them. German research indicates that a tenth of the cirrus cloud over central Europe is produced by aircraft. While there is much concern over the amount of carbon dioxide released into the atmosphere by aircraft, cement kilns contribute more to the world's output of this gas than aircraft.