Global Warming

The Earth is kept warm by it's atmosphere, which acts rather like a woolly coat - without it, the average surface temperature would be about -18 degrees Centigrade. Heat from the sun passes through the atmosphere, warming it up, and most of it warms the surface of the planet. As the Earth warms up, it emits heat in the form of infra-red radiation - much like a hot pan emits heat even after it's taken away from the cooker. Some of this heat is trapped by the atmosphere, but the rest escapes into space. The so-called "greenhouse gases" make the atmosphere trap more of this radiation, so it gradually warms up more than it should, like a greenhouse (although a greenhouse actually does this by stopping warm air rising and escaping from it).

What Causes The Greenhouse Effect?

There are some natural greenhouse gases: water vapour, nitrous oxide, carbon dioxide, methane and ozone. However, over the past fifty years, production of carbon dioxide, nitrous oxide and methane has risen sharply, and a new type of chemical - the chlorofluorocarbon, or CFC - has been introduced as a refrigerant, solvent and aerosol propellant, but it is also a very powerful greenhouse gas, because it can trap a lot of radiation - one molecule of CFC is 12,000 to 16,000 times as effective at absorbing infra-red radiation as a molecule of carbon dixide.

This graph shows how much each gas contributes to the greenhouse effect, taking into account how much of it there is and how much radiation it can absorb.

The carbon dioxide comes mainly from burning fossil fuels in power stations, which also causes acid rain. It is also created by living animals breathing, and is naturally converted by plants back to oxygen. However, deforestation is reducing the planet's carbon dioxide absoring capability. Nitrous oxide is a by-product of nylon production, and is also released by fertiliser use in agriculture. The extra methane is produced in coal mining, natural gas production and distribution (natural gas is methane), and waste disposal. One fifth of all methane generated by human activity comes from microbial decay of organic material in flooded rice fields.

What Causes The Greenhouse Effect?

There are some natural greenhouse gases: water vapour, nitrous oxide, carbon dioxide, methane and ozone. However, over the past fifty years, production of carbon dioxide, nitrous oxide and methane has risen sharply, and a new type of chemical - the chlorofluorocarbon, or CFC - has been introduced as a refrigerant, solvent and aerosol propellant, but it is also a very powerful