Greenhouse gases are naturally found in air. They include carbon dioxide, methane and nitrous oxide. They trap heat in the atmosphere and keep the Earth's surface much warmer than it would be if there was no atmosphere. This warming effect is called the natural greenhouse effect. In the last 200 years, the amount of greenhouse gases in the air has been increasing, due to human activities. Mankind has been increasing the amount of carbon dioxide, methane and nitrous oxide in the air, and has even been adding completely new greenhouse gases to the atmosphere, like the CFCs, which also destroy the ozone layer.
Carbon dioxide is produced naturally through when animals breathe, when dead plants and animals decay, and during natural forest fires. Mankind produces carbon dioxide when coal, oil and gas (the fossil fuels) are burnt for energy and electricity, and when forests are cut down or burnt to make way for agriculture. Trees help to remove carbon dioxide from the atmosphere by using it to make food (photosynthesis). Plant life in the oceans also uses carbon dioxide. Methane is another major greenhouse gas. It is formed naturally in marshes and bogs when dead plant and animal matter decays, and also by termites. Mankind releases methane by growing rice, farming cattle, burying waste and burning fossil fuels. Nitrous oxide is a greenhouse gas naturally produced by oceans and by lightening strikes, but humans have increased the amount in air by the production of nylon and through using agricultural fertilisers. CFCs (chlorofluorocarbons) are a group of man-made substances containing chlorine, fluorine and carbon. They were invented in the 1930s for use in fridges, but have other uses, including aerosols. They are very strong greenhouse gases and remain for a long time in the atmosphere. They break down at high altitudes where they contribute to the destruction of the ozone layer. Fortunately, their use has been banned since 1995.