C13.03: Greenhouse gases

• Carbon dioxide along with methane and water vapour are the main 'greenhouse gases' in the Earth's atmosphere. These gases absorb energy radiated from its surface.
• The Earth is heated by the Sun. Almost 30% of the energy reaching the Earth is reflected back into space from the atmosphere and surface.
• The greenhouse gases let short-wavelength electromagnetic radiation pass through. The surface of the Earth cools down by emitting longer wavelength infrared (thermal) radiation. However, greenhouse gases absorb infrared radiation. The radiation stimulates the bonds in these molecules to vibrate, bend and stretch more vigorously, raising their temperature. So some of the energy radiated from the surface of the Earth gets trapped in the atmosphere and the temperature rises. The higher the proportion of greenhouse gases in the air, the more energy is absorbed.
• Over the past century the amount of carbon dioxide released into the atmosphere has greatly increased. More fossil fuels than ever are used to make electricity, heat homes and run cars. This has enormously increased the amount of carbon dioxide produced. Carbon has been locked up for hundreds of millions of years in fossil fuels which is released as carbon dioxide when used as fuel.
• Methane gets into the atmosphere from swamps and rice fields. Another source of methane is emissions from the growing number of grazing cattle, and from their decomposing waste. The increasing human population produces more waste to dispose of in landfill sites, which are another source of methane.
• The levels of greenhouse gases in the atmosphere can be monitored using an instrumental technique called infrared spectroscopy. Infrared radiation stimulates the bonds in molecules of carbon dioxide and methane to vibrate more vigorously, absorbing some of the radiation, which can be detected and displayed on an infrared spectrum.
• The balance between the carbon dioxide produced and the carbon dioxide absorbed by CO2 'sinks', such as tropical rainforests and then oceans, is affected by human activity. As more trees are cut down for timber and to clear land (deforestation), the carbon dioxide removed from the air as the trees photosynthesise is reduced. Also, as the temperature rises, carbon dioxide gets less soluble in water. This makes the oceans less effective as CO2 'sinks'
• Scientists are searching for 'hard' evidence of the link between the levels of carbon dioxide and the climate. One source is ice cores drilled from Greenland's ice sheet, which have gases trapped inside.