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What is the greenhouse effect?

The Earth’s atmosphere acts in many ways to sustain life. One of these is the warming of the atmosphere to a level suitable for life by a process commonly called the ‘greenhouse effect’. Energy from the sun is absorbed by the Earth, but certain gases (greenhouse gases) in the atmosphere prevent the heat from being fully re-radiated into space, thus warming the lower atmosphere. These are gases such as water vapour, carbon dioxide, methane, ozone and halocarbons. The levels of the latter four have been affected by human activities over the past 300 years. Concentrations of carbon dioxide (from burning of coal, oil and gas, and changes in land-use such as deforestation), methane (from agriculture and natural gas leakage), and ozone in the lower atmosphere (from the products of vehicle exhausts) have increased greatly over this period.

At present, for example, about 6.5 billion tonnes of carbon is [sic] emitted globally into the atmosphere each year, mostly through fossil fuel combustion. Changes in land use result in a further net global annual emission of 1-2 billion tonnes of carbon. The picture, however, is complicated by other gases originating from human activity, such as sulphur dioxide that act to cool climate. In addition, the Earth's climate varies naturally as a result of interactions between the ocean and the atmosphere, changes in the Earth’s orbit, fluctuations in energy from the sun and volcanic eruptions.

From "Climate change: the UK's role in a global challenge" (26 pages), november 2004, British Council Briefing Sheet no. 20.