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The greenhouse effect is a natural increase in the temperature of the planet as energy from sunlight is trapped by so-called greenhouse gases - such as water vapour and CO2 - that have the ability to absorb and reflect energy.
The greenhouse effect is an important natural process, but human activities are increasing its impact.
The two main gases in earth’s atmosphere are nitrogen (N2) at 78% of the total and oxygen (O2) at 21%, but neither is a greenhouse gas. The three most important greenhouse gases, water vapour (H2O), carbon dioxide (CO2) and methane (CH4) constitute a very small fraction of the atmosphere.
Water vapour (H2O) occurs in concentrations of less than 1% to 3% of the atmosphere. Carbon dioxide (CO2) and methane (CH4) occur in concentrations of 0.038% and 0.00018% but are very important greenhouse gases.
Without greenhouse gases, the earth would be about 30 degrees Celsius colder and uninhabitable.
The atmosphere and clouds protect us from solar radiation but some of this radiation is able to pass through and reach the earth's surface. The earth absorbs the sun's energy and converts it into heat, which is then re-emitted into the atmosphere as long-wave (infrared) radiation.
Some of the radiation escapes into space but most of it is absorbed and re-emitted towards the earth by gases in the atmosphere. This is called the greenhouse effect because it is rather like what happens in a greenhouse: the sun's energy can get in but not all of it can escape, so the greenhouse gets warmer.
Human activity on earth, mainly the use of fossil fuels to produce energy, has increased the concentration of greenhouse gases in the atmosphere. Since the beginning of the industrial revolution in the mid 18th century, the concentrations of carbon dioxide (CO2) and methane (CH4) have increased by 31% and 149% respectively (see Climate Change 2001: The Scientific Basis, IPCC).
These concentrations are now the highest they have been for 420,000 years (Ibid). This means that more infrared radiation is being absorbed and re-emitted by these gases and temperatures are rising.
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