The earth is warmed by radiant energy from the Sun. Over time, the amount of energy transmitted to the earth’s surface is equal to the amount of energy re-radiated back into space in the form of infrared radiation, and the temperature of the Earth’s surface stays roughly constant. However, the temperature of the earth is strongly influenced by the existence, density, and composition of its atmosphere. Many gases in the earth’s atmosphere absorb infrared radiation re-radiated from the surface, trapping heat in the lower atmosphere.
Without the natural greenhouse effect, it is likely that the average temperature of the earth’s surface would be of the order of -19 degrees Celsius, rather than the +14 degrees Celsius actually observed. The gases that help trap the sun’s heat close to the earth’s surface are referred to as ‘greenhouse gases.’ All greenhouse gases absorb infrared radiation (heat) at particular wavelengths.
The most important greenhouse gases are water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several engineered gases, such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF6). Water vapour is by far the most common, with an atmospheric concentration of nearly 1%, compared with less than 0.04% for CO2. The effect of human activity on global water vapour concentrations is considered negligible. Anthropogenic (human-made) emissions of water vapour are not factored into national greenhouse gas emission inventories for the purposes of meeting the requirements of the United Nations Framework Convention on Climate Change (UNFCCC) or the Kyoto Protocol. Concentrations of other greenhouse gases, such as methane and nitrous oxide, are a fraction of that for CO2.
Scientists recognised in the early 1960s that concentrations of CO2 in the earth’s atmosphere were increasing every year. Subsequently, they discovered that atmospheric concentrations of methane, nitrous oxide, and many engineered greenhouse gas chemicals were also rising. Because current concentrations of greenhouse gases keep the earth at its present temperature, scientists began to postulate that increasing concentrations of greenhouse gases would make the earth warmer.
In computer-based simulation models, rising concentrations of greenhouse gases nearly always produce an increase in the average temperature of the earth. Rising temperatures may, in turn, produce changes in weather and in the level of the oceans that might prove disruptive to current patterns of land use and human settlement, as well as to existing ecosystems. To date, however, it has proven difficult to disentangle the human impact on climate from normal temporal and spatial variations in temperature on both a global scale and geologic timeframe.