If the Earth had no atmosphere, its average surface temperature would be very low of around -18℃ rather than the about 14.5℃ found today. The difference in temperature is due to a suite of gases called greenhouse gases which affect the overall energy balance of the Earth's system by absorbing infra-red radiation.

The heat content at the surface of the earth is mainly derived from the sun. When solar radiation (visible light) reached the earth, it heats up the earth. The surface temperature of the sun is about 6000℃ but the surface temperature of the earth is only about 15℃. In spite of the large temperature difference, the earth also emits radiation. As the surface temperature of the earth is lower, the radiation emitted has a longer wave length (infra-red) which is not visible to the eye. Nevertheless, it is still capable of scattering heat back to the outer space which in term cools the earth.

If the heating and cooling of the earth is comparable, the long-term mean temperature of the earth will remain more or less unchanged.

The greenhouse gases in the air, such as carbon dioxide, will hinder the emission of heat from the earth to outer space. The greenhouse gases absorb part of the infra-red radiation emitted from the earth and then re-emitted in all directions also in the form of infra-red radiation. Consequently, part of the heat will go into the outer space but part of it will return to the earth. This is what is known as the greenhouse effect.

As such, if the greenhouse gases in the atmosphere increases, the earth surface will receive more radiation energy than before, the air temperature of the earth will rise.

According to the IPCC Fourth Assessment Report issued on 2 February 2007 (Climate Change 2007: The Physical Science Basis. Summary for Policymakers), most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentration. Human affect greenhouse gas levels by introducing new sources or by interfering with natural sinks.

The major greenhouse gases in the atmosphere are carbon dioxide (CO2), methane, (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and ozone (O3). Atmospheric water vapour (H2O) also makes a large contribution to the natural greenhouse effect but it is thought that its presence is not directly affected by human activity.

Global atmospheric concentrations of CO2, CH4 and N2O have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in CO2 concentration are due primarily to fossil fuel use and land-use change, while those of CH4 and N2O are primarily due to agriculture.

Increase of greenhouse gases concentration causes a reduction in outgoing infrared radiation, thus the Earth's climate must change somehow to restore the balance between incoming and outgoing radiation. This lead to global warming of the Earth's surface and the lower atmosphere. A small rise in temperature will induce many other changes, for example, cloud cover and wind patterns. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).

The global mean temperature rose by 0.74 °C during the hundred year period between 1906 and 2005. The rising rate was 0.13°C per decade in the 50 years from 1956 to 2005, nearly twice the rate in the past 100 years. Eleven (1998, 2005, 2003, 2002, 2004, 2006, 2007, 2001, 1997, 2008, 1999) of the last twelve years (from 1997 to 2008) rank among the 12 warmest years on record.

Using complex climate models, the Intergovernmental Panel on Climate Change in their Third Assessment Report forecast that global average surface temperature will rise by 1.4℃ to 5.8℃ by the end of 2100. The latest Fourth Assessment Report has slightly revised the forecast to the likely range of 1.1-6.4℃ with best estimate of 1.8-4.0℃.

Sea level changes showed geographical variations because of uneven changes in ocean temperature, salinity, ocean currents and winds. Regional sea level is affected by climate variability on shorter time scales, leading to regional decadal variations which can be greater than the global change.

According to the IPCC Fourth Assessment Report issued on 6 April 2007 (Climate Change 2007: Climate Change Impacts, Adaptation and Vulnerability. Summary for Policymakers), key impacts as a function of increasing global average temperature change are as follows:

Key impacts as a function of increasing global average temperature change. The black lines link impacts, dotted arrows indicate impacts continuing with increasing temperature. Entries are placed so that the left hand side of text indicates approximate onset of a given impact. (Source : Intergovernmental Panel on Climate Change 2007)