If the Earth had no atmosphere, its average surface temperature would be very low of around -18蚓 rather than the about 14.5蚓 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蚓 but the surface temperature of the earth is only about 14.5蚓. 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.

Condition for no greenhouse gas in the atmosphere

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.

Schematic diagram illustrating the Greenhouse Effect

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.

Atmospheric concentrations of carbon dioxide, methane and nitrous oxide over the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). The corresponding radiative forcings are shown on the right hand axes of the large panel. (Source: Intergovernmental Panel on Climate Change 2007)