The source of energy which drives the Earth's climate is the radiation of the sun. To balance the incoming energy, the Earth itself must radiate on average the same amount of energy back to space. It does this by emitting thermal "long-wave" radiation in the infrared part of the electromagnetic spectrum.

Some atmospheric gases, called greenhouse gases, trap a portion of the outgoing energy, retaining heat somewhat like the glass panels of a greenhouse. Without this natural "greenhouse effect", temperatures would be much lower than they are now (-15°C), and life as known today would not be possible. Instead, thanks to greenhouse gases, the earth's average temperature is a more hospitable 15°C. However, problems may arise when the atmospheric concentration of greenhouse gases increases by human activities

Greenhouse gases are atmospheric gases that have the ability to redirect the outgoing energy. First they absorb the outgoing light, and then reemit it randomly in any direction, so that much of it cannot escape the atmosphere. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occuring greenhouse gases include:

• water vapor
• carbon dioxide
• methane
• nitrous oxide, and
• ozone.

Certain human activities add to the levels of most of these naturally occurring gases:

• Carbon dioxide is released to the atmosphere when solid waste, fossil fuels (oil, natural gas, and coal), and wood and wood products are burned.
• Methane emissions result from the rise production, the decomposition of organic wastes in municipal solid waste landfills, and the raising of livestock. Methane also is emitted during the production and transport of coal, natural gas, and oil.
• Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels.

Greenhouse gases that are not naturally occurring include by products of foam production, refrigeration, and air conditioning called chlorofluorocarbons (CFCs),as well as hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) generated by industrial processes.

The atmospheric concentrations of many greenhouse gases have grown significantly since pre-industrial times (about 1750 A.D.):

• carbon dioxide (CO2) from about 280 to almost 360 ppmv3 (view graphic),
• methane (CH4) from 700 to 1720 ppbv and
• nitrous oxide (N2O) from about 275 to about 310 ppbv.

These trends can be attributed largely to human activities, mostly

• fossil-fuel use: From 1850 - 1990 212 (±21) Gt of carbon were emitted by burning fossil fuels (oil, natural gas, and coal). In 1990, the three largest sectors of energy consumption were industry (45% of total CO2 releases), residential/commercial sector (29%) and transport (21%).
• land-use change (e.g. deforestation): During the period 1850 - 1990, net cumulative global CO2 emissions from land use change are estimated to have been 124 (±50) Gt Carbon. Of these about 108 Gt C were from forest areas and about 16 Gt C from cultivation of mid-latitude grasslands. Note: agriculture ecosystems have a smaller capacity to accumulate carbon than forests (20 to 100 time less).
• agriculture: Agriculture's contribution to the enhanced greenhouse effect is 9 - 15% (e.g. methane emissions by cows).

Concentrations of other anthropogenic greenhouse gases have also increased. An increase of greenhouse gas concentrations leads on average to an additional warming of the atmosphere and the Earth's surface. Many greenhouse gases remain in the atmosphere - and affect climate - for a long time.

Human activities (e.g. combustion of fossil fuels, biomass burning) also affect the amount of aerosol (microscopic airborne particles) in the atmosphere which influences climate in other ways. The main direct effect of aerosols is the scattering of some solar radiation back to space, which tends to cool the Earth's surface.

Because human-made aerosols typically remain in the atmosphere for only a few days they tend to be concentrated near their sources such as industrial regions. The radiative forcing therefore possesses a very strong regional pattern, and the presence of aerosols adds further complexity to possible climate change as it can help mask, at least temporarily, any global warming arising from increased greenhouse gases. However, the aerosol effects do not cancel the global-scale effects of the much longer-lived greenhouse gases, and significant climate changes can still result.