The earth's climate is driven by a continuous flow of energy from the sun. This energy arrives mainly in the form of visible light. About 30% is immediately scattered back into space, but most of the remaining 70% that is absorbed passes down through the atmosphere to warm the earth's surface.

The earth must send this energy back out into space in the form of infrared radiation. Being much cooler than the sun, the earth does not emit energy as visible light. Instead, it emits infrared, or thermal radiation. This is the heat thrown off by an electric fire or grill before the bars begin to glow red.

"Greenhouse gases" in the atmosphere block infrared radiation from escaping directly from the surface to space. Infrared radiation cannot pass straight through the air like visible light. Instead, most departing energy is carried away from the surface by air currentsand clouds, eventually escaping to space from altitudes above the thickest layers of the greenhouse gas blanket.

The main greenhouse gases are water vapour, carbon dioxide, ozone, methane, nitrous oxide, and halocarbons and other industrial gases. Apart from the industrial gases, all of these gases occur naturally. Together, they make up less than 1% of the atmosphere. This is enough to produce a "natural greenhouse effect" that keeps the planet some 30^oC warmer than it would otherwise be - essential for life as we know it.

Levels of all key greenhouse gases (with the possible exception of water vapour) are rising as a direct result of human activity. Emissions of carbon dioxide (mainly from burning coal, oil, and natural gas), methane and nitrous oxide (due mainly to agriculture and changes in land use), ozone (generated by automobile exhaust fumes and other sources) and long-lived industrial gases such as CFCs, HFCs, and PFCs are changing how the atmosphere absorbs energy. Water vapour levels may also be rising because of a "positive feedback". This is all happening at an unprecedented speed. The result is known as the "enhanced greenhouse effect".

The climate system must adjust to rising greenhouse gas levels to keep the global "energy budget" in balance. In the long term, the earth must get rid of energy at the same rate at which it receives energy from the sun. Since a thicker blanket of greenhouse gases helps to reduce energy loss to space, the climate must change somehow to restore the balance between incoming and outgoing energy.

This adjustment will include a "global warming" of the earth's surface and lower atmosphere. But this is only part of the story. Warming up is the simplest way for the climate to get rid of the extra energy. But even a small rise in temperature will be accompanied by many other changes: in cloud cover and wind patterns, for example. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).

Meanwhile, industrially-generated "sulphate aerosols" may man-made aerosols have an overall local cooling effect. Sulphur emissions from coal­ and oil­fired power stations and the burning of organic material produce clouds ofmicroscopic particles that can reflect sunlight back out into space and also affect clouds. This The resultant cooling partly compensates counteracts for greenhouse warming. These sulphate aerosols, however, remain in the atmosphere for a relatively short time compared to the long-lived greenhouse gases, so their cooling effect is localized. They also cause acid rain and poor air quality, problems that need to be addressed. This means we should not rely indefinitely on the cooling effect of sulphate aerosols to keep the climate cool indefinitely.

Climate models estimate that the global average temperature will rise by about 1.4 – 5.8^oC (2.5 – 10.4°F) by the year 2100. This projection uses 1990 as a baseline and assumes that no policies are adopted for minimizing climate change. It also takes into account climate feedbacks and the effects of sulphate aerosols as they are presently understood.

Past emissions have already committed us to some climate change. The climate does not respond immediately to emissions. It will therefore continue to change for many hundreds of years even if greenhouse gas emissions are reduced and atmospheric levels stop rising. Some important impacts of climate change, such as a predicted rise in sea level, will take even longer to be fully realized.

There is new and stronger evidence that climate change has already begun. The climate varies naturally, making it difficult to identify the effects of rising greenhouse gases. However, an increasing body of observation now presents a collective picture of a warming world. For example, the pattern of temperature trends over the past few decades resembles the pattern of greenhouse warming predicted by models; these trends are unlikely to be due entirely to known sources of natural variability. Many uncertainties remain, however, such as how changes in cloud cover will influence future climate.