Climatologists reporting for the UN Intergovernmental Panel on Climate Change (IPCC) say we are seeing global warming caused by human activities. People are causing the change by burning nature's vast stores of coal, oil and natural gas. This releases billions of tonnes of carbon dioxide (CO2) every year, although the changes may actually have started with the dawn of agriculture, say some scientists. The physics of the 'greenhouse effect' has been a matter of scientific fact for a century. CO2 is a greenhouse gas that traps the Sun's radiation within the troposphere, the lower atmosphere. It has accumulated along with other manmade greenhouse gases, such as methane and chlorofluorocarbons (CFCs). Some studies suggest that cosmic rays may also be involved in warming.

If current trends continue, we will raise atmospheric CO2 concentrations to double pre-industrial levels during this century. That will probably be enough to raise global temperatures by around 2℃ to 5℃. Some warming is certain, but the degree will be determined by cycles involving melting ice, the oceans, water vapour, clouds and changes to vegetation. Warming is bringing other unpredictable changes. Melting glaciers and precipitation are causing some rivers to overflow, while evaporation is emptying others. Diseases are spreading. Some crops grow faster while others see yields slashed by disease and drought. Clashes over dwindling water resources may cause conflicts in many regions.

As natural ecosystems--such as coral reefs--are disrupted, biodiversity is reduced. Most species cannot migrate fast enough to keep up, though others are already evolving in response to warming. Thermal expansion of the oceans, combined with melting ice on land, is also raising sea levels. In this century, human activity could trigger an irreversible melting of the Greenland ice sheet. This would condemn the world to a rise in sea level of six metres--enough to flood land occupied by billions of people.

The global warming would be more pronounced if it were not for sulphur particles and other pollutants that shade us, and because forests and oceans absorb around half of the CO2 we produce. But the accumulation rate of atmospheric CO2 has doubled since 2001, suggesting that nature's ability to absorb the gas could now be stretched to the limit. Recent research suggests that natural CO2 'sinks', like peat bogs and forests, are actually starting to release CO2.

At the Earth Summit in 1992, the world agreed to prevent 'dangerous' climate change. The first step was the 1997 Kyoto Protocol, which came into force during 2005. It will bring modest emission reductions from industrialised countries. Many observers say deeper cuts are needed and developing nations, which have large and growing populations, will one day have to join in. Some, including the US Bush administration, say the scientific uncertainty over the pace of climate change is grounds for delaying action. The US and Australia have ren

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