World has not been as warm as it is now for a millennium

Climate change is with us. A decade ago, it was conjecture. Now the future is unfolding before our eyes. It is seen in disappearing Arctic ice and permafrost. The shantytown dwellers of Latin America and South Asia see it in lethal storms and floods. Europeans see it in disappearing glaciers, forest fires and fatal heat waves. Scientists see it in tree rings, ancient coral and bubbles trapped in ice cores. These reveal that the world has not been as warm as it is now for a millennium or more.
The three warmest years on record have all occurred since 1998; 19 of the warmest 20 since 1980. And Earth has probably never warmed as fast as in the past 30 years - a period when natural influences on global temperatures, such as solar cycles and volcanoes should have cooled us down.
Climatologists reporting for the UN Intergovernmental Panel on Climate Change (IPCC) say they 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 man-made 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 degrees Celsius to 5 degree Celsius.
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.
In any case, according to the IPCC, the world needs to quickly improve the efficiency of its energy usage and develop renewable non-carbon fuels like: wind, solar, tidal, wave and perhaps nuclear power. It also means developing new methods of converting this clean energy into motive power, like hydrogen fuel cells for cars.