Average global surface temperatures are estimated to increase by between 1.8°C and 4°C by 2099. However, the range varies under different greenhouse gas emissions scenarios. The lowest likely increase is 1.1°C under a low emissions scenario that encompasses a rapid shift to a service and information economy with reductions in material intensity.
The highest likely increase is 6.4°C under a high emissions scenario where rapid economic growth and the use of fossil intensive energy sources outweighs the rapid introduction of new and more efficient technologies. Both scenarios anticipate a decline in population after 2050.
The summary report outlines findings from six years' research by some 2,000 scientists. It is the first part of the IPCC's fourth assessment of climate change. In its third assessment, the IPCC predicted a rise of 1.4 to 5.8°C, but the latest projections are based on more refined climate models, and new information about carbon cycle feedbacks and the climate response.
Sea levels are likely to rise by 0.18-0.59 metres by 2099 – lower than predicted by the third assessment – but this excludes the full effects of ice flow due to lack of data.
Despite such uncertainties, there is greater confidence in projected changes in temperature, wind patterns and precipitation.
The report forecasts a reduction in snow cover, widespread thawing in most permafrost regions and a decline in sea ice. More frequent heat waves and heavy precipitation are "very likely", with more precipitation in high latitudes. Subtropical regions could see precipitation decline by as much as 20% if emissions remain high, and tropical cyclones are likely to become more intense.
Environment Secretary David Miliband said the report confirmed that international political commitment to take action to avoid dangerous climate change was "urgently needed".
Other findings included:
- Temperatures rose by 0.74°C between 1906 and 2005.
- The rates of rises in carbon dioxide levels, sea level and temperature have increased since 1993.
- It is "very unlikely" that the Gulf Stream "will undergo a large abrupt transition" during this century.