Industrialized countries have agreed to cut their emissions of carbon dioxide, methane and nitrous oxide - the three most important greenhouse gases - by 5.2% below 1990 levels by 2008-12.
They will also reduce emissions of hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride by the same amount, measured against either 1990 or 1995 levels.
The deal was hammered out after 10 days of tough negotiations at the climate summit in Kyoto, Japan. The US, which went into the summit not wanting to make any cuts below 1990 levels, has softened its original hard-line stance and agreed to make a cut of 7%. The European Union will make a slightly larger cut of 8%.
Central and eastern European nations have also agreed to an 8% reduction, while Canada, Hungary, Japan and Poland have agreed to a cut of 6%. However, Russia, New Zealand and the Ukraine will only have to stabilize their emissions at 1990 levels, while Norway will be allowed to increase emissions by 1%, Australia by 8% and Iceland by 10%. Developing nations, which the US originally insisted had to be part of the deal, will not have to take any action to reduce emissions. China has refused to sign the deal.
The Kyoto agreement gives countries flexibility in how they make and measure their emissions reductions. Industrialized nations will receive credit for paying for projects that help to reduce emissions in other developed nations, while an international emissions trading regime will be set up to let rich countries buy and sell excess emissions credits among themselves. Countries will also be given credit for planting forests, as these help to absorb carbon dioxide from the atmosphere.