The Kyoto Protocol is an international agreement imposing limits on emissions of carbon dioxide and other gases.
Scientists blame these gases for raising temperatures across the world, melting glaciers and raising the level of the oceans.
Here are the key points of the treaty::: The Protocol was negotiated in the Japanese city of Kyoto in 1997 and ratified by 140 nations.
:: The Kyoto targets vary by region. The UK is committed to cutting its emissions by 12.5% below 1990 levels by 2012.
The European Union's target is an 8% reduction on 1990 levels by 2012.
The US agreed to a 7% reduction before President Bush denounced the pact in 2001.
Japan is struggling to find ways to meet its obligations which require it to cut its emissions by 6%.
The agreement also allows some countries with low emissions to increase them.
The Protocol has only just come into force, despite being negotiated seven years ago.
The Kyoto agreement was delayed by the requirement that countries accounting for 55% of the world's emissions must ratify it.
The Protocol allows nations to conduct something called emissions trading.
This allows countries to buy and sell their agreed quota of greenhouse gas emissions.
Countries which create large volumes of greenhouse gases are allowed to buy unused "carbon credits" from those who do not emit as much as they are allowed to.
Countries can also gain credits by for activities which increase the environment's capacity to absorb carbon.
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