But the United States, the world's number one industrial power and its biggest polluter, is unlikely to return to the four-year-old pact.

Here is a chronology of some of the major political events since the Earth Summit in Rio de Janeiro in 1992.

1992:

June - World leaders end their Earth Summit in Rio de Janeiro pledging to do their utmost to prevent environmental ruin and alleviate poverty.

The main achievement of the Earth Summit is a treaty to reduce emissions of "greenhouse gases" which help trap heat in the atmosphere and are believed to be a cause of global warming.

- Nations adopt 1990 as the benchmark year at the summit in which industrialised countries agree to take voluntary steps to cut emissions to that year's levels. Most countries, other than Germany and Great Britain, have failed to meet that goal.

Sep 30 - The U.S. space agency reports that the "ozone hole" over Antarctica grew 15 percent in 1992 and is now nearly the size of the entire North American continent. The Antarctic ozone hole, first spotted in 1985, is caused by the depletion of the Earth-shielding ozone layer which is eroded by human-made chemicals such as chlorofluorocarbons, known as CFCs.

1993:

May 20 - U.S. scientists report that they have invented a computer model for predicting the effects of global warming on crops and forests. They find that the growth rate of plants is affected by the amount of carbon dioxide in the air, by nitrogen in soil and by temperature and moisture changes caused by global warming.

Oct 20 - U.S. President Bill Clinton announces an ambitious plan to combat global warming. Under the plan, U.S. greenhouse gas emissions would be cut to 1990 levels by 2000 through over 50 initiatives affecting all sectors of the economy.

1994:

June 1 - The international environmental group, Greenpeace announces in a report entitled "The Climate Timebomb", that global warming is causing severe climatic changes and environmental disasters around the world.

1995:

Feb 3 - Only slight global warming has occurred in the last 100 years and there is no sign of it producing more hurricanes, tornadoes or other extreme climate changes, Accu-Weather Inc. says in a report for the Global Climate Coalition, a business trade association seeking a voice in scientific research.

1996:

Feb 20 - Environment ministers from the world's leading economic powers agree that developing and industrial nations alike must do more to battle global warming and chemicals that deplete the ozone layer after a two-day meeting of the Organisation for Economic Co-operation and Development environment ministers in Paris.

Jul 18 - A pledge by major industrial powers to fight global warming by reducing greenhouse gas emissions from oil and coal wins wide support and is put into the official record at a United Nations Conference on Climate Change in Geneva.

1997:

Dec 11 - The world's first treaty to reduce greenhouse gases finally emerges after marathon talks in Kyoto, Japan. Under the Kyoto agreement, industrial nations are committed to reducing their greenhouse gas emissions by 5.2 percent from 1990 levels by 2008-12.

1998:

November - Around 170 nations gather at the United Nations global warming conference in Buenos Aires to discuss ways of cutting emissions of greenhouse gases by 2008-2012.

- Specialists from the U.S. and Canada tell the summit that global warming is killing the world's coral reefs, and with them the swarming sea life they shelter and support.

1999: