Here is a factfile on the world's most ambitious and complex environmental accord:

What is it?

The Kyoto Protocol legally commits industrialised countries which have signed and ratified it to trim their output of six carbon gases.

These "greenhouse" gases are trapping the Sun's heat, causing the Earth's surface to warm and thus changing the planet's delicately-balanced climate system.

Why was it created?

The protocol can be traced to early scientific evidence in the 1970s and 80s about the peril of man-made global warming. Its framework was adopted on 12 December 1997 in Kyoto, Japan, by 159 countries that are members of the 1992 United Nations Framework Convention on Climate Change (UNFCCC).

But it took almost four more years of negotiations to complete its rulebook and then nearly three more years to get the deal ratified so that it could take effect.

Which are the culprit gases?

The Kyoto Protocol legally commits industrialised countries to trim their output of carbon gases

The biggest villain is carbon dioxide, the byproduct of burning oil, gas and coal. The others are methane (mostly the result of agriculture), nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride.

Which countries have to make cuts?

Thirty-six western and Eastern European industrialised countries, which have to meet individual targets of reducing or stabilising output of greenhouse gas emissions by a timetable of 2008-12 as compared with their 1990 level.

The United States walked away from Kyoto in 2001 and Australia says it will not ratify it. Developing countries are being encouraged to reduce their pollution but do not have to meet specific targets.

How deep are the cuts?

As agreed in 1997, individual goals were 6% for Japan; 8%for the European Union (EU); and 0% for Russia.

Within the EU, the targeted cuts are 21% for Germany, 12.5% for Britain, 6.5% for Italy and 0% percent for France, although Spain can increase output by 15%
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Before it abandoned Kyoto, the United States was committed to a reduction of7%. 

How can countries meet their targets?

Any way they choose. They can use command-and-control measures such as taxes, fuel efficiency laws or awareness campaigns.

But fossil fuels, the biggest source of global warming, are also today's prime energy source, and thus curbing their use carries an economic cost. So, Kyoto includes three incentive mechanisms - including a future market in carbon emissions - to ease the bill.

Why does ratification take so long?

Kyoto can only take effect after ratification by 55 countries accounting for at least 55% of carbon dioxide (CO2) emissions by developed nations as at 1990 levels.

After the decision by the United States (36% of 1990 emissions) to quit Kyoto in 2001, the onus fell on Russia (17.4% of emissions) to push the treaty over the threshold. Russia dragged its feet, eventually ratifying last November.

Will Kyoto work?

The United States walked away from Kyoto in 2001 and Australia says it will not ratify it

A good question, given the absence of the biggest CO2 polluter (the United States) and Kyoto's untested incentives and a rulebook of notorious complexity, including the counting of forests as a means of offsetting CO2 pollution.

In its original form, the deal targeted an overall cut by 38 countries of at least 5.2% by 2012 compared with 1990 levels. If all the 36 remaining countries meet their targets, and if concessions made to bring the treaty to fruition are factored in, their cut will be in the order of only 2 or 3%.

In the meantime, emissions by the United States and China will continue to zoom.

What is the next step?

Scientists say that reductions of around 60% are urgently needed to avoid wreaking potentially catastrophic damage to the world's climate system.

The present "commitment period" under Kyoto runs out in 2012. Negotiations begin under the UNFCCC in November on the post-2012 pact.

Parties will be under pressure to make far deeper cuts, to include China and India in targeted reductions, to coax the United States back into the multilateral fold - and to make the follow-on deal far simpler.