WHAT IS IT?

The Kyoto Protocol is the main achievement of the UN Framework Convention on Climate Change (UNFCCC), the forum set up under the 1992 Rio Summit.

It is the only international treaty to set down legally-binding targets for curbing climate-altering "greenhouse" gases.

It has been ratified by 192 states plus the European Union (EU).

Under its present commitments, 37 industrialised, or "Annex 1," economies pledged to reduce overall emissions of the six main gases by at least five percent by 2012 compared to 1990.

The United States signed the agreement but refuses to ratify it.

Developing economies -- "Annex 2" countries -- were not assigned targeted emissions curbs under the principle that rich countries bore historical responsibility for warming.

HOW DOES IT WORK?

Annex 1 countries can meet their targets any way they choose, including carbon trading.

The Protocol also has two special instruments of its own, known as the Clean Development Mechanism (CDM) and Joint Implementation.

By investing in clean-technology projects in developing economies or the former Soviet bloc, advanced economies get carbon credits that can be sold or offset against their emissions quotas.

Countries which fail to meet their emissions targets must make up the difference, plus a penalty of 30 percent, in any second commitment period.

In 2009, emissions of carbon dioxide (CO2) by Annex 1 countries were 6.5 percent below their 1990 level, according to the International Energy Agency (IEA). But this figure masks big differences within the group, with Canada notably far over its target.

TROUBLED HISTORY

The Protocol was adopted by the UNFCC as a 21-page "framework" accord on December 11, 1997 after 30 months of negotiations.

Several years of wrangling ensued over its complex rulebook, especially the counting of forests as carbon absorbers, or "sinks", that can be set against national emissions -- a provision regarded as a loophole by environmentalists.

In March 2001, the Protocol was almost wrecked when then-president George W. Bush abandoned the pact as being unfair and too costly for the US economy. It survived thanks to championing by the EU.

Further foot-dragging on ratification by Russia delayed implementation, which eventually took effect on February 16 2005.

UNCERTAIN FUTURE

The big question in Durban is whether there will be endorsement of a second commitment period from 2013.

Developing countries are lobbying hard for this. They like Kyoto because of its tough verification and compliance clauses and the "differentiated" burden-sharing between Annex 1 and Annex 2.

But in rich countries, support for Kyoto has drained since the stormy December 2009 Copenhagen Summit, which set up voluntary pledges aimed at limiting warming to 2 degrees Celsius (3.6 Fahrenheit).

Critics say the Kyoto format is out of date, as China and United States, which together account for 41 percent of global CO2 emissions, do not have binding curbs, nor do Brazil, India and Indonesia, which are also fast-growing emitters.

In 2009, developing countries accounted for 54 percent of CO2 emissions and the United States 18 percent. Kyoto's Annex 1 countries accounted for just 25 percent.

The EU says it can envisage a second set of commitments, depending on pledges from the major polluters in the wider UNFCCC forum. The prospects for this are slim, though.

Except where stated, the source is the Fourth Assessment Report published by the UN's Intergovernmental Panel on Climate Change (IPCC) in 2007.

-- Evidence of global warming is "unequivocal," with a more than 90-percent probability that humans are largely responsible. The main culprit is greenhouse gas from fossil fuels, which traps solar heat in the atmosphere, warming Earth's surface.

-- Levels of carbon dioxide (CO2) have risen by around a third since pre-industrial times and are now at their highest in 650,000 years. 2010 saw the largest single-year jump in CO2 emissions, from 8.6 to 9.1 billion tonnes, according to the US Department of Energy.

-- Since 1900, sea level has risen by 10-20 centimetres (four to eight inches). Global average surface temperature has risen by 0.8 degrees Celsius (1.44 degrees Fahrenheit). Average temperatures above land have risen far faster, by 0.91 C (1.64 F) since the mid-20th century, according to the Berkeley Earth Surface Temperature Project.

-- Climate change is already visible in sea-level rise, loss of alpine glaciers and snow cover, shrinking Arctic summer sea ice, thawing permafrost and poleward migration of many animals and plants towards cooler habitats.

-- By 2100, "best estimates" for the rise in global average surface temperatures run from 2.4-4.0 C (4.3-7.8 F) depending on fossil-fuel use. These figures also mask big variations, according to region and country.

-- In 2007, the IPCC projected sea levels will rise by at least 18 cms (7.2 inches) by 2100. Since then, many studies point to the risk of meltoff from the Greenland and West Antarctic ice sheets. Most experts now say a one-metre (39-inch) increase is plausible.

-- 20-30 percent of plant and animal species are threatened with extinction if average global temperatures increase by 1.5-2.5 C (2.7-4.5 F) compared to the average temperature during the two last decades of the 20th century.

-- In Africa, by 2020, up to 75 to 250 million people will be exposed to increased water stress. Yields from rain-fed agriculture in some African countries could be reduced by up to 50 percent. Desert-like areas could expand by five to eight percent by 2080.

-- In Asia, available fresh water will decrease by mid-century. Coastal mega-deltas will be at risk from flooding due to rising seas. Mortality due to diseases associated with floods and droughts will increase.

-- Extreme weather events such as heatwaves, droughts and rainstorms are likely to become more frequent and/or intensive, according to an IPCCC special report published on November 18.

-- To stabilise emissions at 445-535 parts per million (ppm) of CO2 equivalent would limit the overall rise in global warming since pre-industrial times to 2.0-2.8 C (3.6-5.0 F). Concentrations are currently about 390 ppm. A level of 450 ppm corresponds roughly to the target of 2.0 C (3.6 F) embraced by nations at the UN climate talks in Cancun in 2010.

-- Countries have to close a "gigatonne gap" of excess carbon emissions to meet the 2.0 C (3.6 F) target. A 2010 UNEP report set this at five to nine gigatonnes, a figure likely to be higher today. In October, researchers at the Institute for Atmospheric and Climate Science at ETH Zurich calculated emissions will have to fall by 8.5 percent by 2020 compared to 2010 and then continue to decline.