With 191 countries and the European Union as members, the Kyoto Protocol is the only global treaty with binding limits on climate-altering greenhouse gases. The UN treaty commits  nearly 40 developed "Annex 1" economies, emitting about a quarter of the world's greenhouse gas, to an average five-percent cut from 1990 levels by 2012.This forms part of efforts to limit global warming to 2.0 degrees Celsius (3.6 Fahrenheit) from pre-industrial levels at which scientists say the planet may be spared the worst impacts of climate change. A compliance assessment has yet to be done, but initial figures show the global target to be achievable despite vast differences between individual countries' performances.The protocol's first leg runs out on December 31 , and the Doha talks must agree on the modalities for a second commitment period from 2013, which was agreed upon at the last round of UN talks in South Africa a year ago. Developing nations are not bound by the Kyoto Protocol under the principle that rich nations were responsible for pumping the vast majority of Earth-warming gases into the atmosphere since the industrial revolution.The United States signed the UN pact but refuses to ratify it . Canada, having vastly exceeded its emission limits, withdrew from the protocol in December 2011 the first country to do so. Fellow Annex 1 members New Zealand, Japan and Russia have indicated they will not join a second commitment period. This leaves mainly the EU, Switzerland, Norway and Australia countries which jointly emit about 15 percent of the world's greenhouse gases.Many argue the deal is no longer fair, given that China, Brazil, India and Indonesia, which have no targets, are now among the biggest polluters.HOW DOES IT WORK?Annex 1 countries can meet their targets by cutting emissions or buying unused allowances from poorer countries. They can earn carbon credits by funding clean technology projects in developing nations, which they can sell on the carbon market or use to offset emissions. Countries that exceed their targets are supposed to make up the difference, plus a penalty of 30 percent, in a second commitment period.HOW MUCH LONGER?The big question in Doha is how long a second commitment period should last, who will back it, and what its targets will be. Small island countries under the most imminent threat of warming-induced sea level rises, want a five-year commitment which they believe will reflect the urgency. The EU and others want an eight-year period flowing over into a new, global climate deal that must enter into force in 2020.