The world's three biggest emitters of greenhouse gases, China, the US and India, will be legally bound to cut their emissions for the first time under a new global warming treaty prefigured at the close of the Durban climate conference early yesterday.  The move, finally agreed at 5am after the meeting of 194 countries overran by more than 24 hours, is a substantial step forward in fighting climate change, as between them the "Big Three" emit 46 per cent of the 33.5 billion tons of carbon dioxide pouring into the atmosphere annually, yet they are not currently obliged to cut this back in any way.  The fact that their soaring emissions - China's and India's growing by more than 9 per cent annually, america's by 4 per cent - will now be brought into a binding reduction framework, gives some hope that the world may hold the expected rise in global temperatures under the danger threshold of 2C above pre-industrial levels.  Without their participation, the chances of this were zero. Yet only a fortnight ago, at the start of the conference, all were resolutely opposed to any moves forcing them to cut back their CO2, fearing it would harm their economies.  The fact that they were persuaded to change their minds was a victory for the European Union, which came to Durban with the explicit aim of negotiating a "road map" to a new climate pact binding every country of the world - the three biggest polluters included. The current climate treaty, the Kyoto Protocol, does not cover them, and deals with only 15 per cent of global CO2.  The EU succeeded in its three aims - a new legal treaty, with a signing by 2015, and an entry into force by 2020 - because it managed to put together a coalition with developing countries, many of which greatly fear global warming's impending effects.  This "high-ambition coalition" eventually included more than 120 nations, and put what turned out to be irresistible pressure on the three top polluters, although it was a close-run thing. The US agreed first, then China - but the Indians, desperate to keep their surging economic growth which is bringing the country out of poverty, held out until the bitter end over the language which will now commit them to cutting their CO2 back.  The conference teetered on the brink of collapse and was finally saved by a face-to-face deal between two women - the EU Commissioner for Climate action, Connie Hedegaard from Denmark, and the Indian Minister of Environment affairs, Jayanthi Natarajan, who agreed on a final formula.  Formally entitled the "Durban Platform", this agreement commits the world community to "develop a protocol, another legal instrument or an agreed outcome with legal force... applicable to all parties" - which means a wholly new legally binding climate treaty for everyone. Work on it will begin "with urgency" in the first half of next year and it will be signed no later than 2015, and will come into effect and be implemented no later than 2020.  a major fear of environmentalists was that a new treaty coming into force by 2020 would mean "locking in a decade of inaction", but the agreement deals with this in two ways. Firstly, it explicitly recognises for the first time the so-called "emissions gap" - the fact that when all the pledges that all the countries in the world have made about reducing CO2 are combined, that is still far from enough to halt global warming. Secondly, it established a group to work on raising the carbon-cutting ambitions of all countries, in the years before the new treaty comes into force.  In return for the agreement on the new treaty, the EU agreed to a new version of the Kyoto Protocol, which has in the past been important to the developing countries as a talisman of rich countries' good faith, but is increasingly seen as less important than an overarching new treaty.  The concerns of countries such as India and China about climate action harming their development have been recognised by the setting up at Durban of the Green Climate Fund, which will channel much of the $100 billion per year of climate finance which wealthy nations have promised by the end of the decade.  Chris Huhne, the Energy and Climate Change Secretary, led negotiations for Britain at Durban and played a central role in securing the agreement.  He said yesterday: "This deal is a very significant step forward and makes it credible again that we can hold global warming to below two degrees above the pre-industrial level, as long as we use this framework to determine ambitious levels of curbs on greenhouse gases which respect the science.  "It is a very optimistic result at Durban and I think it will be seen as an absolutely key turning point," Mr Huhne added.