 The Durban climate conference may have agreed a deal - or at least a deal to agree a deal - but the scale of the work that still needs to be done became plain yesterday.  although talks are supposed to start immediately, america's special envoy for climate change, Todd Stern, infuriated the EU by warning that much preparatory work had to be done before the negotiators could sit down to haggle.  "(In drawing up) the Kyoto protocol, there was a period of a year to year and a half of scoping out so I expect that will go on . . . for a year or two," Stern said. "Then you still have two to two and a half years to negotiate, and finish in 2015." EU officials are acutely aware that the time to forge a deal is short, and the issues to be resolved vastly complex.  The Durban conference ended on Sunday with a last-ditch deal whereby developed and developing countries will for the first time work on an agreement that should be legally binding on all parties, to be written by 2015 and to come into force after 2020.  But while the UN and most of the countries present hailed the deal as a breakthrough, getting an agreement that all countries sign up to will be intensely complicated. "Many political agreements put off the difficult actions for the next regime and that appears to be the reality for the Durban platform," said David Symons, director of environmental consultancy WSP. "No one should underestimate the difficulty of arriving at a legal agreement between the developed and developing countries, let alone one that for the first time includes China, India, Europe and america.  "The Durban platform provides an anodyne set of words, with much of the detail yet to be agreed and the teeth not really coming for eight years. The real challenge will be in agreeing the fine print."  Jonathan Grant of consultancy PwC said the scale of the task was daunting, as G20 countries would need to cut their carbon intensity (the amount of CO2 released as a proportion of energy produced) by 5% a year to 2050. France's vast nuclear power programme of the 1980s delivered a 4% per year cut for 10 years, he said, and the UK's "dash for gas" to replace coal-fired power stations in the 1990s only produced cuts of 3% a year for a decade.  The timetable is significant, particularly in relation to the US electoral cycle. Striking a deal at Durban was crucial, because by next year's conference there could be another president, and none of the Republican candidates would have signed up to the Durban platform.  an incoming Republican would have to make a public renunciation of the climate talks in order to get out of the 2015 deadline. If Barack Obama wins another term, however, in 2015 he will be facing the final year of his presidency. That may spur him to try to ensure a global climate agreement is part of his legacy.  any new agreement will come down to targets - how far each country will have to cut its emissions. The motivation to increase ambitions could come from several sources, said Michael Jacobs of the London School of Economics, including people power. "By 2015 the world's young people in particular can be expected to demand greater action as the evidence of future damage becomes clear." He also cites the ambition of China's next five-year plan, due in 2015, and demands from investors for stronger, clearer policies as important.  Grant suggests Britons will have a simpler motivation: "It will be people's wallets. If energy bills continue to rise as they have, people will eventually start to manage their demand much more efficiently than now. People are left a bit cold by the climate negotiations but energy bills impact them directly."  The magic number is two - a temperature rise of 2C above pre-industrial levels is estimated to be the limit beyond which climate change becomes catastrophic and irreversible. In order to have even a 50:50 chance of staying within that limit, the Intergovernmental Panel on Climate Change (IPCC) calculates that emissions must peak by 2020 at the latest and fall rapidly thereafter. Carbon output must be roughly halved by mid-century, compared with 1990.  In 2014 the IPCC will produce its fifth assessment report. The overwhelming majority of climate research shows the situation is growing more serious, with increasing evidence that human activity is harming the climate and a clearer picture of what the consequences will be. This may mean governments could have to raise their targets even further.  If life were simple, it might be possible to work out a formula for dividing up the cuts needed among the countries, according to emissions per head of population, perhaps also taking into account emissions per unit of economic output.  That sort of thinking will not work in these talks, which have been running for 20 years. One key issue is historic emissions - industrialised countries started burning fossil fuels earlier and so bear responsibility for most of the CO2 already in the atmosphere. Balancing that, some countries have worked harder to reduce emissions than others - the EU, for instance, has, while China has invested heavily in renewables in recent years - so they will all want credit for these actions. Then there are the differing capabilities of each country - for instance, those with large forests provide a valuable service in absorbing carbon, while others' circumstances afford less opportunity to use low-carbon power. Japan is a case in point: having pledged to phase out nuclear power, it will be hard-pressed to find enough renewable alternatives.  Just how difficult it will be to resolve these issues was apparent in Durban. India's environment minister made an impassioned speech in which she insisted that equity - taking into account developing countries' economic capabilities, large populations still to be lifted out of poverty, and low responsibility for historic emissions - must be the foundation of the negotiations. She said: "Equity is the centrepiece, it cannot be shifted. This is not about India. Does fighting climate change mean we have to give up on equity?"  China's minister Xie Zhenhua backed her up strongly.  The International Energy agency estimates that by 2020 China's emissions per head will be equal to or higher than the EU's. That will make it difficult for China to base its argument for easier targets on its large population. The vast  