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Leading article: A global agreement remains the planet's best hope

The United Nations summit on climate change in Cancun convened two weeks ago amid the lowest expectations of success. And it seems poised to deliver on them. The conference ends today and none of the delegates is even pretending that there is a chance that the meeting will produce a global deal on legally binding reductions to the carbon emissions that are driving global warming.

Todd Stern, representing the United States, says that the world should not get "hung up" on the need for a definitive outcome from Cancun. He need not have worried. When it emerged that several key governments would only be sending junior officials to Mexico, the clear message went out that this would not be a breakthrough summit.

The former UN climate change chief Yvo de Boer is now talking up the chances of the scheduled conference in South Africa in 2011 concluding a deal. But the same hopes were expressed of Cancun when the Copenhagen talks failed last December. According to Mr De Boer "the ingredients for a deal are on the table". But they were too in Copenhagen, in the shape of binding emission cuts for both high-income countries and developing nations (although delayed for the latter), plus a commitment to clean technology transfer from rich to poor countries. What was absent in the Danish capital and now it seems in Mexico is the willingness of national governments to sign up to such a rational deal. Still more depressingly, the scale of the emissions cuts under discussion are lower than the best scientific estimates of the UN Environment Programme of what is necessary to keep the rise in global temperatures to below 2C this century. Even if the deal is done, it might be insufficient to prevent runaway warming.

This is a dark hour. The Kyoto Protocol will end in 2012. With every failed summit, the likelihood grows that there will be no new treaty to replace it. Kyoto was far from perfect. The nations covered by the protocol's targets account for less than a quarter of global emissions. And it did not cover shipping or aviation. But Kyoto did represent a global recognition of the need to tackle climate change. And if the treaty lapses without a replacement the small successes it has delivered, such as finance for developing nations that protect their rainforests, could unravel.

Optimists point out that Spain and India have made constructive moves over the past fortnight. But Japan, Canada and Russia have grown more recalcitrant. And the election of a host of new climate sceptic Republican members to Congress in last month's mid-term US elections has tied President Barack Obama's hands. China, meanwhile, remains the roadblock that it was in Copenhagen.

It is tempting to argue that the search for a binding global deal should now be abandoned and to recommend that governments focus on national emission reductions, bilateral deals where possible or even adaptation to a hotter planet. Yet if nations go their own way, we will likely descend into a beggar-thy-neighbour world, in which countries with laxer emissions controls poach manufacturing capacity from states that take a lead. It is hard to see even the most modest national emission reduction efforts surviving under such circumstances.

Climate change is a global problem. And despite the lamentable failure of international governments to settle on the way forward, a global political agreement remains the only viable means of solving it. We must hope that the optimists are correct and that Cancun, however unsatisfactory, inches us closer to the global accord that the world urgently needs.