If you live in the Maldives, "1.5 to stay alive" is more than just a catchy slogan. The reality is that temperature rises above 1.5C will destroy this island nation from all sides: rising sea levels will swamp the tiny atolls, warmer water will kill its beautiful coral reefs, and an acidic ocean will literally dissolve the islands one by one.
The Maldives is not alone: other atoll countries, like the Marshall Islands, Tuvalu and Kiribati are in the same boat. Other vulnerable states, particularly those in Africa which are prone to drought and harvest failures, and nations in Central America and Asia which could suffer stronger hurricanes and more extreme weather, also know that 1.5C is the key line for them. At the UN climate talks in Copenhagen, more than 100 countries are determined to hold the line on 1.5C.
The problem is that time is rapidly running out. Senior climate scientists have been holding side events in the main conference centre, explaining what different temperature rises mean, and how emissions trajectories need to change to avoid them. When I asked Dr Richard Betts, head of climate impacts at the Met Office Hadley Centre, what would need to happen to restrain the temperature rise to 1.5C, his response was surprising: "The world would need to peak its carbon emissions by last Wednesday," he said.
Was Betts being facetious? Slightly — but he was also underlining a deadly serious point. "Well obviously we can't be that precise," he clarified. "But the truth is that according to some of our latest modelling work, to have a 50-50 chance of staying below 1.5C, we need to be peaking emissions round about now – this month or so." And if we don't, the chances of restraining temperatures to this relatively modest level quickly begin to diminish. If emissions go on rising for another decade, he told me, the window of opportunity for having a 50-50 chance of keeping emissions even below 2C also begins to close.
The debate about whether humanity should aim for 1.5C or 2C is one of the most heated here at Copenhagen. Europe has been committed to 2C for a long time, and at the most recent G20 summit other big nations — including the US — also signed up. In the current draft of the text being considered by negotiators here, both 1.5C and 2C appear in square brackets, showing that they are still being debated. Few seasoned delegates expect the 1.5C to survive the week.
For the last decade the US has been the primary bad guy — but now India and China seem to be assuming that mantle. India strongly opposes any mention in the negotiating text about when global emissions should peak, because it fears that any such commitment would eventually force it to have to take on a mandatory carbon emissions target itself: anathema to a developing country which plans to burn an increasing quantity of coal over future decades.
India and China have for the first time offered numerical targets — but these refer only to emissions intensity (carbon released per unit of GDP) rather than absolute amounts of carbon. So China's intensity cut of 45% will likely lead to a CO2 rise of 100% over the next decade alone. Nor is America's offer much better: just 3% below 1990 levels by 2020 is worse than the target the Clinton-Gore administration signed up to 15 years ago at the Kyoto protocol meeting.
Not everyone's targets are inadequate. The Maldives (which I am currently advising) have pledged to be the world's first carbon neutral country, achieving this by 2019. Costa Rica will be the second, by 2021. But if you add together all the targets offered by the main players, the eventual temperature rise will take us well over 3C: between 3.5 and 3.9C, according to the latest analyses. That's still better than business as usual, which gives a likely temperature outcome of 4.8C. But it is hardly a safe climate either.
In just five days' time, the world will know which way it is headed – not because of any advances in climate science, but because heads of state gathering in Copenhagen will have made their decisions. A temperature rise of 1.5 is still just about possible, but not for much longer. On this, the fate of the Maldives, and many other countries like it, rests.
• Mark Lynas is author of Six Degrees: Our future on a hotter planet, and adviser on climate change to the Maldives. He is also presenting a nightly live webcast from Copenhagen with the director of the Age of Stupid and founder of the 10:10 campaign, Franny Armstrong.