“NEVER before has a responsibility so great been in the hands of so few,” declared Christiana Figueres, head of the United Nations Framework Convention on Climate Change (UNFCCC) at the opening of COP21 in Paris. The UNFCCC, an environmental treaty, has been signed by more than 190 countries since its inception in 1992. Meetings under way in Paris represent the twenty-first time its parties have met to divert global doom, arguing over climate and canapés. Under the treaty, all signatories agree to curb “dangerous anthropogenic interference within the climate system”. And after a deal at another such meeting in 2010, they also agree on what “dangerous anthropogenic interference” actually means: global mean surface temperature increasing more than 2°C above that of pre-industrial times. But the history of this limit raises questions about its suitability to save the world.

The two-degree maximum appeared initially in papers written by the Yale economist William Nordhaus in the mid-1970s. As “a first approximation” he suggested the world should not warm more than it had in the past 100,000 years or so—the period for which ice-core data were available. Given how little was known about the costs and damages of global warming at that time, Dr Nordhaus admitted that the estimate was “deeply unsatisfactory”. Nevertheless, European scientists discussed the two-degree limit during the next decade or so; in 1990 the Swedish Environment Institute produced a report that argued that, on the basis of “the vulnerability of ecosystems to historical temperature changes,” warming above just 1°C was not advisable. The authors knew it was too late to keep within this level, and so suggested 2°C instead. From thence the maximum was adopted by the European Union’s Council of Ministers in 1996; the G8 picked it up in 2009. During the chaos of the UNFCCC talks in Copenhagen that year, the two-degree limit emerged in glory, forming part of the deal made there between the world’s biggest polluters. In 2010 it was enshrined within UN policy.

The simplicity of the two-degree maximum appeals to policymakers. It concentrates the complexity of climate change into one digestible figure, and tells politicians what to do about it. Two-degrees is a safety barrier supposedly separating the world from catastrophic impacts—or some of the world, at least. Ice is already melting and coasts already flooding; communities on low-lying islands such as Tuvalu and Kiribati anticipate migration. These and other vulnerable countries want the warming limit to become 1.5°C in any Parisian deal. Given the assumptions made to argue that limiting warming to 2°C is possible, their demands will probably be glossed over.

But the two-degree maximum is not as simple as it seems. Accurate measurements of the world’s temperature are difficult to obtain: discrepancies mean arguments rage over whether a hiatus in warming occurred in the 15 years or so to 2013. And different parts of the world warm at different rates, for example, with the Arctic heating up twice as quickly as the global average. A limit linked to the concentration of greenhouse gases in the atmosphere could be better: such gases mix quickly and so can be sampled easily. Some argue that levels of soot and methane and ocean-heat content should be watched more closely, too. But to reach new international agreements on these alternative indicators is unfeasible. The two-degree maximum is imperfect, and will almost certainly be breached. But its existence focuses negotiations. And, when more than 190 countries have to agree, that may be enough to prove its worth.T