A scientific critique of the two-degree climate change target

Government representatives are meeting once again at the international climate talks in Paris in December 2015, with the aim of deciding on new steps towards preventing dangerous man-made climate change. Much of the disagreement among the parties can be linked to differing national priorities and burden sharing — that is, how much each country has to contribute to a solution, for example, in terms of emission reductions, financial and technological support, or in compensation payments for loss and damages. The means with which to restrict warming to below the 2 °C limit are not the only issues discussed in policy circles1,2 and by the public3,4, the adequacy of the target itself is also debated. The ultimate objective of the United Nations Framework Convention on Climate Change (UNFCCC), established back in 1992, is “to stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system”5. But it neither specifies what concentration level that would be, nor how to assess what is “dangerous”. Following the Copenhagen Accord in 2009, the UNFCCC formally decided in 2012 to pursue actions in line with a 2 °C global temperature increase target6. This target was a political decision informed by science, but no scientific assessment ever defended or recommended a particular target. Policymakers like to hide behind scientific evidence7, ask for ‘actionable science’ and claim to make ‘science-based decisions’. Some argue that this process “has more in common with a salad bar — where people pick and choose convenient studies — than with the balanced search for truth that science aspires to”8. Ultimately, the 2 °C target is a political consensus that takes into account what policymakers at that time considered to be both realistically achievable and tolerable. It is high time for a scientific assessment of the question whether global mean temperature is a meaningful quantity, and if so, how science can contribute to deciding what level most appropriately reflects the UNFCCC’s ultimate objective.