What are the probabilities of a \(2^{\circ }\) C, \(3^{\circ }\) C or \(4^{\circ }\) C world? In the last decades, researchers and policymakers have worked with a wide range of future climate scenarios, but the likelihood of their realisations has never been quantified. We propose two ways to obtain a probability distribution for the aggressiveness of emission abatement policies: one method is based on the elicitation from economists of estimates of the optimal social cost of carbon; the second on the principle of maximum entropy. In both cases we make use of information on the technological and fiscal feasibility of various policies to bound the distribution, and of the observed difference between average recommended and implemented policies to adjust its mean. We find that the results are robust to methodological choices. They suggest that the likelihood of achieving the Paris Agreement target is small; that there is a significant probability of an end-of-century temperature anomaly above \(3^{\circ }\) C; that the much-studied 8.5 W/ \(\hbox {m}^2\) forcing has low probability, but should not be neglected, at least in tail-event studies. If one considers, as we do, the distribution of end-of-century temperatures we obtain as ‘too dangerous’, the disconnect between economists’ recommendations and policy action should be substantially reduced.
