How below is well-below? Future-proofing interpretations of the Paris Agreement

The Paris Agreement’s fallback temperature goal, keeping global warming well below 2^oC, is typically interpreted as staying below 2^oC with a specified probability, with legal disputes over what the probability should be. Such framing is not futureproof because uncertainty decreases with time, systematically weakening the target towards allowing temperatures to approach 2C itself. We show the science and legal discussion available at the Paris Agreement’s signing guides an interpretation using a level of conserved median warming, with a minimum 66% chance of staying below 2^oC translating to 1.8^oC of expected median warming and a more defensible 83% chance giving 1.6^oC.