Probability of emergence of novel temperature regimes at different levels of cumulative carbon emissions

The United Nations Paris Agreement creates new urgency and importance for understanding the emergence of climate impacts at different levels of cumulative carbon emissions. Using observations and climate models, we quantify the probability that different areas of the globe will experience permanent separation from the historical temperature regime, as a function of both time and cumulative emissions. We find that, in addition to a similar to 50% probability that global warming will exceed 2 degrees C, cumulative emissions of 1000 gigatons of carbon (GtC) are likely to cause large areas of the tropics to transition to a new climate regime in which the annual temperature is never less than four standard deviations above the baseline mean. Although limiting cumulative emissions to 750 GtC substantially reduces these risks, the likelihood that some regions will still permanently experience extremely high annual temperatures suggests a critical role for adaptation, even if the Paris Agreement's mitigation targets are met.