Point of No Return and Optimal Transitions in CMIP5
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The evolution of the climate in terms of global mean surface temperature (GMST) under anthropogenic forcing scenarios is explored with respect to the warming targets of 1.5 K and 2 K as set in the Paris agreement. For the first time we successfully use Linear Response Theory to combine a large number of complex climate models from the CMIP5 ensemble, allowing us to determine the GMST response to any CO2 forcing scenario. We construct a simple stochastic model, forced by CO2 emissions, that well recovers ensemble mean and variance of the CMIP5 GMST. The Point of No Return, the point in time when it is too late to reach a target, is derived, and we show its dependence on cumulative emissions, climate uncertainty, risk tolerance and stringency of efforts to combat climate change. This yields an easy-to-communicate result to inform debate on climate change action. Based on our assessment the 1.5 K target is not deemed reachable anymore, and only a limited time remains to start ambitious efforts to reach the 2 K goal. In addition, using simple economic assumptions we find optimal pathways for energy transitions to a carbon-free era and show how they depend on the warming target.