Higher CO2 concentrations increase extreme event risk in a 1.5 °C world

The Paris Agreement(1) aims to 'pursue efforts to limit the temperature increase to 1.5 degrees C above pre-industrial levels.' However, it has been suggested that temperature targets alone are insufficient to limit the risks associated with anthropogenic emissions(2,3). Here, using an ensemble of model simulations, we show that atmospheric CO2 increase-an even more predictable consequence of emissions than global temperature increase-has a significant direct impact on Northern Hemisphere summer temperature, heat stress, and tropical precipitation extremes. Hence in an iterative climate mitigation regime aiming solely for a specific temperature goal, an unexpectedly low climate response may have corresponding 'dangerous' changes in extreme events. The direct impact of higher CO2 concentrations on climate extremes therefore substantially reduces the upper bound of the carbon budget, and highlights the need to explicitly limit atmospheric CO2 concentration when formulating allowable emissions. Thus, complementing global mean temperature goals with explicit limits on atmospheric CO2 concentrations in future climate policy would limit the adverse effects of high-impact weather extremes.