Climate sensitivity measures how much Earth's surface will warm in response to a doubling of CO2 concentrations. It is one of the most critical metrics in climate science, widely used in economic and policy assessments of future global warming. With current CO2 levels approximately 50% higher than in 1850 and, depending on technology and decisions, we are on track to more than double CO2 concentrations within the 21st century.
A crucial question is whether our climate has the same sensitivity to CO2 increases from pre-industrial levels to twice those levels compared to increases from double to triple or quadruple CO2 concentrations. Most studies so far assume a linear climate response to increasing CO2.
However, our findings indicate that the response is not linear: we may experience different warming rates when moving from 1x to 2xCO2 compared to 2x to 4xCO2. Moreover, if we stabilize the climate with net-negative CO2 emissions or reduce atmospheric CO2 to pre-industrial levels, we might not be able to return Earth's temperature to its pre-industrial state of 1850.
We demonstrate that Earth's response to CO2 perturbations is non-linear, with significant implications for scaling responses from specific CO2 perturbation levels and utilizing evidence from past climate states.