Abstract
Uncertainty of climate sensitivity is one of the critical issues that may affect climate response strategies. Whereas the equilibrium climate sensitivity (ECS) was specified as 2-4.5 °C with the best estimate of 3 °C in the 4th Assessment Report of IPCC, it was revised to 1.5-4.5 °C in the 5th Assessment Report. The authors examined the impact of a difference in ECS assuming a best estimate of 2.5 °C, instead of 3 °C. The current pledges of several countries including the U.S., EU and China on emission reductions beyond 2020 are not on track for the 2 °C target with an ECS of 3 °C but are compatible with the target with an ECS of 2.5 °C. It is critically important for policymakers in Paris to know that they are in a position to make decisions under large uncertainty of ECS.