Abstract

Equilibrium climate sensitivity measures the long-term response of surface temperature to changes in atmospheric CO₂. The range of climate sensitivities in the Intergovernmental Panel on Climate Change Fifth Assessment Report is unchanged from that published almost 30 years earlier in the Charney Report. We conduct perfect model experiments using an energy balance model to study the rate at which uncertainties might be reduced by observation of global temperature and ocean heat uptake. We find that a climate sensitivity of 1.5°C can be statistically distinguished from 3°C by 2030, 3°C from 4.5°C by 2040, and 4.5°C from 6°C by 2065. Learning rates are slowest in the scenarios of greatest concern (high sensitivities), due to a longer ocean response time, which may have bearing on wait-and-see versus precautionary mitigation policies. Learning rates are optimistic in presuming the availability of whole ocean heat data but pessimistic by using simple aggregated metrics and model physics.