On the Utility of the Transient Climate Response

Abstract The Transient Climate Response (TCR) is a metric of climate sensitivity relevant to present‐day climate change, but has been less studied than the Equilibrium Climate Sensitivity. Here, we emphasize the utility of the TCR by demonstrating that temperature and radiative forcing in transient scenarios are linearly related, where the proportionality constant is essentially the TCR. This relationship has been invoked in previous studies but not extensively validated; here it is shown to hold across models as well as forcing scenarios. This linear scaling furthermore allows for a simple estimate of the time to cross a given temperature target, given only a forcing scenario and a value for TCR. Deviations from a strictly linear relationship between temperature and forcing are governed by the “slow” component of global warming driven by declining deep ocean heat uptake, but can only be fully accounted for by incorporating changing patterns of surface warming via ocean heat uptake “efficacy.”