Estimating climate sensitivity based on top-of-atmosphere radiation imbalance

There are large uncertainties in predicting the Earth's future climate with an increased CO2 atmosphere in current climate models. To reduce the uncertainties, a simplified energy balance model constrained by top-of-atmosphere (TOA) radiative energy imbalance is explored. Instead of solving the initial condition problem in previous energy balance analysis, current study focuses on boundary conditions as well as on climate system memory and deep ocean heat transport. Along with surface temperature measurements of the present climate, the climate feedbacks are obtained based on the constraints of the TOA radiation imbalance. Comparing to the feedback factor of –3.3 W/m2/K of the thermal emission of the climate system, the estimated feedback factor for the current climate system ranges from –1.3 to –1.0 W/m2/K when a TOA net radiative heating of 0.85 W/m2 to the climate system is considered. That is, a positive climate feedback is found. The feedback range is caused by the uncertainties in the climate memory length. The estimated time constant of the climate is about 100 years, implying that the climate may be not in an equilibrium state in the last century. For the doubled-CO2 atmosphere, the estimated global warming would be 3.1 K if the estimate of TOA radiation imbalance were correct. These results are consistent with some other model predictions. The significant difference is that current estimates, though with certain uncertainties, may not support model projections of either high or low climate sensitivities.

This study suggests that long-term measurements of the TOA radiation with both high precision and high absolute accuracy are highly required. With long-term, accurate global TOA energy imbalance measurements and the analysis method suggested here, a great potential in accurate climate predictions could be realized, and a physically-based tool in determination of climate change policies can be provided to the public and policymakers.