Past rapid warmings as a constraint on greenhouse-gas climate feedbacks.

There are large uncertainties in the estimation of greenhouse-gas climate feedback. Recent observations do not provide strong constraints because they are short and complicated by human interventions, while model-based estimates differ considerably. Rapid climate changes during the last glacial period (Dansgaard-Oeschger events), observed near-globally, were comparable in both rate and magnitude to current and projected 21st century climate warming and therefore provide a relevant constraint on feedback strength. Here we use these events to quantify the centennial-scale feedback strength of CO, CH and NO by relating global mean temperature changes, simulated by an appropriately forced low-resolution climate model, to the radiative forcing of these greenhouse gases derived from their concentration changes in ice-core records. We derive feedback estimates (95% CI) of 0.155 ± 0.035 W m K for CO, 0.114 ± 0.013 W m K for CH and 0.106 ± 0.026 W m K for NO. This indicates that much lower or higher estimates, particularly some previously published values for CO, are unrealistic.