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 CO2, CH4 and N2O 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 (expressed as dimensionless gain) of 0.14 +/- 0.04 for CO2, 0.10 +/- 0.02 for CH4, and 0.09 +/- 0.03 for N2O. This indicates that much lower or higher estimates of gains, particularly some previously published values for CO2, are unrealistic. The centennial-scale climate feedback strength of greenhouse gases is not consistent with the highest and lowest estimates from CMIP models according to feedback strength estimates based on gas concentrations in ice cores during Dansgaard-Oeschger events