Analysis of fast feedbacks in coupled climate-carbon experiments in response to 2x and 4xCO2

We performed an ensemble of twelve five-year experiments using a coupled climate-carbon-cycle model with scenarios of prescribed atmospheric carbon dioxide concentration; CO2 was instantaneously doubled or quadrupled at the start of the experiments. Within these five years, climate feedback is not significantly influenced by the effects of climate change on the carbon system. However, rapid changes take place, within much less than a year, due to the physiological effect of CO2 on plant stomatal conductance, leading to adjustment in the shortwave cloud radiative effect over land, and a 10-20% enhancement to the effective radiative forcing due to CO2. The implications for calibration of energy-balance models are discussed.