Carbon dioxide induced stomatal closure increases radiative forcing via a rapid reduction in low cloud

We performed an ensemble of twelve five‐year experiments using a coupled climate‐carbon‐cycle model with scenarios of prescribed atmospheric carbon dioxide concentration; CO 2 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 CO 2 on plant stomatal conductance, leading to adjustment in the shortwave cloud radiative effect over land, due to a reduction in low cloud cover. This causes a 10% enhancement to the radiative forcing due to CO 2 , which leads to an increase in the equilibrium warming of 0.4 and 0.7 K for doubling and quadrupling. The implications for calibration of energy‐balance models are discussed.