Global warming increased by the response of land plants to CO2

An increase in atmospheric CO2 influences climate both directly through its radiative effect and indirectly through its physiological effect on land plants. The primary climate effect of CO2 is a result of its absorption of longwave radiation. But increased CO2 reduces plant transpiration, which also causes warming -- a physiological effect on climate. We examine both radiative and physiological effects of increased CO2 in the climate response of the NCAR coupled Community Land and Community Atmosphere Model. In the model, in response to a doubling of CO2, the radiative effect causes a mean land warming of 2.86 ± 0.02 K (±1 standard error) and the physiological effect causes a mean land warming of 0.42 ± 0.02 K. On average, the physiological effect is responsible for 14% of total warming, and is responsible for more than 20% of total warming over 21% of the land surface. Global runoff is the excess of precipitation over evaporation over land. In response to a doubling of CO2, the CO2-radiative effect increases runoff by 5.2 ± 0.6%, primarily by increasing precipitation over the continents. The physiological effect increases global runoff by 8.4 ± 0.6%, primarily a result of diminished evaporation from the continents. Water vapor changes from radiative forcing typically follow a Clausius-Clapeyron relationship. However, water vapor changes associated with warming in response to the CO2-physiological effect are much less than what would be predicted from the Clausius-Clapeyron relationship. Closing plant stomata decrease the water vapor flux to the atmosphere, resulting in lower relative humidity and increased runoff. Our study points to an emerging robust modeling result that the physiological effect of CO2 on land plants will increase global warming beyond what is caused by the radiative effect of CO2 on longwave radiation.