Importance of carbon dioxide physiological forcing to future climate change

Abstract

An increase in atmospheric carbon dioxide (CO(2)) concentration influences climate both directly through its radiative effect (i.e., trapping longwave radiation) and indirectly through its physiological effect (i.e., reducing transpiration of land plants). Here we compare the climate response to radiative and physiological effects of increased CO(2) using the National Center for Atmospheric Research (NCAR) coupled Community Land and Community Atmosphere Model. In response to a doubling of CO(2), the radiative effect of CO(2) causes mean surface air temperature over land to increase by 2.86 +/- 0.02 K (+/- 1 standard error), whereas the physiological effects of CO(2) on land plants alone causes air temperature over land to increase by 0.42 +/- 0.02 K. Combined, these two effects cause a land surface warming of 3.33 +/- 0.03 K. The radiative effect of doubling CO(2) increases global runoff by 5.2 +/- 0.6%, primarily by increasing precipitation over the continents. The physiological effect increases runoff by 8.4 +/- 0.6%, primarily by diminishing evapotranspiration from the continents. Combined, these two effects cause a 14.9 +/- 0.7% increase in runoff. Relative humidity remains roughly constant in response to CO(2)-radiative forcing, whereas relative humidity over land decreases in response to CO(2)-physiological forcing as a result of reduced plant transpiration. Our study points to an emerging consensus that the physiological effects of increasing atmospheric CO(2) on land plants will increase global warming beyond that caused by the radiative effects of CO(2).

Fig. 1.

Changes in latent heat flux, low-level cloudiness, and net solar flux at surface in response to CO₂-radiative forcing, CO₂-physiological forcing, and combined CO₂-radiative and physiological forcing. They are averaged annual mean changes in response to a doubling of atmospheric CO₂ calculated from the last 70-yr results of 100-yr simulations. Hatched areas are regions where changes are not statistically significant at the 5% level using the Student t-test.

Fig. 2.

Changes in surface air temperature, runoff, and near-surface relative humidity in response to CO₂-radiative forcing, CO₂-physiological forcing, and combined CO₂-radiative and physiological forcing. They are averaged annual mean changes in response to a doubling of atmospheric CO₂ calculated from the last 70-yr results of 100-yr simulations. Hatched areas are regions where changes are not statistically significant at the 5% level using the Student t-test. Isolines for temperature changes of 3K (white solid lines) and 4K (white dashed lines) are plotted for the case of CO₂-radiative and combined CO₂-radiative and physiological forcing to highlight the differences between these two.

Fig. 3.

Fraction of total surface warming (warming caused by the combined CO₂-radiative and physiological effects) associated with the physiological forcing of CO₂. The fractional contribution of warming due to CO₂-physiological forcing is calculated as the ratio between changes in surface air temperature in response to the CO₂-physiological forcing and temperature change in response to the combined effect of CO₂-radiative and physiological forcing. These changes are averaged annual mean values in response to a doubling of atmospheric CO₂ calculated from the last 70-yr results of 100-yr simulations. Hatched areas are regions where changes are not statistically significant at the 5% level using the Student t-test.