Are there interactive effects of physiological and radiative forcing produced by increased CO_2 concentration on changes of land hydrological cycle?

摘要

Three coupled climate-carbon cycle models including CESM (Community Earth System Model), CanEsm (the Canadian Centre for Climate Modelling and Analysis Earth System Model) and BCC (Beijing Climate Center Climate System Model) were used to estimate whether changes in land hydrological cycle responded to the interactive effects of CO_2-physiological forcing and CO_2-radiative forcing. No signs could be indicated that the interactive effects of CO_2-physiological forcing and CO_2-radiative forcing on the hydrological variables (e.g. precipitation, evapotranspiration and runoff) were detected at global and regional scales. For each model, increases in precipitation, evapotranspiration and runoff (e.g. 0.37,0.18 and 0.25 mm/year~2) were simulated in response to CO_2-radiative forcing (experiment M3). Decreases in precipitation and evapotranspiration (about -0.02 and -0.09 mm/year~2) were captured if the CO_2 physiological effect was only accounted for (experiment M2). In this experiment, a reverse sign in runoff (the increase of 0.08 mm/year~2) in contrast to M3 is presented. All models simulated the same signs across Eastern Asia in response to the CO_2 physiological forcing and radiative forcing: increases in precipitation and evapotranspiration only considering greenhouse effect; reductions in precipitation and evapotranspiration in response to CO_2-physioIogical effect; and enhanced trends in runoff from all experiments. However, there was still a large uncertainty on the magnitude of the effect of transpiration on runoff (decreased transpiration accounting for 8% to 250% of the increased runoff) from the three models. Two models (CanEsm and BCC) attributed most of the increase in runoff to the decrease in transpiration if the CO_2-physiological effect was only accounted for, whereas CESM exhibited that the decrease in transpiration could not totally explain the increase in runoff. The attribution of the CO_2-physiological forcing to changes in stomatal conductance versus changes in vegetation structure (e.g. increased Leaf Area Index) is an issue to discuss, and among the three models, no agreement appeared.