PNAS:利用植物蒸腾作用改善干旱预测
美国华盛顿西雅图大学一项研究提出，在大气二氧化碳(CO2)不断升高的情况下植物水流失的减少，这样的结果可能会带来比目前指标预测值更小的未来干旱压力。植物开放叶片气孔，吸收二氧化碳，但是这个过程导致水同时流失到了大气中。科研人员假定大气二氧化碳水平的增加可能减少与碳摄取有关的植物水流失，这可能使土壤湿度增加，而且减少干旱。为了预测未来蒸腾作用的变化如何可能改变干旱压力，ABIgail Swann及其同事比较了来自没有纳入二氧化碳对蒸腾作用影响的帕尔默干旱指数的干旱压力预测与考虑了二氧化碳强迫的蒸腾作用变化的干旱指数。帕尔默干旱指数预测了未来干旱压力将很可能在全球超过70%的陆地面积上增加，但是对于考虑到二氧化碳强迫的蒸腾作用变化的干旱指数来说，这个值下降到了37%。这些指标都表明了干旱压力很可能在北美南部、南美东北部和南欧增加，这些地区的降水量不断减少。然而，考虑到二氧化碳强迫的蒸腾作用变化的指数提示，干旱压力很可能在温带的亚洲和中非地区减少。这组作者说，考虑到植物对大气二氧化碳生理响应的干旱指数可能减少干旱估计的不确定性，并且改善对农业、水资源和野火风险的预测。
原文链接：
plant responses to increasing CO2 reduce estimates of climate impacts on drought severity
原文摘要：
Rising atmospheric CO2 will make Earth warmer, and many studies have inferred that this warming will cause droughts to become more widespread and severe. However, rising atmospheric CO2 also modifies stomatal conductance and plant water use, processes that are often are overlooked in impact analysis. We find that plant physiological responses to CO2 reduce predictions of future drought stress, and that this reduction is captured by using plant-centric rather than atmosphere-centric metrics from Earth system models (ESMs). The atmosphere-centric Palmer Drought Severity Index predicts future increases in drought stress for more than 70% of global land area. This area drops to 37% with the use of precipitation minus evapotranspiration (P-E), a measure that represents the water flux available to downstream ecosystems and humans. The two metrics yield consistent estimates of increasing stress in regions where precipitation decreases are more robust (southern North America, northeastern South America, and southern Europe). The metrics produce diverging estimates elsewhere, with P-E predicting decreasing stress across temperate Asia and central Africa. The differing sensitivity of drought metrics to radiative and physiological aspects of increasing CO2partly explains the divergent estimates of future drought reported in recent studies. Further, use of ESM output in offline models may double-count plant feedbacks on relative humidity and other surface variables, leading to overestimates of future stress. The use of drought metrics that account for the response of plant transpiration to changing CO2, including direct use of P-E and soil moisture from ESMs, is needed to reduce uncertainties in future assessment.
作者：Abigail L.S. Swann
