Abstract
Abstract. Anthropogenic warming has been projected to increase global drought for the 21st century when calculated using traditional offline drought indices. However, this contradicts observations of the overall global greening and little systematic change in runoff over the past few decades and climate projections of future greening with slight increases in global runoff for the coming century. This calls into question the drought projections based on traditional offline drought indices. Here we calculate a widely used traditional drought index (i.e., the Palmer Drought Severity Index, PDSI) using direct outputs from 16 Coupled Model Intercomparison Project Phase 5 (CMIP5) models (PDSI_CMIP5) such that the hydrologic consistency between PDSI_CMIP5 and CMIP5 models is maintained. We find that the PDSI_CMIP5-depicted drought increases (in terms of drought severity, frequency, and extent) are much smaller than that reported when PDSI is calculated using the traditional offline approach that has been widely used in previous drought assessments under climate change. Further analyses indicate that the overestimation of PDSI drought increases reported previously using the PDSI is primarily due to ignoring the vegetation response to elevated atmospheric CO2 concentration ([CO2]) in the traditional offline calculations. Finally, we show that the overestimation of drought using the traditional PDSI approach can be minimized by accounting for the effect of CO2 on evapotranspiration.
Highlights
Drought is an intermittent disturbance of the water cycle that has profound impacts on regional water resources, agriculture, and other ecosystem services (Sherwood and Fu, 2014)
By taking meteorological outputs from climate model projections as the inputs to offline drought indices/hydrological impact models, numerous studies have projected increases in future drought, in terms of severity, frequency, and extent, mainly as a consequence of warming associated with anthropogenic climate change
Several recent studies have demonstrated that the drying bias in the offline calculated E trend is primarily due to neglecting the impact of increasing atmospheric CO2 concentration ([CO2]) on the water use efficiency of vegetation (Lemordant et al, 2018; Milly and Dunne, 2016, 2017; Roderick et al, 2015; Swann et al, 2016; Yang et al, 2019). This vegetation[CO2] response only impacts transpiration, not soil evaporation, interception from vegetation surfaces or sublimation in snow environments; it should be noted that transpiration dominates (∼ 65 %; note that a transpiration over an evapotranspiration ratio of 0.41 ± 0.11 is estimated by the Coupled Model Intercomparison Project Phase 5 – CMIP5 – models) global terrestrial evapotranspiration (Lian et al, 2018; Zhang et al, 2016)
Summary
Drought is an intermittent disturbance of the water cycle that has profound impacts on regional water resources, agriculture, and other ecosystem services (Sherwood and Fu, 2014). Several recent studies have demonstrated that the drying bias in the offline calculated E trend is primarily due to neglecting the impact of increasing atmospheric CO2 concentration ([CO2]) (and its resultant vapor pressure deficit increase) on the water use efficiency of vegetation (Lemordant et al, 2018; Milly and Dunne, 2016, 2017; Roderick et al, 2015; Swann et al, 2016; Yang et al, 2019) This vegetation[CO2] response only impacts transpiration, not soil evaporation, interception from vegetation surfaces or sublimation in snow environments; it should be noted that transpiration dominates (∼ 65 %; note that a transpiration over an evapotranspiration ratio of 0.41 ± 0.11 is estimated by the Coupled Model Intercomparison Project Phase 5 – CMIP5 – models) global terrestrial evapotranspiration (Lian et al, 2018; Zhang et al, 2016).
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