Abstract Author: Eleanor J Burke
Abstract Title: Quantifying the impact of plant response to increased CO2 on drought
Abstract: Increasing the carbon dioxide (CO2) concentration in the atmosphere influences the climate system both through radiative forcing and via its impact on plant physiology – physiological forcing. Plants respond to increasing CO2 by decreasing their stomatal conductance. This results in a reduction of transpiration and an increase in water availability locally at the land surface. These changes also feedback into the climate system which results in additional changes at the land surface. The impact of plant physiological forcing on changes in the likelihood of drought on doubling atmospheric CO2 was quantified by comparing two ensembles of the Hadley Centre Climate model: a 48-member ensemble which includes plant physiological response to increased CO2 and a 37-member ensemble with no response. Drought was defined using four different indices: the Palmer Drought Severity Index (PDSI) which is calculated offline driven by output from the climate model; the Standardized Precipitation Index (SPI) which is based solely on precipitation; and indices based on runoff and soil moisture availability calculated from within the climate model. Feedbacks on the precipitation as a result of physiological forcing are either positive or negative, depending on the region. Therefore physiological forcing can cause either an increase or decrease of SPI-based drought. In addition to feedbacks on the precipitation, physiological forcing has a positive feedback on the atmospheric demand. PDSI-based drought increases as a result of the increase in atmospheric demand. However, neither the SPI nor the PDSI drought indices take into account the increased water availability at the land surface, and will both over-estimate any change in drought under increased CO2. Physiological forcing causes a small reduction of the change in drought defined using soil moisture availability or runoff from within the climate model. This highlights a potentially significant impact of plant physiological forcing when projecting future changes in drought.