CO2-plant effects do not account for the gap between dryness indices and
projected dryness impacts in CMIP5 or CMIP6
Abstract
Recent studies have found that terrestrial dryness indices like the
Palmer Drought Severity Index, Standardized Precipitation
Evapotranspiration Index, and Aridity Index calculated from climate
model projections are mostly negative, implying a drier land surface
with future warming. Yet, the same models’ prognostic runoff and bulk
soil moisture projections instead feature regional signals of varying
sign, suggesting that the dryness indices could overstate climate
change’s direct impacts. Observed trends also show this “index-impact
gap.” Most studies have attributed this gap to the indices’ omission of
CO2-driven stomatal closure. However, here we show that the index-impact
gap is still wide even in model experiments that switch off CO2 effects
on plants. In these simulations, mean PDSI, Aridity Index, and SPEI
still decline broadly with warming, while mean runoff and bulk soil
moisture still respond more equivocally. This implies that CO2-plant
effects are not the dominant or sole reason for the index-impact gap.