The curious case of projected 21st-century drying but greening in the American West - PDF

Climate models project significant 21st-century declines in water availability over the American West from anthropogenic warming. However, the physical mechanisms underpinning this response are poorly characterized, as are 
the uncertainties from vegetation’s modulation of evaporative losses. To understand the drivers and uncertainties of future hydroclimate in the American West, a 35-member single model ensemble is used to examine the response 
of summer soil moisture and runoff to anthropogenic forcing. Widespread dry season soil moisture declines occur across the region despite increases in total water-year precipitation and ubiquitous increases in plant water-use 
efficiency. These modeled soil moisture declines are initially forced by significant snowpack losses that directly diminish summer soil water, even in 
regions where water-year precipitation increases. When snowpack priming is 
coupled with a warming- and CO2-induced shift in phenology and increased primary production, widespread increases in leaf area further reduces summer soil moisture and runoff by outpacing decreased stomatal conductance 
from high-CO2. The net effects lead to the co-occurence of both a ‘greener’ and ‘drier’ future across the Western US. Because simulated vegetation exerts a large influence on predicted changes in water availability in the American West, these findings highlight the importance of reducing the substantial uncertainties in the ecological processes increasingly incorporated into numerical Earth System Models. 


REFERENCE

• Mankin, J.S., J.E. Smerdon, B.I. Cook, A.P. Williams and R. Seager, 2017. The curious case of projected 21st-century drying but greening in the American West. J. Climate, 8689-8710, doi:10.1175/JCLI-D-17-0213.1.