Scaling Potential Evapotranspiration with Greenhouse Warming

3 Potential evapotranspiration (PET) is a supply-independent measure of the evaporative de4 mand of a terrestrial climate, of basic importance in climatology, hydrology, and agriculture. 5 Future increases in PET from greenhouse warming are often cited as key drivers of global 6 trends toward drought and aridity. The present work computes recent and business-as-usual7 future Penman-Monteith PET fields at 3-hourly resolution in 13 modern global climate mod8 els. The %-change in local annual-mean PET over the upcoming century is almost always 9 positive, modally low double-digit in magnitude, usually increasing with latitude, yet quite 10 divergent between models. 11 These patterns are understood as follows. In every model, the global field of PET %12 change is found to be dominated by the direct, positive effects of constant-relative-humidity 13 warming (via increasing vapor deficit and increasing Clausius-Clapeyron slope.) This direct14 warming term accurately scales as the PET-weighted (warm-season daytime) local warming, 15 times 5-6% per degree (related to the Clausius-Clapeyron equation), times an analytic factor 16 ranging from about 0.25 in warm climates to 0.75 in cold climates, plus a small correction. 17 With warming of several degrees, this product is of low double-digit magnitude, and the 18 strong temperature dependence gives the latitude dependence. Similarly, the inter-model 19 spread in the amount of warming gives most of the spread in this term. Additional spread 20 in the total change comes from strong disagreement on radiation, relative-humidity, and 21 windspeed changes, which make smaller yet substantial contributions to the full PET %22 change fields. 23