Detectability of Decadal Anthropogenic Hydroclimate Changes over North America

Future hydroclimate changes consist of shifts in mean state resulting from anthropogenic forcing and contributions from natural climate variability. Considering the inherently limited predictability of natural climate variability, our confidence in projections of future hydroclimate changes relies on a robust assessment of anthropogenic shifts in mean state. Assessment of anthropogenic shifts in near-term projections is challenging because the “signal” of anthropogenic changes is modest compared to the “noise” of natural variability; however, this “signal to noise” ratio can be greatly improved in a large model ensemble that contains the same “signal” but different “noise”. Here using multiple large ensembles from two state-of-the-art climate models, we assess the decadal shifts in precipitation-minus-evaporation (PmE) mean state caused by anthropogenic forcing, focusing on North America during 2000~2050.

Anthropogenic forcing is projected to cause significant (against internal climate variability) shifts in PmE mean state relative to the 1950~1999 climatology over 50~70% of North America by 2050. The earliest detectable signals include, during November-April, a moistening over northeastern North America and a drying over southwestern North America and, during May-October, a drying over central North America. The central drying is largely attributable to anthropogenic warming. Changes in submonthly transient eddies account for the northeastern moistening and central drying while monthly atmospheric circulation changes explain the southwestern drying. Despite these significant anthropogenic shifts in PmE mean state, large irreducible uncertainties, caused primarily by atmosphere/land internal dynamics, remain in individual projections and are of substantial relevance for policy planning.