Tripti Bhattacharya , Ran Feng, Jessica Tierney , Natalie J. Burls, Scott Knapp, Minmin J. Fu

Southwestern North America, like many subtropical regions, is predicted to become drier in response to anthropogenic warming. However, during the Pliocene, when carbon dioxide was above pre-industrial levels, multiple lines of evidence suggest that southwestern North America was much wetter. While existing explanations for a wet Pliocene invoke increases in winter rain, recent modeling studies hypothesize that summer rain may have also played an important role. Here, we present the first direct evidence for an intensified Pliocene monsoon in southwestern North America using leaf wax hydrogen isotopes. These new records provide evidence that the Pliocene featured an intensified and expanded North American Monsoon. Using proxies and isotope-enabled model simulations, we show that monsoon intensification is linked to amplified warming on the southern California margin relative to the tropical Pacific. This mechanism has clear relevance for understanding present-day monsoon variations, since we show that intervals of amplified subtropical warming on the California margin, as are seen during modern California margin heat waves, are associated with a stronger monsoon. Since marine heat waves are predicted to increase in frequency, the future may bring intervals of `Pliocene-like' rainfall that co-exist with intensifying megadrought in southwestern North America, with implications for ecosystems, human infrastructure, and water resources