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Abstract During the mid‐Holocene (MH: ∼6,000 years Before Present) and Last Interglacial LIG (LIG: ∼129,000–116,000 years Before Present) differences in the seasonal and latitudinal distribution of insolation drove Northern Hemisphere high‐latitude warming comparable to that projected for the end of the 21st century in low emissions scenarios. Paleoclimate proxy records point to distinct but regionally variable hydroclimatic changes during these past warm intervals. However, model simulations have generally disagreed on North American regional moisture patterns during the MH and LIG. To investigate how closely the latest generation of models associated with the Paleoclimate Model Intercomparison Project (PMIP4) reproduces proxy‐inferred moisture patterns during recent warm periods, we compare hydroclimate output from 17 PMIP4 models with newly updated compilations of moisture‐sensitive North American proxy records during the MH and LIG. Agreement is lower for the MH, with models producing wet anomalies across the western United States (US) where most proxies indicate increased aridity relative to the preindustrial period. The models that agree most closely with the LIG proxy compilation display relative wetness in the eastern US and Alaska, and dryness in the northwest and central US. An assessment of atmospheric dynamics using an ensemble of the three LIG simulations that best agree with the proxies suggests that weaker winter North Pacific pressure gradients and steeper summer North Pacific and Atlantic gradients drive LIG precipitation patterns. Our updated compilations and proxy‐model comparisons offer a tool for benchmarking climate models and their performance in simulating climate states that are warmer than present. 

Southwestern North America is currently experiencing a multidecadal megadrought, with severe consequences for water resources. However, significant uncertainty remains about 21st century precipitation changes in this semi-arid region. Paleoclimatic records are essential for both contextualizing current change, and for helping constrain the sensitivity of regional hydroclimate to large-scale global climate. In this paper, we present a new 2.8 Ma to present compound-specific isotopic record from Clayton Valley, the site of a long-lived paleolake in the southern Great Basin. Hydrogen and carbon isotopes from terrestrial plant leaf waxes provide evidence of past shifts in rainfall seasonality as well as ecosystem structure, and help contextualize the formation of this lithium-rich lacustrine basin. Our results suggest that regional hydroclimates underwent a substantial reorganization at the Plio-Pleistocene boundary, especially between 2.6 and 2.0 Ma. In this interval, a reduced latitudinal temperature gradient in the North Pacific likely resulted in a northward shift in storm tracks, and a reduction in winter rainfall over the southern Great Basin. This occurred against a background of increased summer rainfall and a greater accumulation of lithium in the lake basin. Our interpretation is corroborated by a compilation of Plio-Pleistocene north Pacific sea surface temperature records, as well as an isotope-enabled model simulation. Overall, these results suggest that past shifts in rainfall seasonality helped set the stage for the development and dessication of lithium-rich lacustrine deposits.