A 2,623-year temperature reconstruction from the Colorado Rocky Mountains reveals warm and cool droughts.

Recent hydrological drought in the US Southwest has occurred under exceptional warming, intensifying water deficits in the Colorado River and Rio Grande basins. Yet the long-term role of temperature in modulating severe and persistent droughts remains poorly understood because regional temperature reconstructions are scarce. We present a 2,623-y June-September minimum temperature reconstruction from the southern Colorado Rocky Mountains based on Pinus aristata tree rings, spanning 600 BCE to 2022 CE. The reconstruction reveals decadal-to-multidecadal variability but limited expression of sustained hemispheric climate phases. In contrast, late 20th and early 21st century warming is unprecedented in magnitude, persistence, and spatial coherence across western North America. By pairing this reconstruction with a continuous 2,000-y reconstruction of upper Colorado River streamflow, we evaluate temperature conditions during major droughts. The most severe and persistent drought of the Common Era, during the 2nd century CE, developed under a persistent cool anomaly, indicating that extreme hydrologic deficit can arise from precipitation forcing alone. Over the last two millennia, warm droughts were more frequent than cool droughts (56% vs. 44%), but both produced comparable cumulative flow deficits. In contrast to the conventional characterization of droughts as hot and dry, these findings demonstrate that extreme droughts developed under both cool and warm regimes and suggest that natural climate variability in this region can produce extremely persistent drought without the added influence of anthropogenic warming.