A Link Between Hydroclimate Variability and Biomass Burning During the Last Millennium in the Interior Pacific Northwest.

We present oxygen isotope and charcoal accumulation records from two lakes in eastern Washington that have sufficient temporal resolution to quantitatively compare with tree‐ring records and meteorological data. Hydroclimate reconstructions from tree‐rings and lake sediments show close correspondence after accounting for seasonal‐ to centennial‐ scale temporal sensitivities. Carbonate δ18O measurements from Castor and Round lakes reveal that the Medieval Climate Anomaly (MCA) experienced wetter November‐March conditions than the Little Ice Age (LIA). Charcoal records from Castor, Round, and nearby lakes show elevated fire activity during the LIA compared to the MCA. Increased multidecadal hydroclimate variability after 1250 CE is evident in proxy records throughout western North America. In the Upper Columbia River Basin, multidecadal wet periods during the LIA may have enhanced fuel loads that burned in subsequent dry periods. A notable decline in biomass burning occurred with Euro‐American settlement in the late nineteenth century. Plain Language Summary: Paleoclimate reconstructions are based on proxy records that are sensitive to particular aspects of the climate system. We compare high‐resolution lake sediment records with nearby tree‐ring chronologies and instrumental data to explore hypotheses about climate variability. Here, we confirm that lake‐sediment oxygen isotope ratios in the Upper Columbia River Basin are primarily sensitive to cool‐season precipitation, whereas many tree‐ring records are primarily sensitive to warm‐season conditions. When lake records are compared with winter‐sensitive tree‐ring width chronologies, the reconstructions are similar. Oxygen isotopes indicate wet and stable winter conditions during the Medieval Climate Anomaly (900–1250 CE) and variable but generally dry conditions during the Little Ice Age (1450–1850 CE). Wet periods of fuel buildup, followed by drought, may have driven greater fire activity during the LIA. An abrupt decline in fire begins occurred Euro‐American settlement, after the 1850s, and is attributed to loss of fuel connectivity and less ignition. Key Points: Sediment δ18O records show heightened hydroclimate variability during the Little Ice AgeDiscrepancies between tree rings and lake sediments from the Pacific Northwest stem from different seasonal and temporal sensitivitiesCharcoal records in the region indicate enhanced fire activity during periods of higher multi‐decadal hydroclimate variability [ABSTRACT FROM AUTHOR]