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Arctic precipitation is predicted to increase this century, with dramatic consequences for high-latitude systems. Observations remain spatiotemporally limited, hampering determination of the forcings causing wetter Arctic conditions, although two mechanisms have been proposed: enhanced local evaporation and greater poleward atmospheric moisture transport. Here a subcentennial-resolution multiproxy lake sediment record from western Greenland sheds light on these mechanisms. Cool summers throughout the Northern Hemisphere and in western Greenland 9 to 8 ka are associated with aridity in this region, via reductions in local evaporation and in meridional moisture gradients, which suppressed poleward moisture transport. Summers became more humid starting 8.1 ka, mainly due to increased evaporation from warmer Arctic seas but also to increased poleward moisture transport caused by hemispheric warming. This record provides independent support for predictions of both enhanced local evaporation and increased poleward moisture transport causing wetter Arctic summers in step with global ocean and atmosphere warming.

Plain Language Summary As the Arctic warms, it is getting wetter. This change can amplify warming worldwide by causing more plants to grow and decompose, releasing heat-trapping gases into the atmosphere. Sparse modern weather records in the Arctic make it difficult to pinpoint the forces causing increased rainfall, but scientists are debating two theories: (1) More water evaporates from warm, ice-free Arctic seas, and then falls locally as precipitation, or (2) as Earth warms, humidity rises more at lower latitudes, creating an imbalance that draws moist air up into the drier Arctic. Our research turns to history for insights. We show that in western Greenland, summers became cooler and drier about 9,000 years ago, coinciding with a drop in moisture imported from lower latitudes. Then, around 8,000 years ago, the region warmed rapidly and summers got wetter. A rise in both local evaporation and incoming moisture from lower latitudes may have fueled this change, according to our interpretation of geologic records. Our study suggests that both processes may contribute to a future, wetter Arctic. In addition, we advance scientific inquiry by using a recently developed technique, analysis of the hydrogen isotopes of ancient precipitation, to examine prehistoric humidity and precipitation trends that previously eluded investigation.