Will the Pacific Northwest get wetter or drier as the planet warms? To answer this question, we studied tiny marine fossils from ocean sediments off the Oregon coast that preserve a record of past climate conditions over the last 12,000 years. These microscopic shells contain chemical signatures that tell us about ancient ocean temperatures and the amount of freshwater flowing from the Columbia River into the sea. We developed new statistical methods to ensure we were reading genuine climate signals rather than chemical changes that occurred after the fossils were buried in sediment. Our results show that during past warm periods, the Pacific Northwest was actually drier, not wetter. This happened because when the region warmed, the central North Pacific Ocean cooled, reducing the moisture available for storms that bring rain and snow to the Pacific Northwest. This pattern resembles a well-known climate cycle called the Pacific Decadal Oscillation. Understanding how the Columbia River and regional climate responded to past warming helps us prepare for future climate changes in this economically and ecologically important region.