A century of change in the California Current: upwelling system amplifies acidification

Predicting the pace of acidification in the California Current System (CCS), a productive upwelling system that borders the west coast of North America, is complex because the anthropogenic contribution is intertwined with other natural sources. A central question is whether acidification in the CCS will follow the pace of increasing atmospheric CO₂, or if climate effects and other biogeochemical processes will either amplify or attenuate acidification. Here, we apply the boron isotope pH proxy to cold-water orange cup corals to establish a historic level of acidification in the CCS and the Salish Sea, an associated marginal sea. Through a combination of complementary modeling and geochemical approaches, we show that the CCS and Salish Sea have experienced amplified acidification over the industrial era, driven by the interaction between anthropogenic CO₂ and a thermodynamic buffering effect. From this foundation, we project future acidification in the CCS under elevated CO₂ emissions. The projected change in pCO₂ over the 21^st century will continue to outpace atmospheric CO₂, posing challenges to marine ecosystems of biological, cultural, and economic importance.