Anthropogenic Carbon Concentrations along the West Coast of North America

The continental shelf waters off the west coast of North America are exposed to water with increasing concentrations of anthropogenic CO₂ (C_anthro) from exchanges with the atmosphere and the shoreward transport and mixing of upwelled water. We used a multiple linear regression approach to determine the spatial variations of C_anthro in the California Current Ecosystem based on cruise data sets from 2007, 2011, 2012, 2013, and 2016. Our results show significant horizontal, vertical, and temporal gradients in C_anthro in surface and subsurface waters along the coast, in which the C_anthro uptake increases at the rate of 0.8–1.2 µmol kg^-1 yr^-1, with the rate decreasing from south to north in surface waters. The rates gradually decrease with depth to values of ~0.3 µmol kg^-1 yr^-1 at depths near 400 m. Corresponding rates of pH_T decrease by an average of about 0.002 yr^-1 in surface waters. In future years, the rates will be driven by CO₂ emissions and the changes in carbon chemistry that reduce the buffer capacity of subsurface waters (i.e., increasing Revelle Factor) with increasing remineralized CO₂, with the largest impacts on pH and CO₂ partial pressure (pCO₂) occurring in the colder waters at higher northern latitudes.