Statewide and regional changes in California kelp over the last century.

At the state level, the total area (-6.9%), carbon biomass (+5.3%), and social costs (+5.3%) of harvestable kelp beds (see Methods) were not considerably different from 1910–1912 to 2014–2016 surveys. These trends, however, obscure stark regional differences that encompass a dramatic shift of California kelp over this period. In central California, kelp increased 57.6%, growing 19.7 km2 and adding an estimated 145.6 kt CO2. In all other regions kelp declined. Most notably, northern California saw 63% declines in kelp amounting to an estimated 8.1 km2 and 63.2 kt CO2 lost. The overall decline in kelp canopy area with a simultaneously estimated increase in kelp carbon biomass over time highlights regional differences in species composition and associated bed density and carbon content. The estimated social cost of kelp carbon follows the biomass trends, and in both periods exceeds $US 100M.