Fig. 3: Geochemical and model records of 20th-century acidification in the California Current.

Comparison between modeled historic pCO₂ and measured historic coral pCO₂ in the CCS as a function of modern modeled pCO₂. Lines: The solid 1:1 line indicates no acidification over the past century (historic pCO₂ = modern pCO₂). The magnitude of boreal summertime acidification over the 20th century (ΔpCO₂) produced in the model is dependent on depth, represented by the blue, orange, and green dashed lines to reflect different depth bins, which is consistent with marine biogeochemical processes. Acidification in the upper 25 m (blue line) has largely followed the trend of atmospheric CO₂ (ΔpCO₂ = 80 μatm) over the 20th century (1900–2000). Modeled ΔpCO₂ below 50 m outpaces the rise in atmospheric CO₂ and suggests that the California Current has experienced amplified acidification. As depth increases in the CCS, the model implies that the strength of amplified acidification also increases. Coral data: Squares represent the average coral data from individual locations and align with general patterns of modeled acidification (“Methods” section, Supplementary Data 1). The depths of coral collection are color-coded to match the depth of modeled data. Vertical error bars represent standard errors of the mean. Error bars for locations with only one coral represent the population standard deviation, inferred by locations with 8+ samples (“Methods” section). n = number of corals sampled at each location. The width of the rectangles represents the average modern model standard error of the mean across locations (x = 6 μatm). Only one coral location implies a lower modern pCO₂ than historic pCO₂ (USNM 78638); these corals were collected from a tidepool near Moss Beach, California, where the high variability associated with the intertidal zone likely influenced the δ¹¹B skeletal signatures.