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Abstract Long-term ocean time series have proven to be the most robust approach for direct observation of climate change processes such as Ocean Acidification. The California Cooperative Oceanic Fisheries Investigations (CalCOFI) program has collected quarterly samples for seawater inorganic carbon since 1983. The longest time series is at CalCOFI line 90 station 90 from 1984–present, with a gap from 2002 to 2008. Here we present the first analysis of this 37- year time series, the oldest in the Pacific. Station 90.90 exhibits an unambiguous acidification signal in agreement with the global surface ocean (decrease in pH of −0.0015 ± 0.0001 yr⁻¹), with a distinct seasonal cycle driven by temperature and total dissolved inorganic carbon. This provides direct evidence that the unique carbon chemistry signature (compared to other long standing time series) results in a reduced uptake rate of carbon dioxide (CO₂) due to proximity to a mid-latitude eastern boundary current upwelling zone. Comparison to an independent empirical model estimate and climatology at the same location reveals regional differences not captured in the existing models. 

Abstract. The number and quality of ocean pH measurements have increasedsubstantially over the past few decades such that trends, variability, andspatial patterns of change are now being evaluated. However, comparing pHchanges across domains with different initial pH values can be misleadingbecause a pH change reflects a relative change in the hydrogen ionconcentration ([H+], expressed in mol kg−1) rather than anabsolute change in [H+]. We recommend that [H+] be used inaddition to pH when describing such changes and provide three examplesillustrating why.