Long-term trends and anthropogenic forcing of surface ocean carbon storage and acidification.

Abstract

This study investigates long-term trends and anthropogenic influences on surface ocean carbon storage and acidification across the global ocean during 1985-2022, using a high-resolution (0.25° × 0.25°) product reconstructed by a neural-network framework from the Copernicus Marine Service. Global and regional trends in dissolved inorganic carbon (DIC), total alkalinity (TA), pH, surface partial pressure of CO₂ (spCO₂), and air-sea CO₂ flux (fgCO₂) were quantified using linear regression, Mann-Kendall tests, and Sen's slope estimator. Results reveal a significant global DIC increase of 0.75 ± 0.015 μmol kg^-1 yr^-1, corresponding to an annual carbon sink of ∼2.0 PgC yr^-1, alongside a pH decline of -0.00164 ± 0.000034 units yr^-1, indicating intensified acidification. The fgCO₂ rose from 0.32 to 0.72 mol C m^-2 yr^-1, reflecting a strengthened carbon sink (from 1.2 to 2.6 PgC yr^-1), with a temporary decline during 2021-2022 linked to reduced emissions during the COVID-19 pandemic. Spatial analyses highlight pronounced DIC and fgCO₂ increases in the North Atlantic and Southern Ocean, contrasted by persistent CO₂ outgassing in equatorial regions. Spearman's rank correlations and Bai-Perron breakpoint analyses identify structural shifts in carbon dynamics, particularly in 1997-1998 and 2012-2013, associated with accelerated anthropogenic forcing. Declining TA:DIC ratios signal a weakening of oceanic buffering capacity, raising concerns about the future efficiency of marine carbon sequestration. These findings underscore the ocean's critical role in mitigating atmospheric CO₂, while revealing its increasing vulnerability to sustained anthropogenic pressures.