Four decades of trends and drivers of global surface ocean acidification

The oceans are acidifying in response to the oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere, yet the global-scale progression of this acidification has been poorly documented so far by observations. Here, we fill this gap and use an updated version of the in situ and satellite observation-based product OceanSODA-ETHZ to determine the trends and drivers of the surface ocean aragonite saturation state (Ωar) and pH = –log([H+]) (total scale) over the last four decades (1982–2021). In the global mean, Ωar and pH declined at rates of −0.071 ± 0.006 decade−1 and −0.0166 ± 0.0010 decade−1, respectively, with the errors of the trends largely reflecting the uncertainties in the reconstructed pH and Ωar fields. These global mean trends are driven primarily by the increase in surface ocean concentration of dissolved inorganic carbon (DIC) in response to the uptake of anthropogenic CO2, but moderated by changes in natural DIC. Surface warming enhances the decrease in pH, accounting for ∼15% of the global trend. The long-term trends vary substantially across regions and also differ distinctly between pH and Ωar. The highest trends in pH are found in the high latitudes, while Ωar decreases the fastest in the low latitudes. These regional differences are primarily a consequence of regional differences in the ability of the surface ocean to take up and buffer the anthropogenic CO2. Substantial El Niño-driven interannual variability is superimposed on these trends, with Ωar showing greater variability than pH, resulting in substantially longer time of emergence for Ωar.

Key Points

  • From 1982 through 2021, surface ocean Ωar and pH declined at −0.071 ± 0.006 and −0.0166 ± 0.0010 per decade, respectively
  • The trends vary spatially, with Ωar decreasing most strongly in the low latitudes, while pH decreases the most in the high latitudes
  • The trends are predominantly caused by rising atmospheric CO2, with ocean warming enhancing the pH trend

Plain Language Summary

As the ocean takes up human-made carbon dioxide from the atmosphere, it becomes more acidic, that is, its pH is dropping and so is its saturation state of seawater (Ωar) with respect to aragonite, a type of carbonate mineral. These chemical changes, generally referred to as “Ocean Acidification,” are harming marine organisms. Here, we use an observation-based data set to investigate the trends and drivers of these two important metrics of acidification in the global surface ocean over the last four decades (1982–2021). Our results confirm that pH and Ωar have been declining across the global ocean and that these trends are predominantly driven by the increase in the surface ocean concentration of dissolved inorganic carbon resulting from the accumulation of human-made CO2. We can thus unambiguously demonstrate that it is the human activities-driven emissions of CO2 that is causing these trends in ocean acidification (OA). We also show that the observed ocean warming enhances the decline in pH. Our study provides, for the first time, a global observation-based quantification of the progression and driving factors of OA, which will help to better understand the impact of OA on marine life.

Ma D., Gregor L. & Gruber N., 2023. Four decades of trends and drivers of global surface ocean acidification. Global Biogeochemical Cycles 37: e2023GB007765. doi: 10.1029/2023GB007765. Article.

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