Ocean Acidification in the Planetary Boundaries Framework

The Planetary Boundaries (PB) Framework seeks to identify key Earth system processes that sustain planetary stability and are vulnerable to large-scale perturbations driven by human activities (Richardson et al. 2023). The Planetary Health Check 2025 reports that seven of the nine Planetary Boundaries have already been exceeded, including the recently transgressed boundary of ocean acidification (Sakschewski et al. 2025). Ocean acidification poses substantial risks to marine ecosystems by altering the carbonate system at rates that challenge the capacity of calcifying organisms to adapt to the new conditions. These changes threaten ecosystem functioning, marine carbon sequestration, and the provision of marine ecosystem services. In addition, ocean acidification is associated with a measurable decline in the ocean’s buffer capacity (Müller et al. 2023), which reduces the efficiency of the ocean sink for anthropogenic CO₂ and thereby weakens the ocean’s capacity to mitigate climate change. In this contribution, we examine the rationale underlying earlier assumptions and methodological choices in the assessment of ocean acidification within the PB Framework, and discuss approaches that could improve its representation and evaluation. These include an explicit consideration of subsurface acidification, the consideration of regional variability in the derivation of a global threshold, and the exploration of alternative indicators for evaluating the state and impacts of ocean acidification. We demonstrate how incorporating the best available scientific understanding and the most recent observational evidence into the assessment of the Planetary Boundary of ocean acidification can advance the current methodology and help ensure its scientific robustness and relevance.

 

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Sakschewski, B., Caesar, L., Andersen, L., Bechthold, M., Bergfeld, L., Beusen, A., ... & Rockström, J. (2025). Planetary Health Check 2025: a scientific assessment of the state of the planet. Planetary Boundaries Science (PBScience), 144.

Müller, J. D., Gruber, N., Carter, B., Feely, R., Ishii, M., Lange, N., ... & Zhu, D. (2023). Decadal trends in the oceanic storage of anthropogenic carbon from 1994 to 2014. AGU Advances, 4(4), e2023AV000875.