Ocean Acidification: A Planetary Boundary Already Crossed

Updated September 2025

For over 200 years since the Industrial Revolution, atmospheric carbon dioxide (CO₂) levels have increased due to the burning of fossil fuels and land-use changes. The ocean absorbs about 30% of this CO₂, leading to higher CO₂ levels in seawater. This absorption triggers chemical reactions that increase hydrogen ion concentration, making the water more acidic and reducing carbonate ion availability.

Carbonate ions are crucial for building shells and skeletons in marine organisms like oysters, clams, sea urchins, corals, and plankton. Reduced carbonate ion levels hinder these organisms' ability to maintain their structures. More acidic waters also impair the behaviour of non-calcifying organisms such as some fish, reducing their ability to detect predators and threatening entire food webs. These changes ripple outwards into human societies, as many coastal communities rely on healthy oceans for food, livelihoods, and cultural identity.

The Planetary Boundary Breach

Recent research by Findlay et al. (2025) confirms that ocean acidification has now pushed Earth beyond the safe operating space defined in the Planetary Boundary Framework. By 2020, evidence showed that both surface and subsurface oceans had crossed into the “uncertainty zone” for the aragonite saturation state — the critical threshold for many calcifying organisms. This represents not just a localised concern but a global tipping point, with cascading consequences for marine biodiversity and human wellbeing.

Supporting studies reinforce this urgency.

  • Doney et al. (2009) described ocean acidification as “the other CO₂ problem,” stressing its capacity to drive wholesale shifts in seawater chemistry.

  • Orr et al. (2005) modelled the century-scale impacts, projecting serious declines in calcifying organisms under current emission trajectories.

  • Hoegh-Guldberg & Bruno (2010) warned of coral reef collapse as resilience is eroded by declining pH, with devastating biodiversity consequences.

  • Kleypas et al. (1999) showed how coral reef development depends on tight environmental limits, including carbonate saturation, beyond which reef ecosystems cannot persist.

Broader Implications

Ocean acidification is not just a marine science issue — it links directly to climate, food security, and planetary health. Gattuso et al. (2015) estimated the immense economic costs tied to collapsing fisheries, reef degradation, and biodiversity loss. Ribas-Ribas et al. (2020) documented how coral reef ecosystems are especially vulnerable, underlining the compounded risks when warming and acidification occur together.

Together, these studies show that acidification is not an isolated process but one entangled with climate change and biodiversity decline. It represents a systemic risk that will intensify unless fossil fuel emissions are rapidly reduced.

Why It Matters for Us

For Australians, the impacts are already visible: coral bleaching and acidification threaten the Great Barrier Reef, shellfish industries face stress from changing water chemistry, and marine ecosystems are becoming less resilient. This planetary boundary breach is a clear signal — action on carbon emissions, ecosystem protection, and sustainable fisheries cannot wait.

 

References

  • Doney, S. C., Fabry, V. J., Feely, R. A., & Baldwin, R. J. (2009). Ocean acidification: The other CO₂ problem. Oceanography, 22(4), 16–23. https://doi.org/10.5670/oceanog.2009.104

  • Findlay, H. S., Feely, R. A., Jiang, L., Pelletier, G., & Bednaršek, N. (2025). Ocean acidification: another planetary boundary crossed. Global Change Biology, 31. https://doi.org/10.1111/gcb.70238

  • Gattuso, J. P., Magnan, A., Bille, R., Cheung, W. W. L., Ebi, K. L., & Feely, R. A. (2015). A global estimate of the economic burden of ocean acidification. Oceanography and Marine Biology: An Annual Review, 53, 31–59. https://doi.org/10.1201/b18983-3

  • Hoegh-Guldberg, O., & Bruno, J. F. (2010). Cause for concern: the ongoing decline of coral reef ecosystems. Science, 328(5980), 1523–1528. https://doi.org/10.1126/science.1189930

  • Kleypas, J. A., McManus, J. W., & Menez, L. A. (1999). Environmental limits to coral reef development: Where do we draw the line? American Zoologist, 39(1), 146–159. https://doi.org/10.1093/icb/39.1.146

  • Orr, J. C., et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437(7059), 681–686. https://doi.org/10.1038/nature04095

  • Ribas-Ribas, M., et al. (2020). Rising atmospheric CO₂ and ocean acidification will affect coral reefs. Nature Climate Change, 10(2), 138–144. https://doi.org/10.1038/s41558-019-0689-9