The Other CO2 Problem: Ocean Acidification as a Breached Planetary Boundary

Executive Summary

As the ocean absorbs roughly a quarter of human carbon dioxide emissions, its chemistry changes: dissolved CO2 forms carbonic acid, lowering pH and reducing the carbonate ions that shell-forming organisms need to build skeletons and shells. In 2025, ocean acidification became the seventh of nine planetary boundaries to be formally breached. It threatens coral reefs, fisheries, and coastal economies — yet it is consistently overshadowed by climate warming, receiving a tiny fraction of attention and funding.

The Scale of the Problem

Ocean surface pH has fallen about 0.1 units since the Industrial Revolution — roughly a 30% increase in acidity. This affects calcifying organisms (corals, mollusks, certain plankton) at the base of marine food webs, with cascading risks to fisheries that billions of people depend on for protein and livelihoods. Coral reefs, already stressed by warming, face compounding harm from acidification.

A Distinct Problem from Climate Warming

Acidification is a separate carbon-chemistry pathway from temperature rise. It is its own planetary boundary, driven directly by CO2 dissolution rather than by atmospheric heat-trapping, and it persists even under mitigation strategies focused narrowly on temperature (for example, solar geoengineering would do nothing for ocean pH). This is why it merits distinct attention rather than absorption into general climate work.

Why This Is Hard (Low Tractability)

The root cause is atmospheric CO2, so the fundamental fix is the same hard, slow decarbonization required for climate change. Local and regional mitigation — ocean alkalinity enhancement, hatchery water buffering for shellfish aquaculture, marine carbon dioxide removal (mCDR) — is nascent, unproven at scale, and carries its own ecological uncertainties.

The Neglectedness Gap

Ocean-focused climate solutions receive under 0.05% of global philanthropic funding, and acidification specifically is repeatedly flagged as an under-recognized crisis. Its high neglectedness is a key part of why it warrants consideration despite its dependence on the broader decarbonization agenda.

Tractable Directions

1. Monitoring infrastructure to track pH changes and biological impacts.
2. Research into ocean alkalinity enhancement and other mCDR approaches, with rigorous ecological safeguards.
3. Local adaptation for vulnerable fisheries and aquaculture (e.g., shellfish hatchery buffering).
4. Integration with decarbonization advocacy as the ultimate solution.

Recommendations

1. Fund acidification monitoring and impact research as an under-resourced public good.
2. Support careful, safeguarded research into ocean-based CO2 removal.
3. Help vulnerable coastal and aquaculture communities adapt.
4. Keep decarbonization central — it is the only durable fix.

Further Reading

• NOAA Pacific Marine Environmental Laboratory / Ocean Acidification Program
• US EPA, "Ocean Acidification"
• IPCC Special Report on the Ocean and Cryosphere (2019)
• Stockholm Resilience Centre, Planetary Boundaries (2025 update)