Ocean Acidification Threatens to Undermine Global Carbon Absorption, Study Finds

Carbon dioxide (CO₂) emissions from human activities are not only altering the Earth’s atmosphere but are also rapidly acidifying the oceans. A new study published this week in the Proceedings of the National Academy of Sciences (PNAS) warns that within the next 50 years, this acidification could reduce the ocean’s ability to absorb CO₂ by 10%, primarily due to its harmful effects on phytoplankton.

Phytoplankton—microscopic, single-celled organisms that absorb CO₂ through photosynthesis—play a vital role in the global carbon cycle. However, excessive CO₂ disrupts this natural system. When CO₂ dissolves in seawater, it triggers chemical reactions that increase ocean acidity, potentially harming phytoplankton populations. While some studies have reported increases in phytoplankton abundance, these were mostly conducted in nutrient-rich coastal and high-latitude regions, and may not reflect the situation in tropical and subtropical waters.

To gain a more comprehensive understanding, researchers from Princeton University and Xiamen University conducted a six-year survey across 45 sites in the South China Sea and the North Pacific. Simulating future ocean conditions with atmospheric CO₂ concentrations reaching 700 parts per million (ppm), the team tracked the growth of various phytoplankton species. The findings show that the impact of ocean acidity depends on the availability of nitrate, a key nutrient. Smaller bacterial phytoplankton appeared largely unaffected, while larger eukaryotic phytoplankton experienced up to a 30% reduction in growth rate during summer near the equator.

Based on these results, the researchers estimate that ocean acidification could reduce the annual carbon uptake by eukaryotic phytoplankton in tropical and subtropical waters by as much as 5 trillion kilograms over the next five decades. Further analysis revealed that supplementing nitrate in more acidic waters helped restore phytoplankton growth rates—highlighting a potential mitigating factor amid growing environmental concerns.