Seasonal peak in Arctic Ocean acidity could shift to the summer

**Figure 1 | Simulations of seasonal variation in acidity in the Arctic Ocean.** Orr et al. assessed the seasonal cycle of the partial pressure of CO₂ (pCO₂, which correlates with seawater acidity) in the Arctic Ocean, using simulations from a set of Earth-system models. The simulated data are plotted as the monthly anomaly — the difference between average monthly pCO₂ and the annual average, measured in microatmospheres. Currently, pCO₂ peaks around April, but declines when sea ice melts, reaching a minimum in the summer months when marine phytoplankton consume dissolved CO₂ to grow; darker lines indicate periods of peak growth. Future global warming (simulated data are for 2091 to 2100) causes early melting of sea ice and blooming of phytoplankton, resulting in an earlier seasonal minimum of pCO₂. However, pCO₂ then reaches a maximum in the summer months, as a consequence of the high summer ocean temperatures. The combination of high temperatures and high acidity in the summer could be devastating for marine ecosystems.

The global ocean is gradually acidifying on multidecadal timescales. This acidification occurs when carbon dioxide generated by human activities is absorbed by the ocean, and produces conditions in which many marine organisms cannot thrive. Writing in Nature, Orr et al.¹ present global simulations suggesting that future warming in the Arctic Ocean will cause CO₂ levels to peak seasonally in surface waters in the summer, implying that climate change will further accelerate ocean acidification. The resulting increase in acidification would double down on the already heat-stressed ecosystem, with effects that could creep up the food web — further challenging the food security, culture and well-being of Indigenous peoples in the Arctic.

Ocean acidification varies depending on local environmental conditions and processes. For example, acidification of Arctic waters is enhanced by the freshwater input from melting sea ice, precipitation and rivers². The partial pressure of CO₂ (pCO₂, which quantifies the pressure generated by CO₂ dissolved in seawater, but which can be used as a broad measure of how much CO₂ is dissolved) also varies naturally across days, seasons, years and even decades because it depends on a mixture of biological and physical processes.

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Buschman V. Q. & Hauri C., 2022. Seasonal peak in Arctic Ocean acidity could shift to the summer. Nature 610: 36-37. Article.

Ocean Acidification