Nine global climate models, including two French models cited in the last IPCC report, establish that acidity variations between summer and winter are expected to increase across all ocean regions in the coming century.

In tropical and subtropical regions, marine organisms are exposed during the summer to an increase in acidity associated with rising seasonal temperatures. The variations are reversed in colder ocean regions, with the dominant mechanisms being photosynthesis in summer (CO₂ sinks) and the degradation of organic matter in winter (CO₂ source).

Until now, scientists believed that seasonal pH variability stimulated the ability of marine organisms to adapt to global warming, in particular to increasing acidity (due to dissolved atmospheric CO₂). They now demonstrate that seasonal fluctuations of greater amplitude will impair their ability to adapt and will have a harmful effect.

The researchers wanted to compare their simulations with measurements of day-night pH fluctuations in seawater. To do this, they used continuous pH measurements taken at two marine sites near the Bay of Naples, one representing the present ocean and the other representing the ocean in 2100. The latter site contains an excess of CO₂ of volcanic origin, which bubbles naturally from the depths. As a result, the day-night difference in acidity of this disrupted site is approximately twice that of the reference site, which is consistent with the seasonal simulations.

Interviewed about the reliability of these projections, LSCE co-author James Orr explains: "The nine models are quite different but give consistent results. The seasonal and daily amplifications are similar. Everything points in the same direction." Lead author Lester Kwiatkowski adds: "The long-term challenge will be to determine the cumulative impact that seasonal seawater chemistry has on marine communities."