Ocean acidification: what impact will more extreme seasonal variations have?

According to a study carried out by researchers at the LSCE (CEA-CNRS-UVSQ), by the end of the twenty-first century, seasonal changes in ocean acidification (pH) will be amplified, possibly even doubling.

The seasonality of pH levels, which, until now, was thought to be favorable to marine organisms’ ability to adapt to global warming, may in fact prove to be detrimental. This means that more consideration must now be given to integrating this parameter in climate models. The results of the study will be published in Nature Climate Change on January 29, 2018.

Nine earth system models, two of which have been produced in France, show that variations in pH levels between summer and winter are set to increase in all ocean regions in the course of the twenty-first century. In tropical and subtropical regions, marine organisms are exposed during the summer to an increase in acidity related to the seasonal rise in temperatures. In colder ocean regions, the opposite is the case, with the dominant mechanisms being photosynthesis in the summer (CO2 sink) and a decline in organic matter in the winter (CO2 source).

Until now, scientists thought that this seasonal variability was favorable to organisms’ ability to adapt to more long-term changes, including increased ocean acidification caused by increased levels of dissolved CO2 associated with climate warming. Now however, they have shown that amplified seasonal fluctuations will have a negative effect on this acclimation, thus challenging the previous relative optimism.

The researchers compared their simulations of seasonal changes in ocean acidification with daily measurements (daytime and night-time) of variations in pH levels in a representative seawater reservoir. To this end, they used pH measurements taken continuously at two marine sites near the Bay of Naples, one representing the ocean as it is now and the other representing the ocean as it is expected to be in 2100, in which volcanic activity causes a localized increase in dissolved CO2. They found that day-night cycle differences in acidity at the disturbed site are approximately double those at the benchmark site, thus matching their simulations.

These models were also used to make predictions regarding an indicator for “calcification”, the process whereby coral and shell-forming organisms produce skeletal material, demonstrating that, in the majority of oceans, summer conditions may become less favorable for the formation of calcium carbonate shells and coral.


References: Lester Kwiatkowski and James C. Orr, Diverging seasonal extremes for ocean acidification during the twenty-first century, Nature Climate Change, 2018

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