Growing seasonal extremes in ocean acidity

Scientists have proposed that marine organisms living in regions with large daily or seasonal swings in environmental conditions should more easily acclimate to slow changes over decades such as those caused by climate change. But that optimism might not hold if such short-term variability were also affected. Indeed, a new study published in the journal Nature Climate Change finds that if atmospheric CO2 continues to increase, the differences in extremes in surface-ocean acidity between summer and winter will roughly double by the end of the century.

That amplified seasonality in acidity is projected to occur in all ocean regions. In the tropics and subtropics, associated impacts on organisms are then likely to worsen during summer when acidity is highest and improve during winter when acidity is lowest; in colder ocean regions, these summer-winter tendencies are reversed. Future projections of these seasonal extremes come from nine Earth System Models that participated in the last Intergovernmental Panel on Climate Change Assessment Report. Projections were made not only for acidity (hydrogen ion concentration) but also for a saturation index that indicates how suitable conditions are for calcification, a process by which corals and shell builders produce hard skeletal material. The seasonal amplitude of that index (the aragonite saturation state) was found to generally decline as atmospheric CO2 increases. With time this could affect the ability of shell forming organisms to grow, with summer seawater conditions becoming less suitable for growth over most of the ocean.

Previous studies of future changes in ocean acidification have focused largely on annual-average conditions. They show that as atmospheric CO2 continues to increase, the annual-average surface-ocean acidity will also increase while the saturation index will decline. But they fail to consider impacts on winter and summer extremes. “This is the first study to project future changes to the seasonal cycle of ocean acidity across the global ocean,” said Lester Kwiatkowski, lead author of the study from IPSL in Paris.

For further insight, the authors compared their model projections of seasonal changes in acidity to potential changes in daily variability using observations. They used previously published, continuous pH measurements collected at two nearby sites in the Bay of Naples. One site served as a present-day reference, while the other had acidity and CO2 levels expected for the end of the century, being affected by volcanic CO2 bubbling up naturally from the seafloor. They found that the day-night difference in acidity at the perturbed site was roughly double that at the reference site, consistent with results from the model projections for the seasonal amplification.

When asked about the reliability of the projections, coauthor James Orr said, “The 9 models are diverse but give consistent results, while seasonal and daily amplifications are similar, so the results appear on target.” Kwiatkowski added, “The long-term challenge will be to grasp the aggregate impact of these future changes in the seasonality of seawater chemistry on marine communities.”

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

Laboratoire des Sciences du Climat et de l’Environnement (LSCE), IPSL, CEA/CNRS/UVSQ, 29 January 2018.


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