How ocean acidification will affect marine organisms depends on changes in both the long-term mean and the short-term temporal variability of carbonate chemistry1,2,3,4,5,6,7,8. Although the decadal-to-centennial response to atmospheric CO2 and climate change is constrained by observations and models1, 9, little is known about corresponding changes in seasonality10,11,12, particularly for pH. Here we assess the latter by analysing nine earth system models (ESMs) forced with a business-as-usual emissions scenario13. During the twenty-first century, the seasonal cycle of surface-ocean pH was attenuated by 16 ± 7%, on average, whereas that for hydrogen ion concentration [H+] was amplified by 81 ± 16%. Simultaneously, the seasonal amplitude of the aragonite saturation state (Ωarag) was attenuated except in the subtropics, where it was amplified. These contrasting changes derive from regionally varying sensitivities of these variables to atmospheric CO2 and climate change and to diverging trends in seasonal extremes in the primary controlling variables (temperature, dissolved inorganic carbon and alkalinity). Projected seasonality changes will tend to exacerbate the impacts of increasing [H+] on marine organisms during the summer and ameliorate the impacts during the winter, although the opposite holds in the high latitudes. Similarly, over most of the ocean, impacts from declining Ωarag are likely to be intensified during the summer and dampened during the winter.
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This study was funded by the H2020 CRESCENDO grant (no. 641816), the ERC IMBALANCE-P synergy grant (no. 610028) and the MTES/FRB Acidoscope project. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for the Coupled Model Intercomparison Project (CMIP). For CMIP, the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provided coordinating support and led the development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. To analyse the CMIP5 data, this study benefited from the IPSL Prodiguer-Ciclad facility, which is supported by the National Centre for Scientific Research, the University of Pierre et Marie Curie and Labex L-IPSL, which is funded by the French National Research Agency (no. ANR-10-LABX-0018) and by the European FP7 IS-ENES2 project (no. 312979). We thank B. Le Vu for preliminary discussions.