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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-22
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-22
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Jan 2020

Submitted as: research article | 27 Jan 2020

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This preprint is currently under review for the journal BG.

Increase in ocean acidity variability and extremes under increasing atmospheric CO2

Friedrich A. Burger1,2, Thomas L. Frölicher1,2, and Jasmin G. John3 Friedrich A. Burger et al.
  • 1Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland
  • 2Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 3NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA

Abstract. Ocean acidity extreme events are short-term periods of extremely high [H+] concentrations. The uptake of anthropogenic CO2 emissions by the ocean is expected to lead to more frequent and intense ocean acidity extreme events, not only due to mean ocean acidification, but also due to increases in ocean acidity variability. Here, we use daily output from ensemble simulations of a comprehensive Earth system model under a low and high CO2 emission scenario to isolate and quantify the impact of changes in variability on changes in ocean acidity extremes. We show that the number of days with extreme [H+] conditions for surface waters is projected to increase by a factor of 14 by the end of the 21st century under a high CO2 emission scenario relative to preindustrial levels. The duration of individual events is projected to triple, and the maximal intensity and the volume extent in the upper 200 m to quintuple. Similar changes are projected in the thermocline. At surface, the changes are mainly driven by increases in [H+] seasonality, whereas changes in interannual variability are also important in the thermocline. Increases in [H+] variability and extremes arise predominantly from increases in the sensitivity of [H+] to variations in its drivers. In contrast to [H+] extremes, the occurrence of short-term extremes in low aragonite saturation state due to changes in variability is projected to decrease. An increase in [H+] variability and an associated increase in extreme events superimposed onto the long-term ocean acidification trend will enhance the risk of severe and detrimental impacts on marine organisms, especially for those that are adapted to a more stable environment.

Friedrich A. Burger et al.

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Friedrich A. Burger et al.

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Short summary
While sea water becomes more acidic with the uptake of anthropogenic carbon, it is expected that variations in sea water acidity, for example between seasons, become larger, too. Such an increase in variability leads to an increase in short term extreme events in acidity. We use ensemble simulations of an Earth system model to show that such acidity extreme events are projected to become more frequent, intense, longer lasting, and spatially extent in the future.
While sea water becomes more acidic with the uptake of anthropogenic carbon, it is expected that...
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