Increase in ocean acidity variability and extremes under increasing atmospheric CO2

Type Article
Date 2020-09
Language English
Author(s) Burger Friedrich A.1, 2, John Jasmin G.3, Frolicher Thomas L.1, 2
Affiliation(s) 1 : Univ Bern, Phys Inst, Climate & Environm Phys, Bern, Switzerland.
2 : Univ Bern, Oeschger Ctr Climate Change Res, Bern, Switzerland.
3 : NOAA, Geophys Fluid Dynam Lab, Princeton, NJ USA.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2020-09 , Vol. 17 , N. 18 , P. 4633-4662
DOI 10.5194/bg-17-4633-2020
WOS© Times Cited 3

Ocean acidity extreme events are short-term periods of relatively 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 changes in the long-term mean but also due to changes in short-term variability. Here, we use daily mean output from a five-member ensemble simulation of a comprehensive Earth system model under low- and high-CO2-emission scenarios to quantify historical and future changes in ocean acidity extreme events. When defining extremes relative to a fixed preindustrial baseline, the projected increase in mean [H+] causes the entire surface ocean to reach a near-permanent acidity extreme state by 2030 under both the low- and high-CO2-emission scenarios. When defining extremes relative to a shifting baseline (i.e., neglecting the changes in mean [H+]), ocean acidity extremes are also projected to increase because of the simulated increase in [H+] variability; e.g., the number of days with extremely high surface [H+] conditions is projected to increase by a factor of 14 by the end of the 21st century under the high-CO2-emission scenario relative to preindustrial levels. Furthermore, the duration of individual extreme events is projected to triple, and the maximal intensity and the volume extent in the upper 200 m are projected to quintuple. Similar changes are projected in the thermocline. Under the low-emission scenario, the increases in ocean acidity extreme-event characteristics are substantially reduced. At the surface, the increases in [H+] variability are mainly driven by increases in [H+] seasonality, whereas changes in thermocline [H+] variability are more influenced by interannual variability. Increases in [H+] variability arise predominantly from increases in the sensitivity of [H+] to variations in its drivers (i.e., carbon, alkalinity, and temperature) due to the increase in oceanic anthropogenic carbon. The projected increase in [H+] variability and extremes may enhance the risk of detrimental impacts on marine organisms, especially for those that are adapted to a more stable environment.

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