Abstract
Abstract. 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.
Highlights
Since the beginning of the industrial revolution, the ocean has absorbed about a quarter of the carbon dioxide (CO2) released by human activities through burning fossil fuel and altering land use (Friedlingstein et al, 2019)
The simulations used in this study were made with the fully coupled carbon–climate Earth system model developed at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL ESM2M) (Dunne et al, 2012, 2013)
When using the fixed preindustrial 99th and 1st percentiles to define extreme events in [H+] and A, respectively, large increases in the number of days with [H+] and A extremes are projected over the 1861–2100 period in both low- and high-CO2-emission scenarios (Figs. 4 and A1)
Summary
Since the beginning of the industrial revolution, the ocean has absorbed about a quarter of the carbon dioxide (CO2) released by human activities through burning fossil fuel and altering land use (Friedlingstein et al, 2019). When CO2 dissolves in seawater, it forms carbonic acid that dissociates into bicarbonate ([HCO−3 ]), releasing hydrogen ions ([H+]) and thereby reducing pH (pH = −log([H+])). The rise in [H+] is partially buffered by the conversion of carbonate ions ([CO23−]) to [HCO−3 ]. The associated decline in [CO23−] reduces the calcium carbonate saturation state = [Ca2+] [CO23−]/ [Ca2+] [CO23−]. I.e., the product of calsat cium and carbonate ion concentrations relative to the product at saturation. Undersaturated waters with < 1 are corrosive for calcium carbonate minerals. Each type of calcium carbonate mineral has its individual saturation state due to different solubilities, e.g, C for calcite and A for arag-
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