Increase in ocean acidity variability and extremes under increasing atmospheric CO 2

[摘要] Ocean acidity extreme events are short-term periods of relatively high [ H + ] concentrations. The uptake of anthropogenic CO 2 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- CO 2 -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- CO 2 -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- CO 2 -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.

[学科分类] 大气科学