Increase in ocean acidity variability and extremes under increasing atmospheric CO<sub>2</sub>

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.

Compound high-temperature and low-chlorophyll extremes in the ocean over the satellite period

“…MHWs in the low latitudes also have critical impacts on foundation species such as corals, seagrass, and kelp (Smale et al, 2019). In the high latitudes, where biological production is often light-limited (McClain, 2009), MHWs may be beneficial for some species as long as MHWs are not very abrupt, prolonged, or compounded with other stressors over time (Cavole et al, 2016;Walsh et al, 2018). On the other hand, low chlorophyll, when indicating lower net primary production, results in lower food supply in all oceanic regions with harmful effects on marine biology.…”

“…In addition, phytoplankton includes a diverse range of different species that may respond differently to MHWs. For example, both the phytoplankton and zooplankton community composition have changed from larger species to smaller species during the northeast Pacific 2013-2015 MHW (Cavole et al, 2016), resulting in less energy available for the food web. While some species benefited from the compound MHW and LChl event (e.g., rockfish, subtropical copepods, tuna, and orcas), the mortality of many other species substantially increased (subarctic copepods, crabs and mussels, sea birds, seals, sea lions, and whales).…”

“…Compound events can severely impact marine ecosystems, especially when the hazards act synergistically. While MHWs may enhance mortality of some marine organisms, low productivity also threatens marine ecosystems that rely on phytoplankton as the base of their food web (Cavole et al, 2016). In recent years, interest in compound events has evolved into a rapidly growing research field (Zscheischler et al, 2020).…”

“…The compound high-temperature and low-chlorophyll and low-nutrient event had severe consequences for marine life (Cavole et al, 2016). Ecosystem impacts included low primary productivity (Whitney, 2015), extreme mortality and reproductive failure of sea birds (Jones et al, 2018;Piatt et al, 2020), mass strandings of whales in the western Golf of Alaska and of sea lions in California, and changes in species distribution in favor of warm-water species (Cavole et al, 2016;Cheung and Frölicher, 2020). These changes in biomass and species distribution further impacted socio-economically important fisheries (Cheung and Frölicher, 2020).…”

Characterizing Mean and Extreme Diurnal Variability of Ocean CO2 System Variables Across Marine Environments

“…In the ocean, assessment of extremes has focused on marine heat waves, their increased frequency during the last 30 years, and their projected dramatic increase during this century (Frölicher et al., 2018; Hobday et al., 2016). In contrast, extremes in ocean CO 2 system variables on similarly short timescales are only beginning to be assessed (Burger et al., 2020) although changes in their mean states and seasonal variations are well understood. Here, to bridge the time gap, we have assessed observed diurnal variability of ocean CO 2 system variables, comparing its drivers to those of seasonal variability.…”