title: Modeling extreme events in ocean acidity and compound extreme events using a comprehensive Earth system model
Creators: name: Burger, Friedrich Anton
Publisher: name: Universität Bern
Year: 2022
Subjects: subject: 530 Physics

description: Anthropogenic CO2 emissions not only cause global warming, but also ocean acidification, i.e., a shift towards higher [H+] and lower pH with the accumulation of anthropogenic carbon in seawater. Ocean acidification is a potential threat for many marine species, in particular for calcifying marine invertebrates like mollusks or echinoderms, which are important components of the marine food web. Seasonal variations in [H+] and oceanic carbonate system variables, such as the aragonite saturation state (ΩA) or the partial pressure of CO2 (pCO2), have been identified as important modes of variability along with the long-term changes under ocean acidification. These variations are also known to change in amplitude under ocean acidification and may hence become more important in the future. In contrast to these annually occurring variations, relatively little is known about the occurrence of rare and extreme departures from normal conditions in [H+] and carbonate system variables. Such extreme events may add substantially to the stress experienced by organisms from the long-term changes under ocean acidification. The global ocean is not only getting more acidic, but it is also gets warmer due to the ocean heat uptake under global warming. These co-occurring changes are of particular concern, since interactions between the environmental stressors potentially aggravate the stress for marine organisms. In consequence, also co-occurring extremes (so-called compound extreme events) in temperature and [H+] may pose a higher risk to vulnerable species than extremes in temperature or [H+] alone.

In this thesis, a global overview on the occurrence and characteristics of extreme events in [H+] and aragonite saturation state (ΩA) is given. We quantify changes in these extreme events under climate change based on large ensemble simulations of the fully-coupled Earth system model GFDL ESM2M. The drivers of the extremes are assessed and the causes of changes in their characteristics are analyzed. Furthermore, we quantify the frequency of compound extreme events in [H+] and temperature and how their frequency may change under ocean warming and acidification.

The introduction (Chapter 1) provides an overview of the global carbon cycle and its perturbation under anthropogenic CO2 emissions. The properties of oceanic carbonate chemistry and acid-base state are presented, and their biogeochemical control processes discussed. Ocean acidification, the shift towards more acidic conditions due to the oceanic uptake of anthropogenic carbon, and its biological implications are also discussed. Finally, extreme events in ocean acidity are introduced and their role as a potential additional stressor for marine organisms is discussed.

Chapter 2 introduces the GFDL ESM2M Earth system model and describes how the large ensemble simulations that are analyzed in this thesis were conducted. It describes how extreme events are defined in this thesis and compares different approaches for defining these events. Furthermore, the metrics that are used to characterize the extreme events are defined.

Chapter 3, which is published in Biogeosciences, analyzes the changes in high-[H+] and low-ΩA extremes under climate change. To do so, we define the extremes relative to fixed preindustrial baselines and relative to shifting-mean baselines. Relative to fixed preindustrial baselines, the mean changes in [H+] and ΩA due to ocean acidification cause both variables to transition to a near-permanent extreme state in the 21th century, at the surface and also at 200m depth. Relative to shifting-mean baselines, increases in [H+] variability cause [H+] extremes to become more frequent, intense, long-lasting, and spatially extended. The increases in [H+] variability are primarily caused by increases in mean inorganic carbon concentrations (CT) that make [H+] more sensitive to variations in its drivers. In contrast, extremes in ΩA are shown to become less frequent when defined relative to shifting-mean baselines, reflecting that ΩA becomes less variable as it declines with ocean acidification.

The frequency of compound extreme events in temperature and [H+] in the surface ocean is assessed in Chapter 4. Based on observation-derived data, we show that these compound events occur frequently in the subtropical oceans, while they are much rarer in the equatorial Pacific and the mid-to-high latitudes. This spatial pattern emerges from the regionally varying importance of temperature and carbon for variations in [H+]. The compound events occur frequently where [H+] variations are primarily driven by temperature, and they are relatively rare where [H+] variations are mainly driven by CT. Based on large ensemble simulations that were conducted with the GFDL-ESM2M model, we show that ocean acidification and ocean warming cause large increases in the frequency of these compound events under fixed baselines. Smaller increases are projected relative to shifting-mean baselines, which primarily arise due to the increases in [H+] variability that are discussed in Chapter 3. Finally, we isolate the effect of changes in the statistical dependence between temperature and [H+] on compound event occurrence, finding that an overall reduction in dependence dampens the increases in event frequency, in particular when defined relative to shifting-mean baselines.

The drivers of surface [H+] extreme events within the GFDL ESM2M model are investigated in Chapter 5. In the subtropics, we find that the buildup of [H+] extreme events is mainly caused by positive anomalies in air-sea heat fluxes, which increase temperature and thus [H+]. These temperature increases are dampened by reductions in convective mixing from the nonlocal KPP parameterization, which normally offsets the air-sea heat loss in the subtropics. In the mid-to-high latitudes, we find positive anomalies in the vertical mixing and diffusion of temperature to be the main driver of the buildup of [H+] extremes, either due to reduced heat loss to the subsurface or due to increased upward heat transport from the subsurface. In the equatorial Pacific, event buildup is mainly driven by advection of CT. The decay of [H+] extreme events is predominantly caused by reductions in CT due to outgassing of carbon to the atmosphere. Furthermore, increased heat losses to the atmosphere in the subtropics and enhanced biological production in the tropics contribute substantially to [H+] extreme event decay.

The main results of Chapters 3 - 5 are summarized and discussed in Chapter 6. Caveats of the analyses are listed and an outlook on potential future projects is given.