Technical note: Interpreting pH changes

Fassbender, A. J., Orr, J. C., and Dickson, A. G. (2021). Technical note: Interpreting pH changes, Biogeosciences, 18, 1407–1415, https://doi.org/10.5194/bg-18-1407-2021

Summary:

A decline in upper-ocean pH with time is typically ascribed to ocean acidification. A more quantitative interpretation is often confused by failing to recognize the implications of pH being a logarithmic transform of hydrogen ion concentration rather than an absolute measure. This can lead to an unwitting misinterpretation of pH data. This study provides three real-world examples illustrating this and recommends the reporting of both hydrogen ion concentration and pH in studies of ocean chemical change.

The quiet crossing of ocean tipping points

Heinze, C., Blenckner, T., Martins, H., Rusiecka, D., Döscher, R., Gehlen, M., Gruber, N., Holland, E., Hov, Ø., Joos, F., Matthews, J. B. R., Rødven, R., & Wilson, S. (2021). The quiet crossing of ocean tipping points. Proceedings of the National Academy of Sciences, 118(9), https://doi.org/10.1073/pnas.2008478118

Summary:

Anthropogenic climate change profoundly alters the ocean’s environmental conditions, which, in turn, impact marine ecosystems. Some of these changes are happening fast and may be difficult to reverse. The identification and monitoring of such changes, which also includes tipping points, is an ongoing and emerging research effort. Prevention of negative impacts requires mitigation efforts based on feasible research-based pathways. Climate-induced tipping points are traditionally associated with singular catastrophic events (relative to natural variations) of dramatic negative impact. High-probability high-impact ocean tipping points due to warming, ocean acidification, and deoxygenation may be more fragmented both regionally and in time but add up to global dimensions. These tipping points in combination with gradual changes need to be addressed as seriously as singular catastrophic events in order to prevent the cumulative and often compounding negative societal and Earth system impacts. This study highlights four promising options in Earth system management and societal transformation for minimising the likelihood of encountering high-probability high-impact ocean tipping points.

1) GHG emission reductions need to be realized, 2) A sound global resource management needs to be implemented, 3) The implementation of mitigation measures needs to be enabled through adequate governance structures and seamless interagency action, 4) Transformations need to be carried out increasingly fast. The physical−chemical−biological ocean systems are at the verge of tipping into another state in many oceanic regions. Integrated over the world ocean, this adds up to a global issue of concern.

Policy relevant message:

The physical−chemical−biological ocean systems are at the verge of tipping into another state in many oceanic regions. Integrated over the world ocean, this adds up to a global issue of concern. The adverse impacts of human-induced climate change on the ocean can still be minimised. “Earth system targets,” like global warming or ocean acidification levels, and corresponding emission reduction targets need to be identified, agreed on, implemented, and verified. This study highlights four promising options in Earth system management and societal transformation for minimizing the likelihood of encountering high-probability high-impact ocean tipping points: 1) GHG emission reductions need to be realized, 2) A sound global resource management needs to be implemented, 3) The implementation of mitigation measures needs to be enabled through adequate governance structures and seamless interagency action, 4) Transformations need to be carried out increasingly fast.

A unified theory for organic matter accumulation

Zakem, E. J., Cael, B. B., & Levine, N. M. (2021). A unified theory for organic matter accumulation. Proceedings of the National Academy of Sciences, 118(6). https://doi.org/10.1073/pnas.2016896118

Summary:

Organic matter in the global ocean, soils, and sediments stores about five times more carbon than the atmosphere. Thus, the controls on the accumulation of organic matter are critical to global carbon cycling. However, there is a lack of quantitative understanding of these controls. This prevents meaningful descriptions of organic matter cycling in global climate models, which are required for understanding how changes in organic matter reservoirs provide feedbacks to past and present changes in climate. Currently, explanations for organic matter accumulation remain under debate, characterized by seemingly competing hypotheses. In this study, the authors developed a quantitative framework for organic matter accumulation that unifies these hypotheses. The framework derives from the ecological dynamics of microorganisms, the dominant consumers of organic matter.

Clarifying the Relation between AMOC and Thermal Wind: Application to the Centennial Variability in a Coupled Climate Model

Waldman, R., Hirschi, J., Voldoire, A., Cassou, C., & Msadek, R. (2021). Clarifying the Relation between AMOC and Thermal Wind: Application to the Centennial Variability in a Coupled Climate Model. Journal of Physical Oceanography, 51(2), 343–364. https://doi.org/10.1175/JPO-D-19-0284.1

Summary:

This study aims to clarify the relation between the Atlantic meridional overturning circulation (AMOC) and the thermal wind. We derive a new and generic dynamical AMOC decomposition that expresses the thermal wind transport as a simple vertical integral function of eastern minus western boundary densities. This allows to express density anomalies at any depth as a geostrophic transport in Sverdrups (1 Sv ≡ 106 m3 s−1) per meter and to predict that density anomalies around the depth of maximum overturning induce most AMOC transport. This formalism is then applied to identify the dynamical drivers of the centennial AMOC variability in the CNRM-CM6 climate model. The dynamical reconstruction and specifically the thermal wind component explain over 80% of the low-frequency AMOC variance at all latitudes, which is therefore almost exclusively driven by density anomalies at both zonal boundaries. This transport variability is dominated by density anomalies between depths of 500 and 1500 m, in agreement with theoretical predictions. At those depths, southward-propagating western boundary temperature anomalies induce the centennial geostrophic AMOC transport variability in the North Atlantic. They are originated along the western boundary of the subpolar gyre through the Labrador Sea deep convection and the Davis Strait overflow.

Circulation timescales of Atlantic Water in the Arctic Ocean determined from anthropogenic radionuclides

Wefing, A.-M., Casacuberta, N., Christl, M., Gruber, N., & Smith, J. N. (2021). Circulation timescales of Atlantic Water in the Arctic Ocean determined from anthropogenic radionuclides. Ocean Science, 17(1), 111–129. https://doi.org/10.5194/os-17-111-2021

Summary:

Atlantic Water that carries heat and anthropogenic carbon into the Arctic Ocean plays an important role in the Arctic sea-ice cover decline, but its pathways and travel times remain unclear. This study, used two radionuclides of anthropogenic origin (129I and 236U) to track Atlantic-derived waters along their way through the Arctic Ocean, estimating their travel times and mixing properties. Results help to understand how future changes in Atlantic Water properties will spread through the Arctic.

 

The rise of novelty in marine ecosystems: The Baltic Sea case

Ammar, Y., Niiranen, S., Otto, S. A., Möllmann, C., Finsinger, W., & Blenckner, T. (2021). The rise of novelty in marine ecosystems: The Baltic Sea case. Global Change Biology, 27(7), 1485–1499. https://doi.org/https://doi.org/10.1111/gcb.15503

Summary:

Global environmental changes have accelerated at an unprecedented rate in recent decades due to human activities. Consequently, the incidence of novel physical (abiotic) conditions and biological communities, which have been continuously emerging in the Earth system, has rapidly risen. Despite growing attention to the incidence and challenges posed by novelty in terrestrial ecosystems, novelty has not yet been quantified in marine ecosystems. In this study, authors measured the rate of novelty (RoN) in physical conditions and community structure for three trophic levels, i.e., phytoplankton, zooplankton, and fish, in a large marine system ‐ the Baltic Sea. This study found that over the past 35 years abiotic and biotic RoN showed complex dynamics varying in time and space, depending on the baseline conditions. RoN in abiotic conditions was smaller in the open Central Baltic Sea than in the Kattegat and the more enclosed Gulf of Bothnia, Gulf of Riga, and Gulf of Finland in the north. The authors found a similar spatial pattern for biotic assemblages, which resulted from changes in composition and stock size. The study authors identified sea‐surface temperature and salinity as key drivers of RoN in biotic communities. Hence, future environmental changes that are expected to affect the biogeochemistry of the Baltic Sea, may favor the rise of biotic novelty. The results highlighted the need for a deeper understanding of novelty development in marine ecosystems, including interactions between species and trophic levels, ecosystem functioning under novel abiotic conditions, and considering novelty in future management interventions.

Ocean planning for species on the move provides substantial benefits and requires few trade-offs

Pinsky, M. L., Rogers, L. A., Morley, J. W., & Frölicher, T. L. (2020). Ocean planning for species on the move provides substantial benefits and requires few trade-offs. Science Advances, 6(50), eabb8428. https://doi.org/10.1126/sciadv.abb8428

Summary:

Societies increasingly use multisector ocean planning as a tool to mitigate conflicts over space in the sea, but such plans can be highly sensitive to species redistribution driven by climate change or other factors. A key uncertainty is whether planning ahead for future species redistributions imposes high opportunity costs and sharp trade-offs against current ocean plans. In this study, more than 10,000 projections for marine animals around North America were used to test the impact of climate-driven species redistributions on the ability of ocean plans to meet their goals. Planning for redistributions can substantially reduce exposure to risks from climate change with little additional area set aside and with few trade-offs against current ocean plan effectiveness. Networks of management areas are a key strategy. While climate change will severely disrupt many human activities, the authors find a strong benefit to proactively planning for long-term ocean change.

Hysteresis of the Earth system under positive and negative CO2 emissions

Jeltsch-Thömmes, A., Stocker, T. F., & Joos, F. (2020). Hysteresis of the Earth system under positive and negative CO2 emissions. Environmental Research Letters, 15(12), 124026. https://doi.org/10.1088/1748-9326/abc4af

Summary:

Carbon dioxide removal (CDR) from the atmosphere is part of all emission scenarios of the IPCC that limit global warming to below 1.5 °C. In this study, hysteresis characteristics were investigated in 4 x pre-industrial atmospheric CO2 concentration scenarios with exponentially increasing and decreasing CO2 using the Bern3D-LPX Earth system model. Hysteresis is quantified as the difference in a variable between the up and down pathway at identical cumulative carbon emissions. Due to hysteresis, sustained negative emissions are required to return to and keep a CO2 and warming target. The results suggest, that not emitting carbon in the first place is preferable over carbon dioxide removal, even if technologies would exist to efficiently remove CO2 from the atmosphere and store it away safely.

Policy relevant message:

Not emitting carbon in the first place is preferable over carbon dioxide removal, even if technologies would exist to efficiently remove CO2 from the atmosphere and store it away safely.

North Atlantic Western Boundary Currents Are Intense Dissolved Organic Carbon Streams

Fontela, M., Pérez, F. F., Mercier, H., & Lherminier, P. (2020). North Atlantic Western Boundary Currents Are Intense Dissolved Organic Carbon Streams. In Frontiers in Marine Science (Vol. 7, p. 1050). https://www.frontiersin.org/article/10.3389/fmars.2020.593757

Summary:

In the North Atlantic, there are two main western boundary currents related to the Atlantic Meridional Overturning Circulation (AMOC): the Gulf Stream flowing northward and the Deep Western Boundary Current (DWBC) flowing southward. The authors of this study analysed the data from the OVIDE section (GO-SHIP A25 Portugal-Greenland 40–60°N) that crosses the DWBC and the northward extension of the Gulf Stream, the North Atlantic Current. This study shows that North Atlantic western boundary currents play a key role in the transport of dissolved organic matter, specifically dissolved organic carbon (DOC). Revisited transports and budgets of DOC with new available data identify the eastern Subpolar North Atlantic (eSPNA) as an important source of locally produced organic matter for the North Atlantic and a key region in the supply of bioavailable DOC to the deep ocean. The East Greenland Current, and its upstream source the East Reykjanes Ridge Current on the eastern flank of the mid-Atlantic ridge, are export pathways of bioavailable DOC toward subtropical latitudes. The fast overturning and subsequent remineralization of DOC produced in the autotrophic eSPNA explains up to 38% of the total oxygen consumption in the deep North Atlantic between the OVIDE section and 24°N. Carbon budgets that do not take into account this organic remineralization process overestimates the natural uptake of carbon dioxide (CO2) from the atmosphere by one third. The inclusion of DOC transports in regional carbon budgets reconciles the estimates of CO2 uptake in the North Atlantic between model and observations.

ARIOS: a database for ocean acidification assessment in the Iberian upwelling system (1976–2018)

Padin, X. A., Velo, A., & Pérez, F. F. (2020). ARIOS: a database for ocean acidification assessment in the Iberian upwelling system (1976–2018). Earth System Science Data, 12(4), 2647–2663. https://doi.org/10.5194/essd-12-2647-2020

Summary:

The ARIOS (Acidification in the Rias and the Iberian Continental Shelf) database holds biogeochemical information from 3357 oceanographic stations, giving 17 653 discrete samples. This unique collection is a starting point for evaluating ocean acidification in the Iberian upwelling system, characterized by intense biogeochemical interactions as an observation-based analysis, or for use as inputs in a coupled physical–biogeochemical model to disentangle these interactions at the ecosystem level.