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  • 1
    Publication Date: 2023-10-21
    Description: EMB Future Science Brief No. 10 highlights the most recent science on Ocean oxygen, including causes, impacts and mitigation strategies of Ocean oxygen loss, and discusses whether “every second breath we take comes from the Ocean”. It closes with key policy, management and research recommendations to address Ocean deoxygenation and communicate more accurately about the role of the Ocean in Earth’s oxygen. The sentence “every second breath you take comes from the Ocean” is commonly used in Ocean Literacy and science communication to highlight the importance of Ocean oxygen. However, despite its widespread use, it is often not phrased correctly. In contrast, there is little awareness about the threat of the global oxygen loss in the Ocean, called deoxygenation, particularly in comparison with other important stressors, such as Ocean acidification or increasing seawater temperatures. Deoxygenation is increasing in the coastal and open Ocean, primarily due to human-induced global warming and nutrient run-off from land, and projections show that the Ocean will continue losing oxygen as global warming continues. The consequences of oxygen loss in the Ocean are extensive and include decreased biodiversity, shifts in species distributions, displacement or reduction in fisheries resources, changes in biogeochemical cycling and mass mortalities. Low oxygen conditions also drive other chemical processes which produce greenhouse gases, toxic compounds and further degrade water quality. The degraded water quality directly affects marine ecosystems, but also indirectly impacts ecosystem services supporting local communities, regional economies and tourism. Although there are still gaps in our knowledge, we know enough to be very concerned about the consequences: the impacts might even be larger than from Ocean acidification or heat waves, and three out of the five global mass extinctions were linked to Ocean deoxygenation. The sense of urgency to improve Ocean health is reflected in the UN Decade of Ocean Science for Sustainable Development (Ocean Decade) and the EU Mission: Restore our Ocean and Waters (Mission Ocean), and tackling the loss of oxygen in the Ocean is critical to achieving the aims of these two initiatives.
    Description: Published
    Description: Refereed
    Keywords: Ocean oxygen ; Deoxygenation
    Repository Name: AquaDocs
    Type: Book/Monograph/Conference Proceedings
    Format: 84pp.
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  • 2
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    AGU (American Geophysical Union) | Wiley
    In:  Global Biogeochemical Cycles, 31 (8). pp. 1236-1255.
    Publication Date: 2020-02-06
    Description: There is currently no consensus on how humans are affecting the marine nitrogen (N) cycle, which limits marine biological production and CO2 uptake. Anthropogenic changes in ocean warming, deoxygenation, and atmospheric N deposition can all individually affect the marine N cycle and the oceanic production of the greenhouse gas nitrous oxide (N2O). However, the combined effect of these perturbations on marine N cycling, ocean productivity, and marine N2O production is poorly understood. Here we use an Earth system model of intermediate complexity to investigate the combined effects of estimated 21st century CO2 atmospheric forcing and atmospheric N deposition. Our simulations suggest that anthropogenic perturbations cause only a small imbalance to the N cycle relative to preindustrial conditions (∼+5 Tg N y−1 in 2100). More N loss from water column denitrification in expanded oxygen minimum zones (OMZs) is counteracted by less benthic denitrification, due to the stratification-induced reduction in organic matter export. The larger atmospheric N load is offset by reduced N inputs by marine N2 fixation. Our model predicts a decline in oceanic N2O emissions by 2100. This is induced by the decrease in organic matter export and associated N2O production and by the anthropogenically driven changes in ocean circulation and atmospheric N2O concentrations. After comprehensively accounting for a series of complex physical-biogeochemical interactions, this study suggests that N flux imbalances are limited by biogeochemical feedbacks that help stabilize the marine N inventory against anthropogenic changes. These findings support the hypothesis that strong negative feedbacks regulate the marine N inventory on centennial time scales.
    Type: Article , PeerReviewed
    Format: text
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  • 3
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    AGU (American Geophysical Union) | Wiley
    In:  Global Biogeochemical Cycles, 31 (11). pp. 1656-1673.
    Publication Date: 2020-02-06
    Description: In this pilot study we link the yield of industrial fisheries to changes in the zooplankton mortality in an idealized way accounting for different target species (planktivorous fish—decreased zooplankton mortality; large predators—increased zooplankton mortality). This indirect approach is used in a global coupled biogeochemistry circulation model to estimate the range of the potential impact of industrial fisheries on marine biogeochemistry. The simulated globally integrated response on phytoplankton and primary production is in line with expectations—a high (low) zooplankton mortality results in a decrease (increase) of zooplankton and an increase (decrease) of phytoplankton. In contrast, the local response of zooplankton and phytoplankton depends on the region under consideration: In nutrient-limited regions, an increase (decrease) in zooplankton mortality leads to a decrease (increase) in both zooplankton and phytoplankton biomass. In contrast, in nutrient-replete regions, such as upwelling regions, we find an opposing response: an increase (decrease) of the zooplankton mortality leads to an increase (decrease) in both zooplankton and phytoplankton biomass. The results are further evaluated by relating the potential fisheries-induced changes in zooplankton mortality to those driven by CO2 emissions in a business-as-usual 21st century emission scenario. In our idealized case, the potential fisheries-induced impact can be of similar size as warming-induced changes in marine biogeochemistry.
    Type: Article , PeerReviewed
    Format: text
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  • 4
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    AGU (American Geophysical Union) | Wiley
    In:  Earth's Future, 5 (12). pp. 1252-1266.
    Publication Date: 2020-11-23
    Description: The potential of Coastal Ocean Alkalinization (COA), a carbon dioxide removal (CDR) climate engineering strategy that chemically increases ocean carbon uptake and storage, is investigated with an Earth system model of intermediate complexity. The CDR potential and possible environmental side effects are estimated for various COA deployment scenarios, assuming olivine as the alkalinity source in ice-free coastal waters (about 8.6% of the global ocean's surface area), with dissolution rates being a function of grain size, ambient seawater temperature and pH. Our results indicate that for a large-enough olivine deployment of small-enough grain sizes (10 μm), atmospheric CO2 could be reduced by more than 800 GtC by the year 2100. However, COA with coarse olivine grains (1000 μm) has little CO2 sequestration potential on this time scale. Ambitious CDR with fine olivine grains would increase coastal aragonite saturation Ω to levels well beyond those that are currently observed. When imposing upper limits for aragonite saturation levels (Ωlim) in the grid boxes subject to COA (Ωlim = 3.4 and 9 chosen as examples), COA still has the potential to reduce atmospheric CO2 by 265 GtC (Ωlim=3.4) to 790 GtC (Ωlim=9) and increase ocean carbon storage by 290 Gt (Ωlim=3.4) to 913 Gt (Ωlim=9) by year 2100.
    Type: Article , PeerReviewed
    Format: text
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  • 5
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    AGU (American Geophysical Union) | Wiley
    In:  Earth's Future, 6 (3). pp. 565-582.
    Publication Date: 2021-02-08
    Description: To maintain the chance of keeping the average global temperature increase below 2 degrees C and to limit long-term climate change, removing carbon dioxide from the atmosphere (carbon dioxide removal, CDR) is becoming increasingly necessary. We analyze optimal and cost-effective climate policies in the dynamic integrated assessment model (IAM) of climate and the economy (DICE2016R) and investigate (1) the utilization of (ocean) CDR under different climate objectives, (2) the sensitivity of policies with respect to carbon cycle feedbacks, and (3) how well carbon cycle feedbacks are captured in the carbon cycle models used in state-of-the-art IAMs. Overall, the carbon cycle model in DICE2016R shows clear improvements compared to its predecessor, DICE2013R, capturing much better long-term dynamics and also oceanic carbon outgassing due to excess oceanic storage of carbon from CDR. However, this comes at the cost of a (too) tight short-term remaining emission budget, limiting the model suitability to analyze low-emission scenarios accurately. With DICE2016R, the compliance with the 2 degrees C goal is no longer feasible without negative emissions via CDR. Overall, the optimal amount of CDR has to take into account (1) the emission substitution effect and (2) compensation for carbon cycle feedbacks.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    Format: other
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  • 6
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    AGU (American Geophysical Union) | Wiley
    In:  Earth's Future, 5 (1). pp. 128-134.
    Publication Date: 2020-02-06
    Description: The historical developments are reviewed that have led from a bottom-up responsibility initiative of concerned scientists to the emergence of a nationwide interdisciplinary Priority Program on the assessment of Climate Engineering (CE), funded by the German Research Foundation (DFG). Given the perceived lack of comprehensive and comparative appraisals of different CE methods, the Priority Program was designed to encompass both solar radiation management (SRM) and carbon dioxide removal (CDR) ideas, and to cover the atmospheric, terrestrial and oceanic realm. First key findings obtained by the ongoing Priority Program are summarized and reveal that compared to earlier assessments, such as the 2009 Royal Society report, more detailed investigations tend to indicate less efficiency, lower effectiveness and often lower safety. Emerging research trends are discussed in the context of the recent Paris agreement to limit global warming to less than two degrees and the associated increasing reliance on negative emission technologies. Our results show then when deployed at scales large enough to have a significant impact on atmospheric CO2, even CDR methods such as afforestation – often perceived as ‘benign’ – can have substantial side effects and may raise severe ethical, legal and governance issues. We suppose that before being deployed at climatically relevant scales, any negative-emission or climate engineering method will require careful analysis of efficiency, effectiveness and undesired side effects.
    Type: Article , PeerReviewed
    Format: text
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  • 7
    Publication Date: 2019-09-23
    Description: Understanding the causes of the observed expansion of tropical ocean's oxygen minimum zones (OMZs) is hampered by large biases in the representation of oxygen distribution in climate models, pointing to incorrectly represented mechanisms. Here we assess the oxygen budget in a global biogeochemical circulation model, focusing on the Atlantic Ocean. While a coarse (0.5°) configuration displays the common bias of too large and too intense OMZs, the oxygen concentration in an eddying (0.1°) configuration is higher and closer to observations. This improvement is traced to a stronger oxygen supply by a more realistic representation of the equatorial and off-equatorial undercurrents, outweighing the concurrent increase in oxygen consumption associated with the stronger nutrient supply. The sensitivity of the eastern tropical Atlantic oxygen budget to the equatorial current intensity suggests that temporal changes in the eastward oxygen transport from the well-oxygenated western boundary region might partly explain variations in the OMZs.
    Type: Article , PeerReviewed
    Format: text
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  • 8
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    AGU (American Geophysical Union) | Wiley
    In:  Global Biogeochemical Cycles, 28 (4). pp. 415-422.
    Publication Date: 2018-03-19
    Description: Oceanic uptake and long-term storage of atmospheric carbon dioxide (CO2) are strongly driven by the marine “biological pump,” i.e., sinking of biotically fixed inorganic carbon and nutrients from the surface into the deep ocean (Sarmiento and Bender, 1994; Volk and Hoffert, 1985). Sinking velocity of marine particles depends on seawater viscosity, which is strongly controlled by temperature (Sharqawy et al., 2010). Consequently, marine particle flux is accelerated as ocean temperatures increase under global warming (Bach et al., 2012). Here we show that this previously overlooked “viscosity effect” could have profound impacts on marine biogeochemical cycling and carbon uptake over the next centuries to millennia. In our global warming simulation, the viscosity effect accelerates particle sinking by up to 25%, thereby effectively reducing the portion of organic matter that is respired in the surface ocean. Accordingly, the biological carbon pump's efficiency increases, enhancing the sequestration of atmospheric CO2 into the ocean. This effect becomes particularly important on longer time scales when warming reaches the ocean interior. At the end of our simulation (4000 A.D.), oceanic carbon uptake is 17% higher, atmospheric CO2 concentration is 180 ppm lower, and the increase in global average surface temperature is 8% weaker when considering the viscosity effect. Consequently, the viscosity effect could act as a long-term negative feedback mechanism in the global climate system.
    Type: Article , PeerReviewed
    Format: text
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  • 9
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    AGU (American Geophysical Union) | Wiley
    In:  Global Biogeochemical Cycles, 28 (7). pp. 648-661.
    Publication Date: 2019-09-23
    Description: The widely used concept of constant ”Redfield” phytoplankton stoichiometry is often applied for estimating which nutrient limits phytoplankton growth in the surface ocean. Culture experiments, in contrast, show strong relations between growth conditions and cellular stoichiometry with often substantial deviations from Redfield stoichiometry. Here we investigate to what extent both views agree by analyzing remote sensing and in situ data with an optimality-based model of nondiazotrophic phytoplankton growth in order to infer seasonally varying patterns of colimitation by light, nitrogen (N), and phosphorus (P) in the global ocean. Our combined model-data analysis suggests strong N and N-P colimitation in the tropical ocean, seasonal light, and N-P colimitation in the Northern Hemisphere, and strong light limitation only during winter in the Southern Ocean. The eastern equatorial Pacific appears as the only ocean area that is essentially not limited by N, P, or light. Even though our optimality-based approach specifically accounts for flexible stoichiometry, inferred patterns of N and P limitation are to some extent consistent with those obtained from an analysis of surface inorganic nutrients with respect to the Redfield N:P ratio. Iron is not part of our analysis, implying that we cannot accurately predict N cell quotas in high-nutrient, low-chlorophyll regions. Elsewhere, we do not expect a major effect of iron on the relative distribution of N, P, and light colimitation areas. The relative importance of N, P, and light in limiting phytoplankton growth diagnosed here by combining observations and an optimal growth model provides a useful constraint for models used to predict future marine biological production under changing environmental conditions.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
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  • 10
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    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (2). pp. 492-499.
    Publication Date: 2019-09-23
    Description: The iron hypothesis suggests that in large areas of the ocean phytoplankton growth and thus photosynthetic CO2-uptake is limited by the micronutrient iron. Phytoplankton requires iron in particular for nitrate uptake, light harvesting and electron transport in photosynthesis, suggesting a tight coupling of iron and light limitation. One important source of iron to the open ocean is dust deposition. Previous global biogeochemical modeling studies have suggested a low sensitivity of oceanic CO2-uptake to changes in dust deposition. Here we show that this sensitivity is increased significantly when iron-light colimitation, i.e. the impact of iron bioavailability on light harvesting capabilities, is explicitly considered. Accounting for iron-light colimitation increases the shift of export production from tropical and subtropical regions to the higher latitudes of subpolar regions at high dust deposition and amplifies iron limitation at low dust deposition. Our results re-emphasize the role of iron as a key limiting nutrient for phytoplankton.
    Type: Article , PeerReviewed
    Format: text
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