Abruptly attenuated carbon sequestration with Weddell Sea dense waters by 2100

Nissen, C., Timmermann, R., Hoppema, M., Gürses, Ö., & Hauck, J. (2022). Abruptly attenuated carbon sequestration with Weddell Sea dense waters by 2100. Nature Communications, 13(1), 3402. https://doi.org/10.1038/s41467-022-30671-3

Summary

Antarctic Bottom Water formation, such as in the Weddell Sea in the Southern Ocean, is an efficient vector for carbon sequestration on time scales of centuries. Possible changes in carbon sequestration under changing environmental conditions are unquantified to date, mainly due to difficulties in simulating the relevant processes on high-latitude continental shelves. The authors of this study use a model setup including both ice-shelf cavities and oceanic carbon cycling and demonstrate that by 2100, deep-ocean carbon accumulation in the southern Weddell Sea is abruptly attenuated to only 40% of the 1990s rate in a high-emission scenario, while the rate in the 2050s and 2080s is still 2.5-fold and 4-fold higher, respectively, than in the 1990s. Assessing deep-ocean carbon budgets and water mass transformations, this decline was attributed to an increased presence of modified Warm Deep Water on the southern Weddell Sea continental shelf, a 16% reduction in sea-ice formation, and a 79% increase in ice-shelf basal melt. Altogether, these changes lower the density and volume of newly formed bottom waters and reduce the associated carbon transport to the abyss.

Policy relevant message:

Under the high emissions scenario, carbon sequestration by Weddell Sea dense water formation will reduce by 2100.

The Pan-Arctic Continental Slope as an Intensifying Conveyer Belt for Nutrients in the Central Arctic Ocean (1985–2015)

Oziel, L., Schourup-Kristensen, V., Wekerle, C., & Hauck, J. (2022). The Pan-Arctic Continental Slope as an Intensifying Conveyer Belt for Nutrients in the Central Arctic Ocean (1985–2015). Global Biogeochemical Cycles, 36(6), e2021GB007268. https://doi.org/10.1029/2021GB007268

Summary

Microscopic algae called phytoplankton are the base of the trophic chain, sustaining the entire Arctic Ocean (AO) ecosystem. In the central parts of the AO, multi-year sea-ice used to limit transmission of light in the surface ocean and therefore control phytoplankton growth and primary productivity. However, the massive loss in sea-ice during the last 3 decades allowed more and more light to penetrate the water column, making nutrient availability the main bottom-up control of the AO productivity. A major part of the bio-available nutrients reaching the surface in the central AO are transported with ocean currents from the adjacent North Atlantic and Pacific and from deeper water masses. Using a biogeochemical model resolving processes at high spatial resolution, we were able to quantify the different transport pathways of nutrients with ocean currents and revealed that despite increasing supply along the anticlockwise flowing boundary current, the central AO is still running into more severe nutrient limitation.

Policy relevant message:

The continental slope contributes to the transport of nutrients in the ArcticOcean. Yet, despite an intensification of ocean dynamics, the Arctic Ocean is still shifting from a light-limited to a nutrient-limited system.

Policy Brief: Key findings and recommendations from three H2020 Projects on Tipping Points: TiPES, COMFORT, and TiPACCs

 

There is a threat of imminent abrupt and irreversible transitions in the Earth system, both on land and in the ocean. A reduction in greenhouse gas (GHG) emissions and in land-use change must be implemented urgently to mitigate these changes through political, economic, and societal measures. Yet, considerable knowledge gaps remain concerning the processes underlying the dynamics of tipping elements,

Three EU funded Horizon2020 projects have been investigating tipping behaviour in the Earth system: Tipping Points in the Earth System (TiPES), Our Common Future Ocean in the Earth System (COMFORT), and Tipping Points in Antarctic Climate Components (TiPACCs). In the joint policy brief, you can find key findings of the three projects, persisting knowledge gaps as well as policy recommendations.

The policy brief is available for free download here.

 

Possibility for strong northern hemisphere high-latitude cooling under negative emissions

Schwinger, J., Asaadi, A., Goris, N., & Lee, H. (2022). Possibility for strong northern hemisphere high-latitude cooling under negative emissions. Nature Communications, 13(1), 1095. https://doi.org/10.1038/s41467-022-28573-5

Summary

It is well established that a collapse or strong reduction of the Atlantic meridional overturning circulation (AMOC) would substantially cool the northern high latitudes. In this study the authors show that there is a possibility that such cooling could be amplified under deliberate CO2 removal and result in a temporary undershoot of a targeted temperature level. This behaviour was found in Earth system models that show a strong AMOC decline in response to anthropogenic forcing. Idealised simulations of CO2 removal with one of these models indicate that the timing of negative emissions relative to AMOC decline and recovery is key in setting the strength of the temporary cooling. This study shows that the pronounced temperature-fluctuations at high northern latitudes found in these simulations would entail considerable consequences for sea-ice and permafrost extent as well as for high latitude ecosystems.

Policy relevant message:

Cooling of the northern hemisphere as a result of a collapse or strong reduction of the Atlantic meridional overturning circulation (AMOC) can be amplified by Carbon dioxide removal (CDR) techniques. Therefore, not emitting CO2 into the atmosphere is a preferable action over post emission CO2 removal.

Failures to disagree are essential for environmental science to effectively influence policy development

Norberg, J., Blenckner, T., Cornell, S. E., Petchey, O. L., & Hillebrand, H. (2022). Failures to disagree are essential for environmental science to effectively influence policy development. Ecology Letters, 00, 1–19. https://doi.org/10.1111/ele.13984

Summary

While environmental science, and ecology in particular, is working to provide better understanding to base sustainable decisions on, the way scientific understanding is developed can at times be detrimental to this cause. Locked-in debates are often unnecessarily polarised and can compromise any common goals of the opposing camps. The present paper is inspired by a resolved debate from an unrelated field of psychology where Nobel laureate David Kahneman and Garry Klein turned what seemed to be a locked-in debate into a constructive process for their fields. The present paper is also motivated by previous discourses regarding the role of thresholds in natural systems for management and governance, but its scope of analysis targets the scientific process within complex social-ecological systems in general. The authors of this paper identified four features of environmental science that appear to predispose for locked-in debates: (1) The strongly context-dependent behaviour of ecological systems. (2) The dominant role of single hypothesis testing. (3) High prominence is given to theory demonstration compared to an investigation. (4) The effect of urgent demands to inform and steer policy. This fertile ground is further cultivated by human psychological aspects as well as the structure of funding and publication systems.

Feedbacks Between Ocean Productivity and Organic Iron Complexation in Reaction to Changes in Ocean Iron Supply

Völker, C., & Ye, Y. (2022). Feedbacks Between Ocean Productivity and Organic Iron Complexation in Reaction to Changes in Ocean Iron Supply. Frontiers in Marine Science, 9. https://doi.org/10.3389/fmars.2022.777334

Summary

Low concentrations of iron, an important micronutrient for photosynthetic organisms, limit growth in large parts of the ocean. The solubility and availability of iron are to a large degree determined by organic iron-binding molecules, so-called ligands. While ligands come from a variety of sources, many of them are produced in autotrophic (nutrition produced by organisms themselves) or heterotrophic (nutrition gained externally) production in the ocean, leading to the possibility of feedbacks between marine primary production and iron availability. The diversity of ligands, reaching from siderophores, small molecules involved in bacterial iron uptake, to breakdown products and long-lived macromolecules like humic substances, means that feedbacks could be both negative and positive or there may even be no feedback at all. The authors of this study investigate first, how the cycling of this ligand pool can be described simplistically in a model such that it reproduces the observed global distribution of dissolved iron and phosphorus as closely as possible. They show that the inclusion of a ligand similar to refractory dissolved organic carbon leads to an improved agreement with observations in our model. The inclusion of a second, shorter-lived siderophore-like ligand does not strongly affect this agreement. In a second step, the authors study how feedbacks affect how iron distribution and oceanic productivity react to changes in external supply of iron. To be consistent with present-day iron distribution, the dominant feedback is positive, increasing the sensitivity of global biological productivity and hence carbon cycling to changes in iron supply. The strength of the feedback increases with increasing ligand life-time. The negative feedback associated with siderophore-like ligands has the potential to mitigate the positive feedback, especially at the surface and for global export production, but more research on the production and decay of siderophores is needed for a better quantification. Ocean biogeochemical models that assume a constant ligand concentration and hence neglect possible feedbacks may therefore underestimate the reaction of the global carbon cycle to the strong increase in dust deposition under future or glacial climate conditions.

Ocean Acidification Effect on the Iron-Gallic Acid Redox Interaction in Seawater

Pérez-Almeida, N., González, A. G., Santana-Casiano, J. M., & González-Dávila, M. (2022). Ocean Acidification Effect on the Iron-Gallic Acid Redox Interaction in Seawater. Frontiers in Marine Science, 9. https://doi.org/10.3389/fmars.2022.837363

Summary

Ocean acidification impacts the iron (Fe) biogeochemistry both its oxidation state and complexation reactions with other substances and thus affect its bioavailability to marine organisms. This has a direct effect on the ecosystems since Fe is an essential micronutrient. Some substances such as gallic acid produced by marine microorganisms can also alter the oxidation state of Fe in seawater. The authors of this study found that gallic acid impacts iron’s redox state for longer periods and thus favours its bioavailability.

Iron and copper complexation in Macaronesian coastal waters

Arnone, V., González-Santana, D., González-Dávila, M., González, A. G., & Santana-Casiano, J. M. (2022). Iron and copper complexation in Macaronesian coastal waters. Marine Chemistry, 240, 104087. https://doi.org/10.1016/j.marchem.2022.104087

Summary

Iron and Copper are trace metals that are bioessential micronutrients to marine organisms. In this study the authors studied dissolved concentrations of these two metals and the strength of their complexes with other substances in the surface coastal waters of the Macaronesia region (Cape Verde, Canary Islands, and Madeira). Due to biological activity and water mixing induced by the wind around the islands, dissolved metals and ligand concentrations were greater at the coastal stations than in oceanic water. Variations were observed between the eastern and western parts of Fogo, Tenerife and Gran Canaria. On the east coasts, the increase in dissolved metals and ligand concentrations were related to wind-induced water mixing. The results of this study improve our understanding of the impact of coastal areas on the Fe and Cu biogeochemical cycles.

A strong mitigation scenario maintains climate neutrality of northern peatlands

Qiu, C., Ciais, P., Zhu, D., Guenet, B., Chang, J., Chaudhary, N., Kleinen, T., Li, X., Müller, J., Xi, Y., Zhang, W., Ballantyne, A., Brewer, S. C., Brovkin, V., Charman, D. J., Gustafson, A., Gallego-Sala, A. V, Gasser, T., Holden, J., … Westermann, S. (2022). A strong mitigation scenario maintains climate neutrality of northern peatlands. One Earth, 5(1), 86–97. https://doi.org/10.1016/j.oneear.2021.12.008

Summary

Intact peatlands remove carbon dioxide (CO2) from the atmosphere through photosynthesis and store the carbon in soils in waterlogged conditions, while emitting methane (CH4) to the atmosphere. The net climate impact of peatlands depends on the relative magnitude of these two greenhouse gases. In this study the authors assessed the future CO2 and CH4 balance of northern peatlands using five large-scale, process-based peatland models. Their results suggest that under climate policies and action, northern peatlands are likely be climate neutral because the climate-warming effect of peatland CH4 emissions is offset by the cooling effect of peatland CO2 sinks. However, if action on climate change is not taken, northern peatlands could accelerate global warming because CH4 emissions are projected to increase substantially, and northern peatlands may turn from CO2 sinks to sources driven by strong warming and drying.

Policy relevant message:

If action on climate change is not taken, northern peatlands could accelerate global warming because CH4 emissions are projected to increase substantially, and northern peatlands may turn from CO2 sinks to sources driven by strong warming and drying.

Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S

Bourgeois, T., Goris, N., Schwinger, J., & Tjiputra, J. F. (2022). Stratification constrains future heat and carbon uptake in the Southern Ocean between 30°S and 55°S. Nature Communications, 13(1), 340. https://doi.org/10.1038/s41467-022-27979-5

Summary
The Southern Ocean between 30°S and 55°S is a major sink of excess heat and anthropogenic carbon, but model projections of these sinks remain highly uncertain. Reducing such uncertainties is required to effectively guide the development of climate mitigation policies for meeting the ambitious climate targets of the Paris Agreement. This study shows that the large spread in the projections of future excess heat uptake efficiency and cumulative anthropogenic carbon uptake in this region are strongly linked to the models’ contemporary stratification. This relationship is robust across two generations of Earth system models and is used to reduce the uncertainty of future estimates of the cumulative anthropogenic carbon uptake by up to 53% and the excess heat uptake efficiency by 28%. These results highlight that, for this region, an improved representation of stratification in Earth system models is key to constrain future carbon budgets and climate change projections.