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.

COMFORT at the WCRP discussion series

COMFORT at the WCRP discussion series

In a joint seminar and discussion series organised by AIMES, WCRP, and the Earth commission on ‘Tipping Elements, Irreversibility, and Abrupt Change’ an event on the ocean as a potential tipping element took place on 11 February 2022 with speakers Christoph Heinze (“Ocean tipping points – an overview”) and Stefan Rahmstorf (PIK, Germany) (“Recent insights on AMOC”, AMOC = Atlantic Meridional Overturning Circulation) as well as moderators Didier Swingedouw (Univ. Bordeaux, France) and Helene Hewitt (UK MetOffice). Up to 363 participants followed the talks and subsequent discussions online. A number of important potential regime shifts such as the Arctic sea ice retreat, coral reef decline, and changes in biogeochemical regimes under further progressing deoxygenation were presented and discussed. A recording of the event can be watched below:

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.

Biogeochemical extremes and compound events in the ocean

Gruber, N., Boyd, P. W., Frölicher, T. L., & Vogt, M. (2021). Biogeochemical extremes and compound events in the ocean. Nature, 600(7889), 395–407. https://doi.org/10.1038/s41586-021-03981-7

Summary
The ocean is warming, losing oxygen and being acidified, primarily as a result of anthropogenic carbon emissions. With ocean warming, acidification and deoxygenation projected to increase for decades, extreme events, such as marine heatwaves, will intensify, occur more often, persist for longer periods of time and extend over larger regions. Nevertheless, our understanding of oceanic extreme events that are associated with warming, low oxygen concentrations or high acidity, as well as their impacts on marine ecosystems, remains limited. Compound events—that is, multiple extreme events that occur simultaneously or in close sequence—are of particular concern, as their individual effects may interact synergistically. In this paper authors assess patterns and trends in open ocean extremes based on the existing literature as well as global and regional model simulations. They discuss the potential impacts of individual and compound extremes on marine organisms and ecosystems and propose a pathway to improve the understanding of extreme events and the capacity of marine life to respond to them. The conditions exhibited by present extreme events may be a harbinger of what may become normal in the future. As a consequence, pursuing this research effort may also help to better understand the responses of marine organisms and ecosystems to future climate change.

 

Policy relevant message
Since the pre-industrial times, marine heatwaves have become 10 x more common and low oxygen extremes have become about 5 x more frequent. Ocean acidity extremes have become essentially near permanent. All three types of extreme events will increase in frequency, magnitude and intensity with continuously rising atmospheric carbon emissions and global temperature.

Persistent Uncertainties in Ocean Net Primary Production Climate Change Projections at Regional Scales Raise Challenges for Assessing Impacts on Ecosystem Services

Tagliabue, A., Kwiatkowski, L., Bopp, L., Butenschön, M., Cheung, W., Lengaigne, M., & Vialard, J. (2021). Persistent Uncertainties in Ocean Net Primary Production Climate Change Projections at Regional Scales Raise Challenges for Assessing Impacts on Ecosystem Services. Frontiers in Climate, 3. https://doi.org/10.3389/fclim.2021.738224

Summary

Ocean net primary production (NPP) results from CO2 fixation by marine phytoplankton, catalysing the transfer of organic matter and energy to marine ecosystems, supporting most marine food webs, and fisheries production as well as stimulating ocean carbon sequestration. Thus, alterations to ocean NPP in response to climate change, as quantified by Earth system model experiments conducted as part of the 5th and 6th Coupled Model Intercomparison Project (CMIP5 and CMIP6) efforts, are expected to alter key ecosystem services. Despite reductions in inter-model variability since CMIP5, the ocean components of CMIP6 models disagree roughly 2-fold in the magnitude and spatial distribution of NPP in the contemporary era, due to incomplete understanding and insufficient observational constraints.  Projections of NPP change in absolute terms show large uncertainty in CMIP6, most notably in the North Atlantic and the Indo-Pacific regions, with the latter explaining over two-thirds of the total inter-model uncertainty. While the Indo-Pacific has previously been identified as a hotspot for climate impacts on biodiversity and fisheries, the increased inter-model variability of NPP projections further exacerbates the uncertainties of climate risks on ocean-dependent human communities. Drivers of uncertainty in NPP changes at regional scales integrate different physical and biogeochemical factors that require more targeted mechanistic assessment in future studies. Globally, inter-model uncertainty in the projected changes in NPP has increased since CMIP5, which amplifies the challenges associated with the management of associated ecosystem services. Notably, this increased regional uncertainty in the projected NPP change in CMIP6 has occurred despite reduced uncertainty in the regional rates of NPP for historical period. Improved constraints on the magnitude of ocean NPP and the mechanistic drivers of its spatial variability would improve confidence in future changes. It is unlikely that the CMIP6 model ensemble samples the complete uncertainty in NPP, with the inclusion of additional mechanistic realism likely to widen projections further in the future, especially at regional scales. This has important consequences for assessing ecosystem impacts. Ultimately, we need an integrated mechanistic framework that considers how NPP and marine ecosystems respond to impacts of not only climate change, but also the additional non-climate drivers.

Trivial improvements in predictive skill due to direct reconstruction of the global carbon cycle

Spring, A., Dunkl, I., Li, H., Brovkin, V., & Ilyina, T. (2021). Trivial improvements in predictive skill due to direct reconstruction of the global carbon cycle. Earth Syst. Dynam., 12(4), 1139–1167. https://doi.org/10.5194/esd-12-1139-2021

Summary

Numerical carbon cycle prediction models usually do not start from observed carbon states due to sparse observations. Instead, only physical climate is reconstructed, assuming that the carbon cycle follows indirectly. In this study the authors tested such an assumption and found that indirect reconstruction works quite well and that improvements from the direct method are limited, strengthening the current indirect use.