Rising global temperatures pose significant risks to marine ecosystems, biodiversity and fisheries. Recent comprehensive assessments suggest that large-scale mitigation efforts to limit warming below crucial thresholds are falling short, and all feasible future climate projections, including those that represent ideal emissions reductions, exceed the Paris Agreement’s aspirational <1.5{degree sign}C warming target, at least temporarily. As such, there are a number of proposed climate interventions that aim to deliberately manipulate the environment at large scales to counteract anthropogenic global warming. Yet, there is a high level of uncertainty in how marine ecosystems will respond to these interventions directly as well as how these interventions may impact marine ecosystems’ responses to climate change. Due to the key role the ocean plays in regulating Earth’s climate and ensuring global food security, understanding the effects that these interventions may have on marine ecosystems is crucial. This review provides an overview of proposed intervention methodologies for solar radiation modification and marine carbon dioxide removal and outlines the potential trade-offs and knowledge gaps associated with their impacts on marine ecosystems. Climate interventions have the potential to reduce warming-driven impacts, but could also substantially alter marine food systems, biodiversity and ecosystem function. Impact assessments are thus crucial to quantify trade-offs between plausible intervention scenarios and to identify and discontinue scaling efforts or commercial implementation for those with unacceptable risks.

A document by Morgan Raven. Click on the document to view its contents.

Biomass-based marine CO2 removal (mCDR) aims to harness photosynthetic organisms to remove excess CO2 from the atmosphere and sequester that fixed carbon in a long-lived marine reservoir. This strategy would contribute to a portfolio of climate mitigation efforts. To guide decision-making around testing, deploying, and regulating mCDR, we need to better understand how the deep sea and the broader Earth system would respond to increased biomass addition. The central processes driving this response are sensitive to choices about biomass type and storage site, and they stretch across spatial and temporal scales from microns to kilometers and from minutes to millenia. To organize this immense interdisciplinary challenge, we define five generalizable phases of a biomass-based mCDR project: inputs, placement, short-term response, long-term response, and functional stability. Each phase is associated with high-priority research objectives that could be achieved through thoughtful integration of direct field measurements, investigations of analog sites, experiments, and/or models. In-situ and laboratory experiments can be particularly powerful for isolating key processes; for example, in-situ “closed-system” bottle incubations can amplify small signals and reduce uncertainties created by complex physical flows. Regardless of approach, the overarching goal of biomass-based mCDR research is to develop a process-based understanding of biomass sequestration that is robust enough to project the likely outcomes of alternative choices related to mCDR. Beyond assessing carbon storage and ensuring regulatory compliance, future field experiments should prioritize generating the data required to improve models for impacts of biomass-based mCDR on the deep ocean at climatically-relevant scales.

A document by Morgan Raven. Click on the document to view its contents.

Organic matter (OM) sulfurization can enhance the preservation and sequestration of carbon in anoxic sediments, and it has been observed in sinking marine particles from marine O2-deficient zones. The magnitude of this effect on carbon burial remains unclear, however, because the transformations that occur when sinking particles encounter sulfidic conditions remain undescribed. Here, we briefly expose sinking marine particles from the eastern tropical North Pacific O2-deficient zone to environmentally relevant sulfidic conditions (20C, 0.5 mM [poly]sulfide, two days) and then characterize the resulting solid-phase organic and inorganic products in detail. During these experiments, the abundance of organic sulfur in both hydrolyzable and hydrolysis-resistant solids roughly triples, indicating extensive OM sulfurization. Lipids also sulfurize on this timescale, albeit less extensively. In all three pools, OM sulfurization produces organic monosulfides, thiols, and disulfides. Hydrolyzable sulfurization products appear within ≤ 200-m regions of relatively homogenous composition that are suggestive of sulfurized extracellular polymeric substances (EPS). Concurrently, reactions with particulate iron oxyhydroxides generate low and fairly uniform concentrations of iron sulfide (FeS) within these same EPS-like materials. Iron oxyhydroxides were not fully consumed during the experiment, which demonstrates that organic materials can be competitive with reactive iron for sulfide. These experiments support the hypothesis that sinking, OM- and EPS-rich particles in a sulfidic water mass can sulfurize within days, potentially contributing to enhanced sedimentary carbon sequestration. Additionally, sulfur-isotope and chemical records of organic S and iron sulfides in sediments have the potential to incorporate signals from water column processes.