Samantha A. Siedlecki

Oxygen concentrations are of fundamental importance for organisms and geochemical cycling in the oceans. Since the middle of the 20th century, oxygen concentrations have been declining in the open ocean and the coastal ocean, with implications for productive ecosystems. The northwest Atlantic (NWA) shelf from the coast of Labrador to Cape Hatteras is located near the intersection of subtropical and subpolar circulation in the Atlantic basin and is sensitive to climate variability. Here, we used in situ oxygen observations to determine the multidecadal trend of subsurface oxygen concentrations below the mixed layer on the NWA shelf from 1988 to 2019. Our results identified an overall decline in oxygen concentrations that varied across the region from 0.0 to −1.8 µ mol kg ⁻¹ yr ⁻¹ . These rates surpass the established Atlantic basin-wide trend. The greatest subregional oxygen loss occurred on the Eastern Scotian Shelf (−1.8 ± 0.2 µ mol kg ⁻¹ yr ⁻¹ ). By separating and comparing the contribution of oxygen solubility and apparent oxygen utilization (AOU) to the trends, the oxygen trends on the NWA shelf were determined to be driven by changes in the AOU. In the subregions where AOU dominates, the AOU change was driven by the decreased presence of Labrador Current water and the increased presence of Gulf Stream water in the region, while the rest was unexplained by our potential predictors, implicating local biological changes and changes in ice-cover.

Scientific assessments play a critical role in synthesizing evidence to support evidence‐based decision‐making in many sectors and topics. For climate change, large‐scale assessments like the Intergovernmental Panel on Climate Change (IPCC) and the United States (U.S.) National Climate Assessment (NCA) reports help many different users—from national government to local governments, Indigenous Peoples and communities, the private sector, the social sector, and the scientific community itself—navigate the complex field of climate risks, impacts, and integrated solutions. While co‐creation with users is essential to developing actionable assessments, this paper lays the groundwork for future processes by identifying many advancing and emerging areas of relevant knowledge ready for assessment or additional near‐term research. Drawing on the expertise of many scholars who were formerly tasked with producing the Sixth NCA and authors of other national or sub‐national climate assessment reports, this paper highlights select U.S.‐relevant topics in which research advances since the 2023 Fifth NCA make new syntheses particularly valuable for future national, sub‐national, and sectoral climate assessments. It also identifies critical research topics in those select areas that would be useful for future climate assessment efforts. Overall, this research priorities roadmap aims to catalyze collaborations, research, and a network of climate assessments to synthesize the state of climate impacts, risks, and responses and to provide widely accessible and foundational information for sectoral, regional, and cross‐cutting climate action plans and decisions in the U.S.

Cross‐shelf interactions and the associated lateral transport of organic carbon remain poorly quantified and constrained by observations, despite their importance for the ocean carbon cycle. Filaments generated by instabilities of coastal currents are one pathway for this exchange, but their transport is difficult to observe at basin scales. Here, we provide the first satellite‐based estimate of offshore organic carbon transport and filament contribution in Eastern Boundary Upwelling Systems. Using a 21‐year (2003–2023) data set of chlorophyll‐a and sea surface temperature, we apply unsupervised k‐means clustering to detect offshore‐directed filaments and estimate their carbon content across isobaths of 250–2,000 m. We find an offshore transport of 239 ± 25 to 272 ± 28 TgC yr ⁻¹ , with filaments contributing 8 ± 2 to 10 ± 3 TgC yr ⁻¹ (<5%). Filament transport shows a seasonal cycle linked to shelf carbon stock and mesoscale/submesoscale activity. Our results provide a scalable observational framework for monitoring lateral carbon transport.

Scientific assessments play a critical role in synthesizing evidence to support evidence-based decision-making in many sectors and topics. For climate change, large-scale assessments like the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Climate Assessment (NCA) reports help many different users—from national government to local governments, Indigenous Peoples and communities, the private sector, the social sector, and the scientific community itself—navigate the complex field of climate risks, impacts, and integrated solutions. While cocreation with users is essential to developing actionable assessments, this paper lays the groundwork for future processes by identifying many advancing and emerging areas of relevant knowledge ready for assessment or additional near-term research. Drawing on the expertise of many scholars who were formerly tasked with producing the Sixth NCA and authors of other national or subnational climate assessment reports, this paper highlights select U.S.-relevant topics in which research advances since the 2023 Fifth NCA make new syntheses particularly valuable for future national, subnational, and sectoral climate assessments. It also identifies critical research topics in those select areas that would be useful for future climate assessment efforts. Overall, this research priorities road map aims to catalyze collaborations, research, and a network of climate assessments to synthesize the state of climate impacts, risks, and responses and to provide widely accessible and foundational information for sectoral, regional, and cross-cutting climate action plans and decisions in the United States. PLAIN LANGUAGE SUMMARY: Scientific assessments are critical in informing households, communities, businesses, institutions, and governments as they respond to intensifying climate hazards. As climate change impacts accelerate, the pace of assessment and integration of science with decision-making and action must accelerate as well. A network of U.S. climate assessment efforts could rapidly synthesize actionable science and case studies to inform climate response strategies. To support future processes, this paper presents a selection of foundational topics needing assessment and near-term research across climate impacts, risks, and actions from the perspective of the United States and some of its key regions, environments, and economic sectors. For example, assessment is needed for the increasing threats posed by compound extremes and cascading climate risks, which can overwhelm systems developed for independent events. The effectiveness of climate responses likewise requires continuous evaluation and improvement, with consideration of the broader impacts (e.g., economic, environmental, health) associated with those strategies. System-level understanding of integrated climate response strategies can be advanced by combining observations and modeling with mitigation, adaptation, climate interventions, disaster management, and resilience decision scenarios. Overall, these research priorities aim to encourage new research and future national, regional, local, and sectoral climate assessments that support climate-informed decision-making in the United States.

Cross-shelf interactions and the associated lateral transport of organic carbon remain poorly quantified and constrained by observations, despite their potential importance for the ocean carbon cycle. Here, we provide the first systematic, satellite-based estimate of offshore organic carbon transport and the contribution of filamentary structures in Eastern Boundary Upwelling Systems (EBUS). Using a 21-year (2003–2023) dataset of chlorophyll-a and sea surface temperature, we apply an unsupervised k-means clustering approach to detect offshore-directed filaments and estimate their associated carbon content across coast–offshore reference depths ranging from 200 to 2000 m. We find an offshore transport of 46±6 to 58±7 TgC yr⁻¹ , with filaments contributing 9±3 –12±3TgC yr⁻¹ (15–30%) despite their limited spatial extent. Transport decreases with increasing reference depth and shows a clear seasonal cycle, particularly in Pacific systems, linked to variations in shelf carbon stock and mesoscale/submesoscale activity. Our results provide a scalable observational framework for monitoring lateral carbon transport.

The ocean plays a major role in controlling atmospheric carbon at decadal to millennial timescales, with benthic carbon representing the only geologic‐scale storage of oceanic carbon. Despite its importance, detailed benthic ocean observations are limited and representation of the benthic carbon cycle in ocean and Earth system models (ESMs) is mostly empirical with little prognostic capacity, which hinders our ability to properly understand the long‐term evolution of the carbon cycle and climate change‐related feedbacks. The Benthic Ecosystem and Carbon Synthesis (BECS) working group, with the support of the US Ocean Carbon & Biogeochemistry Program (OCB), identified key challenges limiting our understanding of benthic systems, opportunities to act on these challenges, and pathways to increase the representation of these systems in global modeling and observational efforts. We propose a set of priorities to advance mechanistic understanding and better quantify the importance of the benthos: (a) implementing a model intercomparison exercise with existing benthic models to support future model development, (b) data synthesis to inform both model parameterizations and future observations, (c) increased deployment of platforms and technologies in support of in situ benthic monitoring (e.g., from benchtop to field mesocosm), and (d) global coordination of a benthic observing program (“GEOSed”) to fill large regional data gaps and evaluate the mechanistic understanding of benthic processes acquired throughout the previous steps. Addressing these priorities will help inform solutions to both global and regional resource management and climate adaptation strategies.

Benthic biogeochemical models are critical for understanding and predicting seafloor processes that regulate ocean chemistry, carbon sequestration, benthic habitat conditions, and climate feedbacks. However, current sediment models have limited predictive capabilities with widely variable complexity, structure, and underlying assumptions, highlighting a lack of consensus on essential process representations. To address this issue, this paper introduces the Sediment Biogeochemistry Model Intercomparison Project (SedBGC_MIP), a community-driven initiative aimed at systematically comparing existing benthic models against available observational constraints to refine key parameterizations and assess structural uncertainties. We review the state of sediment biogeochemical modeling, highlighting discrepancies in the representation of carbon cycling, burial, and redox remineralization processes across different model complexities. Through case studies, we demonstrate how varying model structures and ecosystem dynamics create uncertainty in global predicted biogeochemical feedbacks. We outline the objectives of SedBGC_MIP, including the need for standardized benchmarking, observational datasets, and cross-disciplinary collaboration to improve model skill and integration into Earth System Models. Ultimately, SedBGC_MIP aims to advance our ability to simulate benthic processes with greater accuracy, enhancing projections of ocean biogeochemistry under climate change scenarios with new capacity to address emerging living marine resource and geoengineering applications.

Benthic biogeochemical models are critical for understanding and predicting seafloor processes that regulate ocean chemistry, carbon sequestration, benthic habitat conditions, and climate feedbacks. However, current sediment models have limited predictive capabilities with widely variable complexity, structure, and underlying assumptions, highlighting a lack of consensus on essential process representations. To address this issue, this paper introduces the Sediment Biogeochemistry Model Intercomparison Project (SedBGC_MIP), a community-driven initiative aimed at systematically comparing existing benthic models against available observational constraints to refine key parameterizations and assess structural uncertainties. We review the state of sediment biogeochemical modeling, highlighting discrepancies in the representation of carbon cycling, burial, and redox remineralization processes across different model complexities. Through case studies, we demonstrate how varying model structures and ecosystem dynamics create uncertainty in global predicted biogeochemical feedbacks. We outline the objectives of SedBGC_MIP, including the need for standardized benchmarking, observational datasets, and cross-disciplinary collaboration to improve model skill and integration into Earth System Models. Ultimately, SedBGC_MIP aims to advance our ability to simulate benthic processes with greater accuracy, enhancing projections of ocean biogeochemistry under climate change scenarios with new capacity to address emerging living marine resource and geoengineering applications.

The Interagency Working Group on Ocean Acidification monitoring Prioritization Plan 2024 calls for Coastal Acidification Networks to identify the ocean and coastal acidification (OCA) monitoring needs most important for their regions. The Northeast Coastal Acidification Network (NECAN) organized a webinar series to study regional needs, which culminated with a workshop in November 2023. This workshop led to the identification of six priority new Monitoring Needs in addition to the maintenance of current monitoring efforts: -Improve spatial and temporal scale of monitoring co-located OCA variables and biological measurements to better resolve variability of acidification dynamics in concert with biological processes -Increase subsurface monitoring to understand how conditions vary at depth -Increase the number of high-frequency monitoring assets that measure at least two of four carbon parameters -Increase near-real-time and rapid response observing capacity to capture episodic events -Determine fluxes and rates that would help parameterize and constrain regional modeling efforts to understand past conditions and project future trends -Increase spatial coverage of “climate”-quality observations This report presents monitoring needs and opportunities for consideration by coastal managers, decision makers, researchers, and monitoring groups. It offers options to apply new capacity or funding to the expansion of OCA monitoring in the NECAN region. Writing the report led to the identification of a number of cross-cutting actions which will lead to the implementation of these Monitoring Needs: ● Expand monitoring beyond carbonate chemistry to provide a complete assessment of OCA, its effects, and future trends. ● Enhance or leverage existing monitoring platforms for a cost-effective and collaborative approach to creating a more complete OCA monitoring system in the NECAN region. ● Expand the NECAN membership to include protected area experts, terrestrial biogeochemists and hydrologists, fisheries experts, social scientists, Tribal liaisons, project leads from large assessments, and other important stakeholders, rights holders and decision makers. ● Increase funding in the Northeast to both sustain currently-stretched efforts and grow a more robust ocean acidification monitoring program. ● Pursue immediate implementation of proxy approaches or interim strategies for measurements with technological or capacity limitations, while new technologies are being developed. ● Synthesize monitoring information to advance the collective understanding of OCA in the NECAN region. ● Deploy monitoring assets strategically, with end-user needs in mind, ensuring that the collected data is accessible, relevant, and useful for decision-making. ● Share NECAN’s experience in developing these recommendations with other regional CANs.