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1. Metrics for quantifying the efficiency of atmospheric CO 2 reduction by marine carbon dioxide removal (mCDR)

   https://doi.org/10.1088/1748-9326/ad7477  

   Yamamoto, Kana ; DeVries, Tim ; Siegel, David_A ( September 2024 , Environmental Research Letters)

   Marine carbon dioxide removal (mCDR) is gaining interest as a tool to meet global climate goals. Because the response of the ocean–atmosphere system to mCDR takes years to centuries, modeling is required to assess the impact of mCDR on atmospheric CO₂reduction. Here, we use a coupled ocean–atmosphere model to quantify the atmospheric CO₂reduction in response to a CDR perturbation. We define two metrics to characterize the atmospheric CO₂response to both instantaneous ocean alkalinity enhancement (OAE) and direct air capture (DAC): the cumulative additionality (α) measures the reduction in atmospheric CO₂relative to the magnitude of the CDR perturbation, while the relative efficiency (ϵ) quantifies the cumulative additionality of mCDR relative to that of DAC. For DAC,αis 100% immediately following CDR deployment, but declines to roughly 50% by 100 years post-deployment as the ocean degasses CO₂in response to the removal of carbon from the atmosphere. For instantaneous OAE,αis zero initially and reaches a maximum of 40%–90% several years to decades later, depending on regional CO₂equilibration rates and ocean circulation processes. The global meanϵapproaches 100% after 40 years, showing that instantaneous OAE is nearly as effective as DAC after several decades. However, there are significant geographic variations, withϵapproaching 100% most rapidly in the low latitudes whileϵstays well under 100% for decades to centuries near deep and intermediate water formation sites. These metrics provide a quantitative framework for evaluating sequestration timescales and carbon market valuation that can be applied to any mCDR strategy.

    

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2. Meeting GHG reduction targets requires accounting for all forest sector emissions

   https://doi.org/10.1088/1748-9326/ab28bb  

   Hudiburg, Tara W. ; Law, Beverly E. ; Moomaw, William R. ; Harmon, Mark E. ; Stenzel, Jeffrey E. ( August 2019 , Environmental Research Letters)

   Atmospheric greenhouse gases (GHGs) must be reduced to avoid an unsustainable climate. Because carbon dioxide is removed from the atmosphere and sequestered in forests and wood products, mitigation strategies to sustain and increase forest carbon sequestration are being developed. These strategies require full accounting of forest sector GHG budgets. Here, we describe a rigorous approach using over one million observations from forest inventory data and a regionally calibrated life-cycle assessment for calculating cradle-to-grave forest sector emissions and sequestration. We find that Western US forests are net sinks because there is a positive net balance of forest carbon uptake exceeding losses due to harvesting, wood product use, and combustion by wildfire. However, over 100 years of wood product usage is reducing the potential annual sink by an average of 21%, suggesting forest carbon storage can become more effective in climate mitigation through reduction in harvest, longer rotations, or more efficient wood product usage. Of the ∼10 700 million metric tonnes of carbon dioxide equivalents removed from west coast forests since 1900, 81% of it has been returned to the atmosphere or deposited in landfills. Moreover, state and federal reporting have erroneously excluded some product-related emissions, resulting in 25%–55% underestimation of state total CO₂emissions. For states seeking to reach GHG reduction mandates by 2030, it is important that state CO₂budgets are effectively determined or claimed reductions will be insufficient to mitigate climate change.

    

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3. Expedition 383 Preliminary Report: Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC)

   https://doi.org/10.14379/iodp.pr.383.2019  

   Lamy, F. ; Winckler, G. ; Alvarez Zarikian, C.A. ( October 2019 , Preliminary report)

   null (Ed.)

   The Antarctic Circumpolar Current (ACC) is the world’s strongest zonal current system that connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and constricted to its narrowest extent in the Drake Passage. Transport of fresh and cold surface and intermediate water masses through the Drake Passage (cold-water route) strongly affects the Atlantic Meridional Overturning Circulation together with the inflow of Indian Ocean water masses (warm-water route). Both oceanographic corridors are critical for the South Atlantic contribution to Meridional Overturning Circulation changes. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, the Pacific sector of the ACC lacks information on its Cenozoic paleoceanography from deep-sea drilling records. To advance our knowledge and understanding of Miocene to Holocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program (IODP) Expedition 383 recovered sedimentary sequences at (1) three sites located in the central South Pacific (U1539, U1540, and U1541), (2) two sites at the Chile margin (U1542 and U1544), and (3) one site from the pelagic eastern South Pacific (U1543) close to the entrance to the Drake Passage. Because of persistently stormy conditions and the resulting bad weather avoidance, we were not successful in recovering the originally planned Proposed Site CSP-3A in the central South Pacific in the Polar Frontal Zone. The drilled sediments at Sites U1541 and U1543 reach back to the late Miocene, and those at Site U1540 reach back to the early Pliocene. High sedimentary rate Pleistocene sedimentary sequences were drilled both in the central South Pacific (Site U1539) and along the Chile margin. Taken together, the sites represent a depth transect from ~1100 m at the Chile margin site (U1542) to ~4070 m in the central South Pacific (Site U1539) and allow investigation of changes in the vertical structure of the ACC, a key issue for understanding the role of the Southern Ocean in the global carbon cycle. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic-, carbonate-, and opal-based proxies to address our objectives of reconstructing with unprecedented stratigraphic detail surface to deep-ocean variations and their relation to atmosphere and cryosphere changes through stadial to interstadial, glacial to interglacial, and warmer than present time intervals. 

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4. Freshening of the western Arctic negates anthropogenic carbon uptake potential

   https://doi.org/10.1002/lno.11421  

   Woosley, Ryan J. ; Millero, Frank J. ( February 2020 , Limnology and Oceanography)

   As human activities increase the atmospheric concentration of carbon dioxide (CO₂), the oceans are known to absorb a significant portion. The Arctic Ocean has long been considered to have enormous potential to sequester anthropogenic CO₂, and mitigate emissions. The frigid waters make CO₂more soluble, and as sea ice melts, greater surface area is exposed to absorb CO₂. However, sparse data have made quantifying the amount of anthropogenic CO₂in the Arctic difficult, stimulating much debate over the basin's contribution to CO₂sequestration from the atmosphere. Using three separate cruises in 1994, 2005, and 2015 in the Canada and Makarov basins, we analyze the decadal variability in anthropogenic CO₂uptake in the central western Arctic. Here we show, from direct carbon system measurements spanning two decades, that despite increased atmospheric CO₂, total dissolved inorganic carbon has actually decreased, with minimal anthropogenic CO₂uptake. The reduction in dissolved CO₂results from a dilution of total alkalinity by increased freshwater supply, particularly river water. Changes in the freshwater budget of the western Arctic override its uptake potential, resulting in a weak sink, or possibly source of CO₂.

    

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5. Dynamics of the Pacific Antarctic Circumpolar Current

   https://doi.org/10.14379/iodp.proc.383.2021  

   Lamy, F. ; Winckler, G. ; Alvarez Zarikian, C.A. ( July 2021 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   The Antarctic Circumpolar Current (ACC), the world’s strongest zonal current system, connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and is constricted to its narrowest extent in the Drake Passage. Fresh and cold Pacific surface and intermediate water flowing through the Drake Passage (cold-water route) and warm Indian Ocean water masses flowing through the Agulhas region (warm-water route) are critical for the South Atlantic contribution to Meridional Overturning Circulation changes. Furthermore, physical and biological processes associated with the ACC affect the strength of the ocean carbon pump and therefore are critical to feedbacks linking atmospheric CO2 concentrations, ocean circulation, and climate/cryosphere on a global scale. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, there are no deep-sea drilling paleoceanographic records from the Pacific sector of the ACC. To advance our understanding of Miocene to Holocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program Expedition 383 recovered sedimentary sequences at (1) three sites in the central South Pacific (CSP) (U1539, U1540, and U1541), (2) two sites at the Chilean margin (U1542 and U1544), and (3) one site from the pelagic eastern South Pacific (U1543) close to the entrance to the Drake Passage. Because of persistently stormy conditions and the resulting bad weather avoidance, we were not successful in recovering the originally planned Proposed Site CSP-3A in the Polar Frontal Zone of the CSP. The drilled sediments at Sites U1541 and U1543 reach back to the late Miocene, and those at Site U1540 reach back to the early Pliocene. High sedimentation rate sequences reaching back to the early Pleistocene (Site U1539) and the late Pleistocene (Sites U1542 and U1544) were recovered in both the CSP and at the Chilean margin. Taken together, the sites represent a depth transect from ~1100 m at Chilean margin Site U1542 to ~4070 m at CSP Site U1539 and allow investigation of changes in the vertical structure of the ACC, a key issue for understanding the role of the Southern Ocean in the global carbon cycle. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic-, carbonate-, and opal-based proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep-ocean variations and their relation to atmosphere and cryosphere changes. 

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