Constraining the Past and Future Ocean Sink of Anthropogenic Carbon with Observations

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The release of fossil fuel CO2 to the atmosphere by human activity has been implicated as the predominant cause of global climate change. The ocean plays a crucial role in mitigating the effects of this perturbation to the climate system, sequestering around a third of anthropogenic CO2 emissions. While much progress has been made in recent years in understanding and quantifying the ocean sink, considerable uncertainties also remain, especially regarding the partitioning of CO2 emissions between the ocean and terrestrial sinks, changes in ocean chemistry, and the future ability of the ocean to sequester CO2.In this project, researchers at the Lamont-Dougherty Earth Observatory of Columbia University will develop and apply an observationally-based approach to constraining the past and future ocean sink of anthropogenic CO2 (Cant). The essential idea is that the anthropogenic CO2 perturbation in the ocean can be treated as a conservative tracer transported by ocean circulation from the mixed layer into the interior. This transport can be described by a Green function, which, convolved with the time-varying surface concentration of Cant, can be used to calculate the concentration in Cant in the ocean. Inverse methods, based on the 'maximum entropy' approach and insights from carbon cycle models, will be used to estimate the ocean's Green function and the Cant surface boundary condition from observations, resulting in an approach that accounts for both the ocean?s complex 3-d circulation and the time-varying air-sea disequilibrium of CO2 over the industrial period.Intellectual merit: This study will directly lead to an improved quantification and understanding of the ocean sink of anthropogenic CO2. Its main outcome will be an observationally-based reconstruction of the historical, time-evolving distribution, uptake, and transport of anthropogenic CO2 in the ocean over the industrial period, and projections for future uptake in response to different emission scenarios. Analysis of these estimates will help constrain potential changes in marine chemistry, quantify the relative roles of the ocean and terrestrial sinks, and lead to new insights into the role of ocean ventilation in the carbon cycle. Broader Impacts: By providing a quantitative estimate of the past and future evolution of the ocean sink of anthropogenic CO2, the proposed work is highly relevant to current efforts to more accurately constrain human impacts on the climate system, an issue of societal importance. Both the Cant fields and Green functions estimated as part of this project will be made freely available to other researchers. The Green functions will be valuable for a variety of problems, ranging from ocean ventilation to marine biogeochemical cycles. This research will contribute to the training and education of a graduate student, and the results will be incorporated into the PI's teaching and disseminated more broadly via a website.