Quantifying mechanisms of variability in ocean CO2 uptake 1980-present

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Over time, the ocean has absorbed roughly one third of all human carbon dioxide (CO2) emissions due to fossil fuel burning and deforestation. However, observations show that the rate at which the ocean absorbs CO2 from the atmosphere can vary greatly from one year to the next, and on even longer timescales. These variations result from some combination of natural and human-caused climate variability, but our understanding of the reasons for this variability remains incomplete, which hampers our understanding of the global carbon budget and hinders our ability to make accurate climate forecasts. In this project, we will perform targeted experiments with several different state-of-the-art models of the ocean carbon cycle to determine what factors are responsible for variability in the rate of CO2 uptake by the ocean over the past four decades. These experiments will determine which models best represent the important climate processes driving variability in ocean CO2 uptake, and will identify areas where the models can be improved, which will lead to better constraints on the climate sensitivity of ocean carbon uptake, and lead to better climate projections. This work will also help to identify how and why the variability of CO2 uptake in models differs from the observations and help to resolve imbalances in the global carbon budget. Finally, this will lead to improved constraints on CO2 emissions to limit the damaging effects of global warming. The broader impacts of this work include the training of a graduate student in ocean carbon cycle modeling, and training of two undergraduate students from historically black colleges and universities.The ocean is one of the two main sinks for anthropogenic CO2 emissions (the terrestrial biosphere being the other) and absorbs roughly 25% of current anthropogenic CO2 emissions. Recent work has shown that the variability of the ocean CO2 sink rivals that of the terrestrial CO2 sink on decadal timescales, but the mechanisms driving this variability remain unclear. Identifying the drivers of variability in the ocean CO2 sink is important for understanding how the ocean carbon sink will respond to future climate changes, for developing more accurate prognostic climate models, and for reconciling decadal imbalances in the global carbon budget. This project will undertake targeted simulations with ocean biogeochemical models and ocean inverse models to answer the overarching science question: “What are the drivers of interannual to decadal variability in the ocean CO2 sink over the past four decades?” To answer this question this work focusses on three main objectives: (i) quantifying mechanisms of variability in ocean CO2 uptake over the past four decades in a suite of ocean biogeochemical models and an ocean circulation inverse model, (ii) comparing across models and methods to determine the most robust mechanisms driving variability in ocean CO2 uptake at both global and regional scales, and (iii) comparing the variability of ocean CO2 uptake in biogeochemical models and ocean inverse models to determine which models and mechanisms best agree with the observations. This work will determine which models best represent the important processes driving variability in ocean CO2 uptake and will identify areas where the models can be improved. This work will also help to identify how and why the variability of CO2 uptake in models differs from the observations, and help to resolve imbalances in the global carbon budget. Last, our results will lead to an improved mechanistic understanding of variability in the ocean carbon sink, which will lead to better constraints on the climate sensitivity of ocean carbon uptake, and to better climate projections.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.