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Abstract

KEY FINDINGS
1. Observing networks and high-resolution models are now available to construct coastal carbon budgets. Efforts have focused primarily on quantifying the net air-sea exchange of carbon dioxide (CO₂), but some studies have estimated other key fluxes, such as the exchange between shelves and the open ocean.

2. Available estimates of air-sea carbon fluxes, based on more than a decade of observations, indicate that the North American margins act as a net sink for atmospheric CO₂. This net uptake is driven primarily by fluxes in the high-latitude regions. The estimated magnitude of the net flux is 160 ± 80 teragrams of carbon per year (medium confidence) for the North American Exclusive Economic Zone, a number that is not well constrained.

3. The increasing concentration of CO₂ in coastal and open-ocean waters leads to ocean acidification. Corrosive conditions in the subsurface occur regularly in Arctic coastal waters, which are naturally prone to low pH, and North Pacific coastal waters, where upwelling of deep, carbon-rich waters has intensified and, in combination with the uptake of anthropogenic carbon, leads to low seawater pH and aragonite saturation states in spring, summer, and early fall (very high confidence, very likely).

4. Expanded monitoring, more complete syntheses of available observations, and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.

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