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More bubbles means more variation in ocean carbon storage

The ocean absorbs carbon from the atmosphere, but exactly how much is uncertain. For instance, estimates from the ranged from 2.2 billion to 4 billion metric tons of carbon per year. One source of this uncertainty may be that the effects of bubbles have not been incorporated into air-sea carbon flux estimates, according to a new by P. Rustogi and colleagues published in Global Biogeochemical Cycles.

When waves break, they create multitudes of tiny bubbles that carry gases such as carbon dioxide back and forth between the atmosphere and water. Models used to evaluate how fast this exchange occurs typically rely on measurements of wind speed, assuming that wind speed directly relates to the prevalence of bubble-forming waves. However, waves can be affected by other factors as well, meaning this assumption doesn't always hold.

To assess the role of bubbles in air-sea carbon exchange in more detail, scientists applied a recently developed "bubble-mediated gas transfer theory" to the ocean. As with other models, the bubble-mediated approach incorporates wind strength, but uniquely, it also accounts for wave conditions that form gas-carrying bubbles. The researchers compared the results from their new model to a simpler, wind-only model that ignores the effect of bubbles.

The two models yielded similar estimates for total annual ocean carbon storage, but the bubble-mediated model showed much higher variability, both seasonally and regionally; in some instances, local fluxes that it indicated differed by 20%–50% from the wind-only model. The bubble-mediated model also suggested that intense wave activity in the Southern Hemisphere leads to much higher carbon storage than in the relatively calm Northern Hemisphere—a difference that's not obvious in the wind-only model.

That north-south difference could have implications for interpreting and projecting carbon cycle dynamics in a changing climate. With average wind speeds and wave heights likely to increase with global warming, it is essential to anticipate accurately how these changes will influence ocean carbon storage, the authors say.

The work is also important for marine carbon dioxide removal projects aiming to enhance carbon uptake to mitigate climate change effects, they note. A prerequisite for these efforts is quantifying how much carbon the ocean takes up naturally. Without a comprehensive understanding of the processes affecting uptake, the impacts of such interventions may be vastly under- or overestimated.