"Exploring the Surprising Growth of Coastal Sink Compared to Open Ocean"
In a recent study published in Nature Climate Change, Moritz Mathis and colleagues have found that the ocean's carbon sink has been strengthening faster in coastal waters than in the open ocean due to human-driven changes in ocean circulation and reduced sea-ice cover, leading to enhanced biological activity.
The global ocean carbon sink has been absorbing about 30% of the carbon emitted by human activities, regulating atmospheric CO2 levels and the Earth's climate. However, the traditional view of the ocean carbon sink as a passive solubility-driven process breaks down in the coastal ocean. Coastal oceans are a hub of carbon exchange, and they are very diverse and complex environments, encompassing vast shallow margins and narrow shelves, and coastlines that can be punctuated by deep fjords or dense networks of mangroves, as well as systems influenced by the upwelling of deep waters, intense boundary currents, or large riverine plumes.
Mathis and colleagues used a global ocean model with refined spatial resolution in coastal areas and a series of twentieth-century simulations isolating the influence of human factors, in particular the rise in atmospheric CO2, eutrophication, and changes in sea-ice, circulation, and biology. They found that the CO2 uptake per unit area has increased at a rate about one-third faster in coastal ocean waters than in the open ocean since 1900. They attribute 60% of this strengthening to biological changes, while the rise in atmospheric CO2 controls the remaining 40%.
The disproportionate contribution of biology in the coastal ocean relative to the open ocean arises from two factors in their model. First, the stimulation of marine photosynthesis and biological carbon drawdown in response to enhanced nutrient delivery by rivers, enhanced nutrient upwelling from the deep ocean due to changes in coastal winds and reduced sea-ice cover. Second, the amplification of this biological drawdown as the ocean buffer capacity goes down and the air-sea CO2 flux becomes more sensitive to changes in carbon content.
However, several sources of uncertainty need to be considered when interpreting the biological boost simulated by the model of Mathis and co-authors. The intensification of winds and upwelling that support a large fraction of the additional biological drawdown identified is highly uncertain. Another key factor is the fate of this biologically fixed carbon, that is, whether it is buried in sediments, exported to the open ocean and isolated from the atmosphere, or in contrast respired rapidly and released back to the atmosphere in shallow coastal waters.
Coastal oceans absorb CO2 but are hotspots of N2O and CH4 emissions, two highly potent greenhouse gases. Observations suggest that N2O and CH4 emissions have offset ~60% of the effect of coastal CO2 uptake in the atmospheric radiative balance, that is, warming avoided, over the past two decades. Increases in coastal biological productivity would yield a higher flux of organic matter to the deep ocean, higher respiration and lower oxygen concentrations, all potentially promoting N2O and CH4 production in coastal waters. Thus, if coastal CO2 uptake has indeed increased due to biological activity in the twentieth century, much of the effect of this CO2 uptake on the atmospheric radiative balance might have been offset by enhanced N2O and CH4 emissions, emphasizing the need to consider the three greenhouse gases when evaluating the influence of coastal waters on climate.
The study by Mathis and colleagues provides a solid argument in support of a biologically strengthened coastal ocean sink that is at odds with the traditional view of a passive ocean sink controlled by the rise in atmospheric CO2. Efforts to measure and evaluate changes in biological productivity, carbon chemistry, wind, and sea ice will be key to determine if this biological boost of the coastal ocean carbon sink occurs in nature.
Source: <https://www.nature.com/articles/s41558-024-01968-6>
Comments
Post a Comment