Carbon flow through inland and coastal waterways, implications for climate

A recent study by an international team of scientists including Raymond Najjar, professor of oceanography at Penn State, found that the flows of carbon through the complex network of water bodies that connect land and ocean has often been overlooked and that ignoring these flows overestimates the carbon storage in terrestrial ecosystems and underestimates sedimentary and oceanic carbon storage.
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Terrestrial and marine ecosystems have a powerful influence on the Earth’s climate by regulating the level of atmospheric carbon dioxide. A recent study found that the flows of carbon through the complex network of water bodies that connect land and ocean has often been overlooked and that ignoring these flows overestimates the carbon storage in terrestrial ecosystems and underestimates sedimentary and oceanic carbon storage.

Carbon storage by the ocean and by land is usually quantified separately and does not fully consider the land-to-ocean transport of carbon through inland waters, estuaries, tidal wetlands and continental shelf waters — referred to as the land-to-ocean aquatic continuum or LOAC. In a detailed analysis of the LOAC, researchers from Belgium, the United States and France provide a perspective on the global carbon cycle and identify key knowledge gaps that have significant implications for enforcing the carbon calculations that are part of international climate accords. They reported their findings in the journal Nature.

“There are scientists who develop terrestrial models and scientists who develop marine models, and the continuum between them is poorly represented in global earth system models,” said Raymond Najjar, professor of oceanography at Penn State and co-author on the paper. “There are different communities of scientists studying land and ocean. Our assessment indicates that more attention needs to be paid to the LOAC, and that our models need to better represent it.”

The researchers discovered that the LOAC carries a substantial amount of carbon of anthropogenic origin. Thus, the carbon removed from the atmosphere by terrestrial ecosystems is not all stored locally, as is commonly assumed, which has implications for global agreements that require countries to report their carbon inventories. The researchers also found that the land-to-ocean carbon transfer of natural origin was larger than previously thought, with far reaching implications for the assessment of the anthropogenic carbon dioxide uptake by the ocean and the land.

“The complexity of the LOAC has made it challenging to assess its influence on the global carbon cycle,” said Pierre Regnier, a professor at the University of Brussels and lead author on the paper.

The scientists said that because of that complexity, important global carbon budgeting efforts, such as those of the U.N. Intergovernmental Panel on Climate Change and the Global Carbon Project, typically assume a direct “pipeline” transfer of carbon from river mouths to the open ocean. Another common assumption is that all the transported carbon is natural, neglecting the impacts of human perturbations on the LOAC, such as damming of rivers and the destruction of coastal vegetation.

In this study, the scientists synthesized more than 100 individual studies of the various components of the continuum. From this synthesis, LOAC carbon budgets were developed for two time periods: the pre-industrial period and the present day. Their results confirm the well-known pre-industrial carbon “loop” in which carbon is taken up from the atmosphere by terrestrial ecosystems, transferred by rivers to the ocean, and then outgassed back to the atmosphere.

“We find the amount of carbon carried by this natural land-to-ocean loop, 0.65 billion tons per year, is roughly 50% greater than previously thought,” said Laure Resplandy, an assistant professor at Princeton University and co-author on the paper.

The researchers also found that this loop is comprised of two smaller loops, one that transfers carbon from terrestrial ecosystems to inland waters and another from coastal vegetation — referred to as blue carbon ecosystems — to the open ocean.

“A larger pre-industrial land-to-ocean carbon transport implies that the ocean uptake of anthropogenic carbon dioxide previously inferred from observations was underestimated,” Resplandy said.

“The flip side is that the land uptake of anthropogenic carbon dioxide was overestimated,” said Regnier.

The study demonstrates that anthropogenic carbon carried by rivers is either outgassed back to the atmosphere or eventually stored in aquatic sediments and the open ocean.

“This new view of the anthropogenic carbon dioxide budget may have a silver lining because sediments and the ocean offer arguably more stable repositories than terrestrial biomass and soil carbon, which are vulnerable to droughts, fires and land use change,” said Philippe Ciais, a research director at the Laboratory for Sciences of Climate and Environment in France and co-author on the paper.

The researchers also have shown that humans have decreased the uptake of atmospheric carbon dioxide from blue carbon ecosystems by up to 50%.

“If left unprotected from sea level rise, pollution and coastal development, blue carbon uptake of atmospheric carbon dioxide will further decline and contribute to additional climate warming,” said Najjar. “The damage that we have done to blue carbon ecosystems and continue to do to them is reducing their capacity to store carbon and will contribute to higher levels of atmospheric carbon dioxide and climate warming.”

Source/Credit: Pennsylvania State University / Patricia Craig

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