The majority of the global carbon-budgeting measures assume a linear flow of water from the land to the sea. This avoids the complicated interplay between rivers, lakes, streams, estuaries, groundwater, mangroves and more.
Study led by Princeton climate expert Laure Resplandy details how carbon is stored in or transported through inland and coastal waterways, a work with significant implications for enforcing international climate accords. Image Credit: iStock.
A study co-led by Laure Resplandy — climate scientist and assistant professor of geosciences — and the High Meadows Environmental Institute (HMEI) at Princeton University, elaborates how carbon is stored and transported via the intricacy of inland and coastal waterways.
Reported in the current issue of the journal Nature, the work has considerable impacts for imposing the carbon calculations that are considered to be a part of international climate accords.
Marine and terrestrial ecosystems have a major impact on the climate by controlling the level of atmospheric carbon dioxide (CO2). But these ecosystems are frequently viewed as being disconnected from each other. This avoids the transfer of carbon from land to the open ocean via a complicated network of water bodies — the continuum of rivers, streams, estuaries and other bodies taking water from land to the sea.
In an elaborate analysis, the group of scientists from Belgium, the United States, and France have found that this land-to-ocean aquatic continuum (LOAC) carries a considerable amount of carbon of anthropogenic (for example, fossil-fuel) origin.
Therefore, the carbon withdrawn from the air by terrestrial ecosystems is not all stored locally, as is generally assumed. This has impacts on global agreements that need countries to report their carbon inventories.
Also, the researchers discovered that the land-to-ocean carbon transfer of natural origin was larger compared to what was previously thought. This has far-reaching impacts on the evaluation of the anthropogenic CO2 uptake by the land and ocean.
The complexity of the LOAC, which includes rivers, groundwater, lakes, reservoirs, estuaries, tidal marshes, mangroves, seagrasses, and waters above continental shelves, has made it challenging to assess its influence on the global carbon cycle.
Pierre Regnier, Professor, University of Brussels
Regnier co-led the study with Resplandy.
Due to that complexity, significant global carbon-budgeting efforts, such as those of the UN Intergovernmental Panel on Climate Change and the Global Carbon Project, normally assume a direct “pipeline” transfer of carbon from river mouths to the open ocean.
One more common assumption is that all the shifted carbon is natural. This neglects the effects of human perturbations on this aquatic continuum, like damming and the decimation of coastal vegetation.
In this study, the scientists synthesized over 100 separate studies of the numerous components of the continuum. From this synthesis, LOAC carbon budgets were made for two time periods: the pre-industrial period and the present day.
Their outcomes substantiate the well-known pre-industrial carbon “loop” in which carbon is taken up from the air by terrestrial ecosystems. This is transferred by rivers to the ocean, and further outgassed back to the air.
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.
Laure Resplandy, Assistant Professor, Geosciences, High Meadows Environmental Institute, Princeton University
Moreover, this loop is made of two smaller loops, one that transfers carbon from terrestrial ecosystems to inland waters and another from coastal vegetation (known as “blue carbon ecosystems”) to the open ocean.
A larger pre-industrial land-to-ocean carbon transport implies that the ocean uptake of anthropogenic CO2 previously inferred from observations was underestimated.
Laure Resplandy, Assistant Professor, Geosciences, High Meadows Environmental Institute, Princeton University
“The flip side is that the land uptake of anthropogenic CO2 was overestimated,” added Regnier.
The study illustrates that anthropogenic carbon that is carried by rivers is either outgassed back to the air or stored in aquatic sediments and the open ocean.
Philippe Ciais, a research director at the Laboratoire des Sciences du Climat et de l’Environnement and a co-author of the study explained: “This new view of the anthropogenic CO2 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.”
Also, scientists have revealed that humans have reduced the uptake of atmospheric CO2 from blue-carbon ecosystems by up to 50%.
“If left unprotected from sea-level rise, pollution, and coastal development, blue-carbon uptake of atmospheric CO2 will further decline and contribute to additional climate warming,” stated Raymond Najjar, a professor from the Pennsylvania State University who also co-authored the study.
Journal Reference:
Regnier, P., et al. (2022) The land-to-ocean loops of the global carbon cycle. Nature. doi.org/10.1038/s41586-021-04339-9.