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Coastal Ecosystems Help Store Greenhouse Gases

New research, published in Nature Climate Change, shows more detailed role of salt marshes, fjords

Coastal ecosystems globally lock away more carbon dioxide than they release, but emissions of two other greenhouse gases – methane and nitrous oxide – counteract that to a degree, according to international researchers led by Australia’s Southern Cross University that include a University of Florida biogeochemist.

The new findings of the coastal greenhouse gas balance in 10 regions globally are outlined in the paper published today in Nature Climate Change.

From tropical lagoons to polar fjords, from coastal mangrove forests to underwater seagrass communities, many coastlines around the world show high diversity in greenhouse gas sinks and emissions.

“Understanding how and where greenhouse gases are released and absorbed in coastal ecosystems is an important first step for implementing effective climate mitigation strategies,” said lead researcher Judith Rosentreter, senior research fellow at Southern Cross University.

Salt marsh on Assateague Island
Salt marshes, like this one on Assateague Island, off the eastern coast of the United States, help store greenhouse gases that otherwise could be released into the atmosphere and contribute to global climate change. Photo by Chris King on Unsplash.

Other activities to curb human impact, like reducing nutrients, organic matter, and wastewater inputs into coastal waterways, can reduce the amount of methane and nitrous oxide released into the atmosphere.

“We have long known that the coastal ocean is where over 80% of all the organic carbon is buried in the global ocean, but this new study shows critical information on where the regional hotspots are for greenhouse gas fluxes,” said Thomas Bianchi, a UF biogeochemist in the Department of Geological Sciences who contributed to the study.

“Our new research shows that fjords around the world take up about 40% of carbon dioxide that would otherwise be released from tidal systems, deltas and lagoons. Most (86%) of this important carbon dioxide uptake by fjords comes from the North America region, mostly Greenland,” said co-author Bradley Eyre, professor of biogeochemistry at Southern Cross University.

The global team of scientists looked at regions in North America, South America, Europe, Africa, Russia, West Asia, South Asia, East Asia, Southeast Asia and Australasia.

They found the strongest coastal reservoir of greenhouse gases was in Southeast Asia because of its extensive and productive tropical coastal wetlands that take up carbon dioxide. A second sink hotspot is North America, with its large areas of coastal wetlands but also fjords where deep sediments store carbon dioxide.

“As the climate warms, many organisms are migrating towards the poles; for example, mangroves are now replacing salt marshes as they expand their range. This creates even more complexity for scientists when trying to develop better models for predicting greenhouse gas fluxes in coastal habitats around the world,” Bianchi said.

The findings will inform the efforts of the Global Carbon Project’s RECCAP2.

Illustration of study areas around the globe
The coastal greenhouse gas (GHG) budget (CO2 + CH4 + N2O) as CO2-equivalent fluxes (in teragrams of CO2-equivalent per year) in 10 regions around the world: Southeast Asia (9), North America (1), Africa (4) are strong coastal GHG sinks. South America (2), Australasia (10), and West Asia (6) are moderate coastal GHG sinks. East Asia (7) and South Asia (8) are weak coastal GHG sinks, and Europe (3) and Russia (5) are weak coastal GHG sources. :Figure from Rosentreter et al. (2023), Nature Climate Change.

Snapshot: coastal greenhouse gas sinks and sources around the world 

A dataset of observations from 738 sites from studies published between 1975 and 2020 was compiled to quantify CO2, CH4, and N2O fluxes in estuaries and coastal vegetation in 10 global regions.

Special coastal features (climate, hydrology, abundance) in each region around the world drive the GHG uptake and/or release from coastal systems.

Strongest coastal greenhouse gas sinks: 

  • Top: the archipelagic region of Southeast Asia, because of its extensive and productive tropical mangrove forests and seagrasses that take up large amounts of CO2.
  • Next: North America because of its large areas of salt marshes, mangroves, and seagrasses but also CO2-uptaking fjords.
  • Third: Africa with large CO2 uptake by mangroves and seagrasses that is moderately reduced by estuarine GHG emissions.

Moderate coastal greenhouse gas sinks: 

  • South America: moderate CO2 uptake by coastal wetlands, especially mangroves, and some estuarine GHG emissions.
  • Australasia: long stretches of coastal wetlands that take up CO2, but this region also has a large number of estuaries along its coasts, many of which are a source of CO2, CH4 and N2O.
  • West Asia: weak estuarine GHG source and moderate CO2 uptake by coastal wetlands, mostly seagrasses.

Weak coastal greenhouse gas sinks: 

  • East Asia and South Asia: the moderate coastal wetlands CO2 sink is largely reduced by estuarine GHG emissions.

Weak coastal greenhouse gas sources: 

  • Europe and Russia: both regions release more coastal GHG than they can take up from the atmosphere. These regions have many impacted tidal estuaries that release greenhouse gases; a colder climate also means they have fewer coastal wetlands (eg, mangroves) that would otherwise take up large amounts of CO2.

This story was adapted from a press release from Southern Cross University, Lismore, Australia.