Global study reveals the extensive loss of salt marshes

• According to a recent study based on satellite images, the planet lost 1,453 square kilometers of salt marshes between 2000 and 2019, which is twice the size of Singapore.
• In addition to providing habitat and many ecosystem services for wild fauna and flora, salt marshes are significant carbon sinks.
• According to the study, the loss of salt marshes led to the emission of 16.3 teragrams or 16.3 million metric tons of carbon per year, which corresponds to the emissions of about 3.5 million cars.
• One of the biggest problems for the wetlands is climate change. Other factors contributing to their reduction in their worldwide range include conversion to aquaculture, coastal erosion, eutrophication, reclamation works, occupation of mangrove forests, and invasive species.

Analyzing satellite images from around the globe, NASA scientists determined that between the late 20th and early 21st centuries, the area of ​​salt marshes lost was twice the size of Singapore. Severe storms contributed to that loss, which led to "significant" carbon emissions, according to a recent map-based study.

Published in the journal Nature at the end of November, the study showed that 2,733 square kilometers of wetlands were lost worldwide over a 19-year period between 2000 and 2019, as well as 1,278 km 2 recovered, a result of the restoration works carried out by humans. The result is a net loss of 1,453 km 2. Globally, salt marsh loss was 0.28% per year, according to the study.

In the past, the availability of up-to-date information on how rapidly salt marshes are being lost and where they are most threatened globally was limited, as were data on resulting carbon emissions, Anthony told Mongabay in an interview. Campbell, lead author of the study. Past assessments have suggested that salt marshes are shrinking much faster at a rate of between 1% and 2% per year.

“We used Landsat data to arrive at these results and then computed an updated carbon balance for salt marshes, which shows less loss but still significant emissions in salt marsh areas,” said Campbell, a research fellow. postdoctoral fellow at NASA's Biospheric Sciences Laboratory in Maryland.

According to the study, the loss of salt marshes led to annual carbon emissions of 16.3 teragrams equivalent to 16.3 million metric tons. This corresponds to about 3.5 million cars emitted annually. The loss of salt marshes has also reduced their own ability to store carbon.

The results are "reason for thought," observes Peter Macreadie, in an email addressed to Mongabay, who was not involved in the study and heads the Blue Carbon Lab at Deakin University in Burwood (Australia). They clarify the picture about the distribution and extent of salt marshes, he adds. "They are sadly losing ground and future climate change will make these ecosystems even more vulnerable."

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Salt marshes: “hidden protagonists”

Coastal wetlands are often known as Salt marshes and They depend on tidal flows to sustain the fragile balance of seawater. Found mainly in temperate areas of the planet, marshes are "hidden players" that provide different ecosystem services, Macreadie points out.

“This modest coastal [ecosystem] rich in vegetation decisively favors fish species and fishing activities, is the habitat of some of the rarest species on the planet, helps to stabilize the coastal strips, prevents erosion, and represents one of the most efficient and long-term carbon sinks on the planet,” he writes.

Along with other coastal ecosystems, however, salt marshes are under pressure from human activities. According to a study published earlier this year, only 15.5% of coastal strips worldwide are unaffected by human activities while a previous study found that salt marshes have lost between 25% and 50 % of the area occupied by them in the past globally.

Climate change is one of the biggest threats they face. Sea-level rise can outpace salt marshes' adaptive capacity or push them further inland, where artificial reefs can halt their expansion and ultimately reduce their extent in a process known as "coastal compression.". Other factors threatening the world's wetlands include conversion to aquaculture, coastal erosion, eutrophication, land reclamation, mangrove forest occupation, and invasive species.

By analyzing past satellite images and comparing them over time, Campbell and his team found that the majority (64%) of salt marsh losses occurred in the United States and Russia, often in caused by hurricanes and coastal erosion. The largest documented marsh loss in history and globally occurred between 2005 and 2009 in North America, where approximately 283 km2 was lost, much of it in hurricane-affected areas.

Uncertainties remain regarding, for example, the total extent of salt marsh ecosystems around the world, whether factors other than coastal erosion may be driving the loss of marshlands in the Arctic, and the extent of occupation of mangrove forests in areas such as Oceania. More analyzes of satellite imagery are needed to fill those gaps in our knowledge, Campbell notes.

Protection of salt marshes

Salt marshes are known as a potential natural solution to climate change as well as a way to mitigate some of its effects. Activities are underway for the restoration and conservation of these marshes as well as other coastal environments, in order to exploit their ecosystem services. The restoration of tidal flows, the reintroduction of vegetation, and the reduction of coastal erosion are some of the objectives set.

Campbell notes that, at the moment, it's difficult to know whether overall marshes still serve as carbon sinks or are losing enough to become a net source of carbon emissions. It is unclear whether humanity will be able to reverse their global loss to unequivocally return them to carbon sinks. One reason is that it's not known exactly how much carbon swamps can store, says Campbell.

William Austin, an expert on 'blue' carbon at the University of St Andrews, considers it 'far-fetched' to believe that improving the condition of wetlands through 'management actions' could reduce emissions and increase their carbon storage capacity. Even if that were the case, though, he notes that the study clearly urges salt marshes to be protected. “While we probably don't yet have enough monitoring data to support such a far-reaching policy change, they appear to be a very meaningful nature-based solution for BOTH carbon AND nature,” he said in an e-mail.

Tracking salt marshes and other coastal environments, such as oceanic meadows, with satellite technology is critical to "consistently and safely monitor such important blue carbon systems," says Campbell.

“With greater spatial knowledge of where they are and how they are changing, we have a better ability to protect them and manage the areas most threatened by change,” he added.