New Delhi: Salt marshes, seagrass meadows and coral reefs around the world have been popular tourist destinations for ages, but what if one were to say that these coastal habitats also play a key role in reducing carbon emissions?
Researchers have found that coastal habitats act as a reservoir for storing carbon dioxide and, in turn, reduce atmospheric pollution, making them vital greenhouse gas sinks. Although, other emissions of methane and nitrous oxide gases offset this impact by 9–20 per cent globally.
A greenhouse gas (GHG) sink facilitates a reduction in harmful atmospheric gases by storing carbon in a different form. These sinks reduce annual greenhouse gas emissions by 11.4 per cent. Ten per cent of these offsets are effected by chemical processes that take place in forests and underneath soils.
This was established in a study was conducted by a group of international researchers led by Australia’s Southern Cross University and was published in the peer-reviewed journal Nature on 22 May.
According to the study, eight of ten coastal regions throughout the world are net greenhouse gas sinks for these three primary greenhouse gases, with Southeast Asia, North America and Africa being major hotspots.
Ecosystems such as coastal wetlands take up atmospheric carbon dioxide and convert it into new leaves, roots and other organic matter. This can help battle rising CO₂ levels in the atmosphere.
By analysing data collected from 738 sites in 10 global regions and collated in studies published between 1975 and 2020, researchers looked at the various ecological impacts of a variety of regions like lagoons, polar fjords, salt marshes, coastal mangrove forests, and underwater seagrass communities.
They assessed the carbon dioxide, methane and nitrous oxide emissions of the various habitats.
Due to human activity, atmospheric concentrations of gases like carbon dioxide, methane, and nitrous oxide have increased by 47, 156 and 23 per cent, respectively, since the beginning of the Industrial era. And while coastal regions offer a respite, methane and nitrous oxide counteract that effect, making this region a net source for greenhouse gases.
“The GHG radiative balances of estuaries and coastal vegetation are a complex spatial and temporal combination of GHG sources and sinks, which complicates the estimate of the net global warming effect and makes the implementation of efficient mitigation strategies difficult,” the researchers wrote, underlining the objective of their study.
An estuary is a coastal water body where rivers and streams meet the sea. The radiation balance, on the other hand, is a measure of the amount of energy available on the earth’s surface to drive the photosynthetic process.
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Spatial & temporal differences
Even though estuaries or tidal systems are well-connected to marshes or areas of vegetation, there is a big difference in how they release or absorb greenhouse gases.
Southeast Asia boasts of the largest and most prolific tropical mangrove forests and seagrass meadows. They absorb a significant quantity of CO2, making it the strongest coastal greenhouse gas sink. The region also has relatively fewer estuaries, which are a source of greenhouse gases emitted as a result of the decomposition of organic waste.
The North American coast is another good sink, owing to its salt marshes, mangroves, seagrass, and fjords (valleys created by glaciers and filled with saltwater).
“Fjords globally absorb approximately 40 per cent of CO2 otherwise released from tidal systems, deltas, and lagoons. North America, particularly Greenland, is responsible for a whopping 86 per cent of this critical CO2 uptake,” said Bradley Eyre, one of the authors of the study.
Africa, too, is a large greenhouse gas sink due to its coastal vegetation.
However, that is partially counteracted by estuaries, making it the third greatest net GHG sink, globally. Long stretches of lagoons found along Africa’s coastline contribute 24 per cent to global lagoon CO2 emissions.
Long stretches of coastal wetlands in Australia and New Zealand absorb CO2 as well, which is also countered by a significant number of estuaries that emit these potent gases.
The researchers also pointed out that the coasts of Europe and Russia emit more greenhouse gases overall than they take in. Because of the region’s cooler temperature and numerous polluted estuaries, there are not enough wetlands to absorb the greenhouse gases that are released.
The researchers concluded that even though three times as much methane is released by coastal wetlands than there is from all estuaries combined, coastal wetlands are still significant CO2 sinks. Some of them can also absorb nitrous oxide, thus making them a net greenhouse gas sink. They called this a “cooling effect” on the global radiative balance.
“The future role of coastal ecosystems as a sink or source of GHGs in each world region will depend on the adoption of best practices to reduce CH4 and N2O emissions while strengthening CO2 uptake,” they wrote.
(Edited by Amrtansh Arora)