How coasts protect the climate

How coasts protect the climate

Salt marsh on the island of Assateague off the US coast. © Sara Cottle via Unsplash

Coastal areas in different regions of the world make an important contribution to climate protection. A study shows that most coastal ecosystems absorb more greenhouse gases from the atmosphere than they emit. Tropical coastal wetlands with mangrove forests and seagrass meadows are particularly effective, as are polar fjords, for example. Only the coasts of Europe and Russia emit net greenhouse gases - mainly due to the many estuaries that bring nutrients from the land. The findings can help to include coastal protection in climate protection strategies and to design it in a meaningful way.

Greenhouse gases such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) contribute to global warming. In addition to human emissions, natural buffers such as oceans, forests or bodies of water also play an important role in the Earth's greenhouse gas balance. They can store large amounts of greenhouse gases and thus act as a sink. However, especially when temperatures rise, many of these natural sinks emit more greenhouse gases and thus additionally increase the warming. Under current conditions, the oceans are a carbon sink, while inland lakes and rivers emit more greenhouse gases than they absorb. So far, however, it has been unclear which roles the coastal regions will play.

Studies on coastal ecosystems worldwide

A team led by Judith Rosentreter from Southern Cross University in Australia has now evaluated for the first time on a global scale the influence of coastal structure and vegetation on the absorption and release of greenhouse gases. To do this, the team evaluated studies from 738 locations where the exchange of gases between coastal ecosystems and the atmosphere was investigated and published between 1975 and 2020. "Understanding how and where greenhouse gases are released and absorbed in coastal ecosystems is an important first step in implementing effective climate protection strategies," says Rosentreter.

For ten regions of the world - including North and South America, Europe, Africa, Russia, East, West, South and Southeast Asia and Australia - the researchers analyzed how the climate, hydrology and existing plants affect whether the the bottom line is that the respective coastal areas absorb or emit more greenhouse gases. They took into account structural features such as estuaries, lagoons and fjords, as well as different types of vegetation, including mangrove forests, seagrass and salt marshes.

Mangrove forests as CO2 sinks

The results: Mangrove forests are particularly effective CO2 sinks. “At 60 percent, highly productive mangrove forests make the largest contribution to global CO2 uptake by coastal vegetation,” the researchers report. Around 359 million tons of CO2 are stored annually by mangrove forests. This more than compensates for the fact that around 340,000 tons of methane and 8,900 tons of nitrous oxide are released there at the same time. "Since mangrove forests are mainly found in tropical Southeast Asia, this region accounts for 37 percent of the global CO2 absorption by mangroves, followed by Africa with 20 percent and tropical North and South America with 15 percent each," explains the team. Seagrass and salt marshes also absorb more greenhouse gases than they emit.

The remaining coastal structures, on the other hand, show a mixed picture. Estuaries, deltas and lagoons emit more of all three greenhouse gases than they absorb. One of the reasons is that the rivers carry numerous nutrients from the interior of the country – including carbon and nitrogen from the remains of animals and plants and from agricultural fertilization of fields. In addition, they can also release nutrients from the sediments, which are also converted by microorganisms into greenhouse gases, dissolved in the water and finally released into the atmosphere. "In addition, the significant influence of tides can increase water turbulence and thus gas transfer rates, which in turn increases CO2 escape from tidal systems," explain Rosentreter and her team.

Eight out of ten regions as greenhouse gas sinks

Fjords, on the other hand, emit small amounts of nitrous oxide, but absorb large amounts of CO2. "Our research results show that fjords around the world absorb up to 40 percent of the CO2 that would otherwise be released from tidal systems, deltas and lagoons," says co-author Bryce Van Dam from the Helmholtz Center Hereon in Geesthacht. "The majority, namely 86 percent of this important CO2 uptake by fjords, comes from the North American region, above all from Greenland." Greenland is politically part of Denmark, but is counted geographically on the North American continent.

Overall, the combination of various factors makes the coastal regions of Southeast Asia the largest sink for greenhouse gases, in particular due to the predominant mangrove forests and seagrass beds. In second place is North America, which is also home to extensive areas of mangroves, seagrass and salt marshes in tropical coastal areas and contributes to the absorption of greenhouse gases in the polar regions through fjords. "If we take all three greenhouse gases into account, the coasts in eight of the ten regions of the world we examined are net sinks for greenhouse gases," says Rosentreter. The only exceptions are Europe and Russia, where the negative effect of estuaries and tidal systems outweighs uptake by plants and fjords.

Implications for climate protection

From the perspective of the researchers, important implications for climate protection can be derived from the study. It is important to protect and restore ecosystems with mangrove forests and salt marshes in order to increase the CO2 uptake by these coastal wetlands. In addition, it is important to reduce nutrient inputs from organic matter and sewage into coastal waters and thus mitigate outgassing. "The future role of coastal ecosystems as sinks or sources of greenhouse gases in different regions of the world will depend on the application of best practices to reduce CH4 and N2O emissions while increasing CO2 uptake," say the research team.

Source: Judith Rosentreter (Southern Cross University, Lismore, New South Wales, Australia) et al., Nature Climate Change, doi: 10.1038/s41558-023-01682-9

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