Coastal Ecosystems Serve as a Crucial Sink for Greenhouse Gases, from Mangroves to Fjords, and Can Also Emit Them.

Coastal ecosystems can absorb or emit three major greenhouse gases: carbon dioxide, methane, and nitrous oxide.

We explored how coastlines in ten regions of the world differ in the uptake and emissions of greenhouse gases. Our research Posted today in The nature of climate change He found that, globally, our coastal ecosystems are net sinks of greenhouse gases, but smaller emissions of the powerful methane and nitrous oxide reduce some of the carbon dioxide uptake.

We found that coasts in Europe and Russia are net emitters, while coasts in Southeast Asia and North America have significant uptake of these gases.

Like upland forests and rainforests, ecosystems such as coastal wetlands can absorb atmospheric carbon dioxide and convert it into new leaves, roots and other organic matter. When some of this carbon stored for the long term In deeper sediments, it could help combat rising levels of carbon dioxide in the atmosphere.

The absorption of greenhouse gases in coastal areas should not be confused with carbon storage. Only part of the long-term coastal uptake of greenhouse gases is stored in coastal sediments, while another part is transported to the ocean where it can be stored or returned to the atmosphere.

Not all coasts are the same

Africa and Australia have large stretches of sandy coasts and coastal wetlands. By contrast, the cold coast of Europe and Russia lack mangrove forests or tropical coastal wetlands. These differences drive the changes we found in how different coasts absorb or release greenhouse gases.

The strongest coastal sink of greenhouse gases is Southeast Asia, due to its extensive and productive tropical mangrove forests and seagrasses that absorb large amounts of carbon dioxide. The North American coast is another excellent sink for greenhouse gases, with salt marshes, mangroves and seagrass beds – and Canadian fjords, glacial valleys filled with seawater.

While Australia and New Zealand have long stretches of coastal wetlands that absorb carbon dioxide, this is offset by a large number of estuaries, many of which are sources of greenhouse gases from decomposing organic matter.

Coastlines in Europe and Russia actually release more greenhouse gases than they absorb. Many polluted tidal estuaries release greenhouse gases, but the cooler climate means that this region has fewer coastal wetlands to absorb these gases back up.

But across the three major greenhouse gases, eight out of the world’s ten coastal areas are net greenhouse gas sinks.

Some coasts absorb greenhouse gases while others emit more

The world’s coastal ecosystems are extremely diverse, ranging from tropical lagoons to polar fjords to coastal mangrove forests to underwater seagrass meadows. This huge diversity means that they differ greatly in how they absorb or release greenhouse gases.

For example, we show that fjords around the world take up about 40% of the carbon dioxide that would be emitted by tidal systems, deltas, and lakes. Most (86%) of the carbon dioxide uptake by the fjords comes from the North American region.

By contrast, salt marshes and mangrove forests are home to trillions of microorganisms that live in deep, anoxic sediments, feeding on dead organic matter and emitting methane and nitrous oxide. Some of these gases reach the atmosphere, making many coastal waters a source of methane and nitrous oxide.

Coastal wetlands release three times more methane than all estuaries in the world. But coastal wetlands, also called “blue carbon” coastal wetlands, can be powerful sinks of carbon dioxide and some of them absorb nitrous oxide. In general, these coastal ecosystems become a net sink for global warming when we consider the net impact of the three most important greenhouse gases.

What does the big picture look like? Globally, our research shows that the ability of our coasts to absorb carbon dioxide is offset by between 9% and 20% of coastal methane and nitrous oxide emissions.

Why is this knowledge important?

If we understand how coastal ecosystems absorb or emit greenhouse gases, we can target the ecosystems most important for conservation or restoration.

This is why many researchers are interested in blue carbon as a way to enhance long-term carbon storage. By protecting and restoring the mangroves and salt marshes of blue carbon-rich countries, such as Indonesia, we can expand their capacity to remove these gases from the atmosphere and eventually store some carbon long-term in their sediments.

And by reducing nutrient overload, the influx of organic matter and wastewater into our coastal waterways, we can cut the greenhouse gases emitted from polluted estuaries.

We should not only view our coastal ecosystems as a boon for carbon credit markets. They have a lot to offer, including many valuable ecosystem services. Our coastline protects our shores from bad weather and tides. They are a nursery for many fish and plants. And they are vital to us as a place to be in nature.

Protecting our coasts is good for us, nature and the Earth system as it plunges into the climate crisis.

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