June 1, 2023

Seagrasses and mangroves can suck carbon from the air

Seagrasses and mangroves can suck carbon from the air
Seagrasses and mangroves can suck carbon from the airSeagrasses and mangroves can suck carbon from the air

OFF THE coast of Formentera, an island in the Spanish Mediterranean, lives an organism that stretches 15km from one end to the other. Posidonia oceanica, more prosaically known as seagrass, spreads by sending shoots out beneath the sediment. Entire meadows, covering several hectares, can thus be made up of a single organism. The grasses are long-lived, too. The vast meadow in Formentera is thought to have been spreading for tens or hundreds of thousands of years.

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But the seagrass is more than just a biological curiosity. Along with two other kinds of coastal ecosystem—mangrove swamps and tidal marshes—seagrass meadows are particularly good at taking carbon dioxide from the air and converting it into plant matter. That makes all three ecosystems important for efforts to control climate change.

This role was highlighted in a report published on March 2nd by UNESCO, an arm of the United Nations, on “blue carbon”—the sort captured by Earth’s oceanic and coastal ecosystems. In total around 33bn tonnes of carbon dioxide (about three-quarters of the world’s emissions in 2019) are locked away in the planet’s blue-carbon sinks. Research by Carlos Duarte, the report’s author and a marine ecologist at King Abdullah University in Saudi Arabia, has shown that one hectare of seagrass can soak up as much carbon dioxide each year as 15 hectares of rainforest.

All this is attracting interest in blue carbon from those keen to use natural processes, rather than human technologies such as direct-air capture, to suck greenhouse gases from the atmosphere. In 2018 Apple partnered with Conservation International, a charity, to protect 11,000 hectares of mangroves on the Colombian coast. The firm estimates the project could lock away around 1m tonnes of carbon.

One reason that blue-carbon ecosystems make such effective sinks is that submerged forests are denser than their land-based equivalents. They can also trap floating debris and organic matter, which settles on the sea floor and can double the amount of carbon stored away.

They posses another advantage, too. Unlike forests on land, blue-carbon ecosystems do not burn. Climate change is intensifying wildfires around the world. As forests burn, their carbon stocks are released back into the atmosphere. And fires can impede a forest’s ability to capture carbon even after they have burned out. In a study published on February 25th in Nature Ecology and Evolution, researchers at Stanford University found that repeated fires favour slow-growing tree species. These are better able to survive blazes, but they are also less effective at soaking up carbon than faster-growing species.

Submerged forests may be impervious to fires, but they remain vulnerable to other sorts of disasters. In May 2020 cyclone Amphan destroyed 1,200 square kilometres of mangrove forest on the border between Bangladesh and the Indian state of West Bengal. A marine heatwave in Australian waters in 2010 and 2011 damaged around one third of the world’s largest seagrass meadow, in Shark Bay. Over the next three years field studies showed that uprooted plants were releasing their carbon back into the atmosphere.

Fortunately, an older, man-made ecological disaster suggests that restoring damaged blue-carbon ecosystems is possible. During the Vietnam war, napalm and a cocktail of weaponised herbicides destroyed more than half of the mangroves in the Mekong delta. A report published in 2014 by the International Society for Mangrove Ecosystems showed that an intense post-war replanting programme was able to restore it within two decades.

And there is more to such ecosystems than simply acting as sponges for greenhouse gasses. They also serve as buffers for vulnerable shorelines, shielding them from storms that barrel in from the high seas. One study of 59 subtropical countries estimated that by dampening waves and providing natural barriers to storm surges, mangrove forests prevent more than $65bn in property damage each year, and help shelter more than 15 million people. Protecting and expanding them, then, appears to be a no-brainer.

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This article appeared in the Science & technology section of the print edition under the headline “Supergrass”