Seagrasses 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.

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...