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When policymakers discuss planting trees to slow climate change, they usually picture forests on land. Yet some of the most efficient carbon stores on the planet lie where the land meets the sea. Mangrove forests, tidal salt marshes and seagrass meadows, collectively described as blue carbon ecosystems, occupy less than two percent of the ocean floor but are responsible for roughly half of all carbon buried in marine sediments each year. As governments search for credible ways to meet their climate commitments, these overlooked coastal habitats have moved from the margins of ecology to the centre of policy debate.
The secret of blue carbon lies in the soil. In waterlogged coastal sediments, oxygen is scarce, so the microbes that normally decompose dead plants work extremely slowly. Instead of rotting away and releasing carbon dioxide back into the atmosphere, roots and leaves accumulate in thick layers of sediment, locking carbon away for centuries or even millennia. A terrestrial forest stores much of its carbon in trunks and branches that can burn or be felled within a human lifetime, whereas a salt marsh buries most of its carbon underground, where it is far harder to disturb.
Measurements confirm how dense these stores are. Studies summarised by the International Union for Conservation of Nature suggest that mangroves can hold three to five times more carbon per hectare than tropical forests on dry land, with some sites in Indonesia storing more than one thousand tonnes of carbon per hectare in the top few metres of sediment. Seagrass meadows, although less celebrated, can bury carbon thirty-five times faster than tropical rainforests of the same area. These figures explain why scientists argue that protecting a single hectare of mangrove can deliver the climate benefit of protecting several hectares of jungle.
Yet blue carbon habitats are disappearing faster than almost any other ecosystem on Earth. Between one and two percent of the world's tidal marshes, mangroves and seagrasses are estimated to be destroyed each year, cleared for shrimp farms, harbours, marinas and waterfront housing. Destruction does not merely halt carbon capture; it reverses it. When coastal sediments are drained or dredged, the oxygen-starved carbon they contain is suddenly exposed to air and microbes, and centuries of accumulated carbon can be released within decades. Researchers calculate that degraded coastal wetlands may already contribute a measurable share of global greenhouse gas emissions, comparable to the annual output of a mid-sized industrial nation.
The economics of protection are increasingly compelling. Because blue carbon gains can be measured and verified, coastal restoration projects can generate credits that are sold on carbon markets, channelling private money into conservation. Several countries, including Australia and the United Arab Emirates, have begun to include mangrove restoration in their national climate pledges under the Paris Agreement. Verification remains technically demanding, however, because carbon burial rates vary greatly from one coastline to another, and a credit issued against a marsh in one region may overstate or understate the true benefit.
Carbon is only part of the argument. Coastal wetlands shelter the young of commercially important fish, filter pollutants from rivers before they reach coral reefs, and blunt the force of storm surges. After Hurricane Sandy struck the United States in 2012, one widely cited analysis estimated that intact wetlands had prevented more than six hundred million dollars of flood damage along the affected coastline. Such figures allow economists to argue that blue carbon projects pay for themselves several times over, even before the carbon stored underground is counted.
Enthusiasm must still be tempered by caution. Some salt marshes release methane, a potent greenhouse gas that can partly offset their carbon gains, although mangroves and seagrasses appear largely free of this problem. Long-term governance also matters: a restored mangrove that is cleared again in thirty years delivers little lasting benefit. Most researchers nevertheless agree that protecting what remains is the cheapest and most reliable blue carbon strategy available, and far easier than rebuilding an ecosystem once it has been lost.