The world's coastal seaweed forests may be doing far more to slow climate change than anyone realized — and a new study has finally caught them in the act.
An international team led by Plymouth Marine Laboratory has tracked 8,000 individual seaweed rafts drifting away from southwest Greenland and shown, for the first time at this scale, how the underwater forests deliver atmospheric carbon to the deep ocean. The findings, published this month, sharpen one of the most important — and least understood — pieces of Earth's natural carbon cycle.
A natural conveyor belt
Large seaweeds, known as macroalgae, soak up carbon dioxide as they grow. Previous research estimated that between 4 and 44 million tonnes of macroalgae-derived carbon sink each year to depths of up to 200 metres, where it can stay locked away for at least a century. The wide range reflected just how little scientists could actually observe.
To narrow it down, researchers from Germany, Portugal, Saudi Arabia, Denmark and the UK combined satellite imagery, ocean current modelling and physical drifters to follow free-floating seaweed mats across hundreds of kilometres of cold Atlantic water.
What they found was striking. Offshore currents carry seaweed far from where it grew. As surface waters cool, the floating vegetation loses buoyancy, sinks, and breaks down in the dark — transporting carbon, almost on a schedule, to the deep sea.
"Our findings illustrate a tangible oceanic conveyor belt that links thriving coastal macroalgal forests with the deep ocean's carbon reservoir," said Prof Ana Queirós, marine climate change ecologist and climate change lead at Plymouth Marine Laboratory. "Recognising these natural transport and mixing pathways enhances how we understand macroalgae's vital role in the Earth's carbon cycle."
Why Greenland matters
Greenland's coastline is fringed with vast brown algae beds — kelp species that thrive in cold, nutrient-rich water. As the Arctic warms, these forests are expanding northward into newly ice-free zones. The new data suggests that, far from being a curiosity at the margins of climate models, these northern seaweed belts may be among the planet's most active carbon shuttles.
The team's tracking also revealed something climate scientists have long suspected but rarely measured: a single seaweed raft can travel astonishing distances before sinking. Some drifted out of view of satellites still mostly intact, only to be picked up again by current monitors hundreds of kilometres downstream.
What this means for climate policy
Coastal "blue carbon" ecosystems — mangroves, salt marshes, seagrasses — are already recognised in national climate accounts. Kelp and other macroalgae have been the awkward outliers: clearly important, but hard to credit because their stored carbon doesn't stay in one place. The Greenland study is part of a growing body of work showing that the carbon doesn't need to stay put to count. It just needs to reach the deep ocean.
That has direct implications for conservation. Kelp forests have been hammered in recent decades by sea-urchin booms, warming waters and storms in some regions, but they're also one of the easiest ecosystems to restore. Replanting projects from California to South Korea to Tasmania are already proving the concept. If their carbon work is as significant as the new data suggests, those restoration projects may turn out to be among the highest-leverage climate investments around.
Eyes on the deep
The next step, researchers say, is to extend the satellite-and-drifter approach to other major coastlines: Norway, Patagonia, Tasmania, the Aleutians. With cheaper sensors and better ocean models, the team believes the full size of the macroalgae carbon pump could be pinned down within a few years.
For now, the message is simple. Underwater forests no one ever sees from land are quietly doing some of the planet's most important climate work — and we're finally learning how.
