23.04.20 Help from kelp Hans-Joachim Ziegler • 5 min.

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Underwater forests can help slow climate change.

Forests are on the front line of climate change. For one thing, they’re highly susceptible to the repercussions of global warming. For example, Australia’s 2019-20 bushfire season alone destroyed more than 20% of its forests. But they’re also one of the best weapons in our climate-protection arsenal. Forests are the earth’s lungs: they use photosynthesis to remove carbon-dioxider from the atmosphere and turn it into glucose and oxygen.

But trees are only temporary carbon sinks. When they die and decompose, the carbon is released back into the atmosphere. Deforestation, disease, and especially fire can quickly negate a forest’s climate benefits.

That’s why scientists worldwide are looking for alternative and more permanent ways to sequester carbon. A particularly promising one will never go up in smoke. Because it lives underwater.

Sequestering with seagrass

Vast seagrass beds line the coasts of every continent except Antarctica. A team of researchers led by biologist Marianne Holmer of the University of Southern Denmark recently discovered that 1 hectare of seagrass can bind as much climate-surly carbon as 10 hectares (25 acres) of terrestrial forest. Moreover, seagrass meadows can sequester carbon in hermetically sealed sediments on the seafloor, where it can remain for hundreds of years thanks to the plant’s durable root system, which can live for over a thousand years.

Unfortunately, large-scale aquaculture of seagrass is highly resource-intensive because each seedling has to be manually planted by divers. And any carbon sequestered in this way will only stay in place if the seabed isn’t disturbed by anchors, fisher boats’ drag nets, or other human activity. The United Nations Environment Program (UNEP) therefore views the protection, not the cultivation, of seagrass beds as a “secret weapon” in the fight against climate change. It likely makes more sense to store carbon where it will stay put, like the deep sea.

Seaweed’s superpowers

That’s where seaweed, or kelp, comes in. Seaweed is a large brown algae that grows in shallow coastal waters and is inedible to most fish species. It has a central stipe, or stem, from which grow irregular-shaped blades, or leaves. Gas bladders, usually near the top of the plant, provide buoyancy. 

These bladders enable dislodged kelp to drift far out into the ocean. When the bladders inevitably burst, kelp sinks to the icy, oxygen-poor depths of the ocean, where it remains indefinitely. Each year, this process is responsible for removing around 200 million tons of carbon-dioxide—roughly half of the United Kingdom’s annual carbon emissions—from the atmosphere and permanently storing it on the ocean floor.


Underwater forests

Sink or swim

Marine biologist Halley Froehlich of the University of California at Santa Barbara would like to accelerate this process. Froehlich is the lead author of a paper published in the August 2019 issue of Current Biology that proposes establishing large-scale marine farms where seaweed is cultivated for harvesting and deep-sea disposal. According to Froehlich, nearly 50 million square kilometers of the world’s oceans are suitable for such farms

Carlos Duarte, professor of marine science at King Abdullah University of Science and Technology on Saudi Arabia’s Red Sea cost, is equally sold on the climate-protecting potential of seaweed. But he thinks it should be left swimming on the surface, not sunk. “Seaweed,” Duarte told National Geographic, “is a very valuable material and there are better ways of using this material, while contributing to mitigating climate change, than disposing of it in the deep sea.”

The magazine goes on to point out that seaweed improves water quality, provides a valuable habitat for a multitude of marine organisms, and protects shorelines by slowing wave momentum. It can also be used as a biofuel and an environmentally friendly fertilizer. In addition, research has shown that adding seaweed to cattle feed can cut the animals’ methane emissions by up to 70 percent. These many applications make seaweed aquaculture a potentially promising business proposition, which seems to argue against deep-sea disposal.

A cathartic Arctic?

All in all, large-scale seaweed aquaculture seems like an idea worth exploring. Like most living things on the planet, however, seaweed hasn’t escaped the effects of climate change. Seaweed stocks have declined by nearly 40 percent since the middle of last century. But new research shows that climate change is also opening up new niches for seaweed. In the Arctic, for instance, retreating ice sheets are allowing more sunlight to penetrate the ocean and creating vast swathes of ideal seaweed habitat. It’s of course no reason to welcome or trivialize global warming. But it does show that the effects of rising temperatures are diverse and, in some cases, may even be minimally countervailing. As the Arctic loses ice, at least it may be able to help purge the atmosphere of some carbon.


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