Experts agree that removing significant amounts of carbon dioxide from the atmosphere is essential to avoiding the worst threats of global warming. Therefore, the cultivation of algae has come into focus in recent years. The idea: They should suck CO₂ out of the air and bind it in the sea. Governments as well as private companies like Amazon have jumped on it and have contributed significant amounts of funding.
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But, as is so often the case, there is a catch with this plan: it may not be possible to grow enough algae to achieve the climate goals. This is illustrated by a study published in “Nature Communications Earth & Environment”. The team of authors estimates that about one million square kilometers of ocean (about three times the area of Germany) would have to be managed to remove one billion tons of carbon dioxide from the atmosphere over the course of a year. With competing uses along the coasts, such as shipping and fishing, it is not easy to find this area in places where algae grows easily.
For comparison, meeting climate targets would require capturing between 2.5 billion and 13 billion tons of carbon dioxide from the atmosphere each year, in addition to drastically reducing greenhouse gas emissions, the study’s authors write.
Calculations of how climate targets can be achieved
Various scientific models suggest that we should remove between 1.3 billion tons of carbon dioxide per year and 29 billion tons by 2050 to prevent global warming from exceeding 1.5°C.
“Industry is ahead of science,” says Isabella Arzeno-Soltero, a postdoctoral researcher at Stanford University who worked on the project. “Our immediate goal was to find out whether, under optimal conditions, we could actually achieve the carbon harvest magnitude that is being talked about. And the answer is no, not really.”
Seagrass removes carbon dioxide from the atmosphere through photosynthesis, and then sequesters a significant portion of it—possibly for millennia—when the plant mass eventually sinks to the depths of the ocean. The idea is that the mass could be grown and then sunk on purpose to sequester the carbon long enough. This should reduce the pressure on the climate.
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Arzeno-Soltero and her colleagues at the University of California, Irvine, used a software model to estimate how many algae of four different species could be grown in the world‘s oceans. They focused on tropical red algae, tropical structural algae and red and brown algae in temperate zones.
Where to find the best algae growing areas
The model took into account factors such as the algae’s nitrate uptake (essential for growth), water temperature, solar radiation intensity and sea wave height, using global ocean data from previous years and taking into account current farming practices. The researchers ran more than 1,000 growth and harvest simulations for each of the algal species, which they say represent the “optimistic upper bounds” for algal production.
For example, the new estimates assumed that ample crops could be found in the most productive seaweed waters in the equatorial Pacific, about 200 nautical miles offshore. In less productive locations, it would be even more difficult to grow enough algae to meet climate targets: it would take three times as much area to grow algae to sequester the same amount of carbon.
The results suggest that cultivating algae to meet these goals is beyond the industry’s current capacities, although achieving climate targets requires far more than just using algae.
Agnes Mols-Mortensen, a macroalgae biologist who grows seaweed in the Faroe Islands and was not involved with the project, says companies looking to scale up their seaweed farming projects should also consider how it might affect the marine ecosystem.
“We should be careful not to overexploit the ocean as much as we do the land,” she says. “We need to develop really solid, research-based methods before we dream of saving the planet with algae. There’s a lot of hype.”
(jl)
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