If we’re going to prevent the gravest dangers of global warming, experts agree, removing significant amounts of carbon dioxide from the atmosphere is essential. That’s why, over the past few years, projects focused on growing seaweed to suck CO2 from the air and lock it in the sea have attracted attention—and significant amounts of funding—from the US government and private companies including Amazon.
The problem: farming enough seaweed to meet climate-change goals may not be feasible after all.
A new study, published today in Nature Communications Earth & Environment, estimates that around a million square kilometers of ocean would need to be farmed in order to remove a billion tons of carbon dioxide from the atmosphere over the course of a year. It’s not easy to come by that amount of space in places where seaweed grows easily, given all the competing uses along the coastlines, like shipping and fishing.
To put that into context, between 2.5 and 13 billion tons of atmospheric carbon dioxide would need to be captured each year, in addition to dramatic reductions in greenhouse-gas emissions, to meet climate goals, according to the study’s authors.
A variety of scientific models suggest we should be removing anything from 1.3 billion tons of carbon dioxide each year to 29 billion tons by 2050 in order to prevent global warming levels from rising past 1. 5˚C. An 2017 report from the UN estimated that we’d need to remove 10 billion tons annually to stop the planet from warming past 2˚C by the same date.
“The industry is getting ahead of the science,” says Isabella Arzeno-Soltero, a postdoctoral scholar at Stanford University, who worked on the project. “Our immediate goal was to see if, given optimal conditions, we can actually achieve the scales of carbon harvests that people are talking about. And the answer is no, not really.”
Seaweed pulls carbon dioxide from the atmosphere through photosynthesis, and then a significant amount is sequestered—potentially for millennia—when the plant matter eventually sinks down into the ocean depths. The idea is that it could be grown and then intentionally sunk to lock away that carbon long enough to ease the pressure on the climate.
Arzeno-Soltero and her colleagues at the University of California, Irvine, used a software model to estimate how much seaweed, of four different types, could be grown in oceans around the world.
The model considered things like the seaweed’s nitrate uptake (which is essential for growth), the water temperature, the sun’s intensity, and height of the sea’s waves, using global ocean data gathered from past years, while accounting for current farming practices. The researchers performed more than 1,000 seaweed growth and harvest simulations for each of the seaweed types, which they said represented the “optimistic upper bounds” for seaweed production.
For example, the new estimates assumed that farming space could be found within the most productive waters for seaweed in the equatorial Pacific, around 200 nautical miles off the coast. In less productive locations, growing enough seaweed to reach climate targets would be even more challenging: three times as much space would have to be devoted to seaweed farming to sequester the same amount of carbon.
Their findings suggest that cultivating enough seaweed to reach these targets is beyond the industry’s current capacity, although meeting climate goals will require much more than reliance solely on seaweed.
Agnes Mols-Mortensen, a macroalgal biologist who farms seaweed in the Faroe Islands and was not involved in the project, says that companies hoping to expand their seaweed farming projects also needed to consider how that could affect the ocean ecosystem.
“We should be careful not to overexploit the ocean like we have the land,” she says. “We need to build really solid methods based on research before we dream about saving the planet with seaweed. There’s a lot of hype.”
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