On 14 November, I toured a shipping container bristling with tubes and wires, perched beside New York City鈥檚 East river. It is the test site of a start-up called Vycarb, which recently began adding crushed rocks and other chemicals to the water to remove carbon dioxide from the atmosphere.
On one side of the container, an hourglass-like device mixed a fine stream of calcium carbonate mineral powder with water pumped from below. This alkaline, grey-green slurry was then released into an underwater pipe flowing with several thousand litres of water per minute and carried into the river, an estuary between Manhattan and Long Island.
Centuries of pollution have given the East river a reputation for industrial blight, but Vycarb chose the site because the water there contains higher levels of CO2 than average 鈥� higher even than the atmosphere鈥檚 roughly 420 parts per million and rising. By making the water less acidic, the alkaline slurry helps convert this greenhouse gas into a stable mineral form, so it stays in the river instead of getting into the air.
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鈥淲e started with little Tupperwares removing milligrams [of CO2],鈥� says , a scientist at the company. At this test site in Brooklyn鈥檚 Navy Yard, the company aims to remove 60 tonnes of CO2 per year.
To monitor the project鈥檚 progress, the container has additional tubes that carry river water to two cylindrical tanks equipped with sensors. This ability to directly measure how the added alkalinity affects the quantity of carbon in the water is a key reason for the company鈥檚 approach. Other methods of what is known as ocean alkalinity enhancement (OAE) require complex modelling to determine the effects of the added alkalinity. 鈥淲e know exactly how the water is when it comes in, and how it is when it comes out,鈥� says Keppler.
As I watched, the sensor tanks filled with water, one from the inflow of the pipe and one from the outflow. After a few minutes, lines of code began appearing on a screen the researchers call 鈥淭he Matrix鈥�. It showed that the water flowing in had 720 parts per million of CO2, twice as much as the water flowing out. This was raw evidence that the whole contraption was working.
Vycarb鈥檚 test is one of a around the world removing CO2 from the air by tweaking the chemistry of the oceans. Past tests of OAE have proved controversial: opponents say it is a form of geoengineering with unknown environmental risks.
For instance, at large scales, raising alkalinity with certain minerals could affect how carbon moves through ocean ecosystems, putting marine life at risk. Another concern is that the increased alkalinity might favour certain species to the detriment of others. And minerals might also contain toxic heavy metals.
But OAE supporters argue that tests are needed to understand those risks and determine whether the method effectively removes CO2. 鈥淭his is the natural next step of trying to gain real-world information,鈥� says at Ocean Visions, a non-profit organisation that advocates for ocean-based CO2 removal. He points to suggesting that low levels of OAE may be safe for ecosystems.
To avoid controversy, OAE efforts like Vycarb鈥檚 are now proceeding more carefully, spending time engaging with communities near test sites and scaling up incrementally to understand how much alkalinity might safely be added to the oceans. 鈥淲e need to research this, but it鈥檚 not a current-day solution,鈥� says at the University of Hawai鈥檌 at Manoa. 鈥淭his is useless unless we stop burning fossil fuels.鈥�
