On 14 November, I toured a shipping container bristling with tubes and wires, perched beside New York City’s 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…