‘They’ve got a plan to fight global warming’, the New York Times reported last week ‘It could alter the oceans’. The plan was devised by a new company in Nova Scotia, Canada. It is basically a simple idea: if you increase the alkalinity of the water in rivers and the ocean, the water will soak up more carbon dioxide. Adding limestone to rivers could do the trick.

The Canadians are building a machine that will grind up limestone and release the powder into a river, creating a chalky plume that should dissolve within minutes. This way, the river should absorb carbon dioxide from the air. When applied on a much larger scale, rivers and oceans could be transformed into giant sponges and help to slow global warming. The basic principle is that limestone converts carbon dioxide into a stable molecule that washes into the sea and then stays there for thousands of years.

The writers of this article also address the fact that ‘toying with ocean chemistry also carries unknown risks’, with possible consequences for fish and other aquatic life. The technique, however, has proven to be safe for the aquatic environment in the past. Decades ago, when acid rain caused major ecological damage to lakes and streams around the world, limestone was added to waterways to restore the pH balance and help fish populations recover. And it worked.

Illustrating that it works is relatively simple. Just add limestone to a river and sample the water up- and downstream to measure the resulting conversion of carbon dioxide into a stable bicarbonate. The tricky part is to scale it up. Roughly two tons of rock are needed to remove one ton of carbon. If scaling up is successful, there are plenty of opportunities to apply this technology. According to the New York Times, ‘there are hundreds of acidified rivers from Maine to Indonesia close to limestone deposits, potentially allowing for hundreds of millions of tons of carbon dioxide to be captured each year’.