This one is scientifically unproven and practically difficult, but seems to have great potential.

Summary:

The problem: To keep climate change at tolerable levels, countries will probably have to do more than just slash emissions — they’ll also need to remove hundreds of billions of tons of carbon dioxide from the atmosphere this century.

The fix: In Nova Scotia, a startup called CarbonRun is grinding up limestone and sprinkling it into rivers poisoned by acid rain. Not only does this technique help local salmon populations, it also helps pull greenhouse gases from the sky.

Typically, rivers contain carbon dioxide that is constantly escaping into the air, where it traps heat and warms the planet. But adding limestone converts some of that carbon dioxide into a stable molecule that instead stays underwater and washes into the sea, where it should remain trapped for thousands of years.

The potential: There are hundreds of acidified rivers worldwide, and CarbonRun estimates that this process, known as river liming, could remove hundreds of millions of tons of carbon each year.

That might just be a start. If similar techniques could be done in the ocean — adding an alkaline substance like limestone or magnesium oxide to seawater, for example — humanity could potentially remove billions of tons each year.

So far, oceans have been tougher to work in. It’s tricky to prove that adding alkalinity actually removes carbon in the turbulent high seas, and early research efforts have faced public backlash. Some scientists hope CarbonRun can be a pioneer, because it’s easier to verify carbon removal happens in rivers, and river liming has been highly popular with local communities, since it can help revive devastated fisheries.

The obstacles: CarbonRun needs to cut costs, which means figuring out how to inexpensively mine and transport limestone. The company needs about two tons of rock for every ton of carbon it removes.

The company also needs buyers. Right now, only a handful of wealthy tech companies, like Microsoft and Stripe, are willing to pay startups to remove carbon from the air. Some experts say carbon removal should be considered a public good and governments should pay for it, but it’s still early days.

One policy twist: Congress offers a generous tax credit — worth $180 per ton — for companies that remove carbon from the air using complex direct air capture machines, but that subsidy does not cover techniques that involve oceans or rivers.

What’s next: CarbonRun has received $25 million to remove an initial 55,442 tons of carbon dioxide from the atmosphere and is starting to scale up. Elsewhere, researchers in Halifax and Massachusetts are studying whether alkalinity enhancement can work in the oceans. — Brad Plumer