In late January, after briefly surpassing Jeff Bezos as the world’s richest person, Elon Musk announced his latest venture: The Tesla CEO planned to donate $100 million, or about 0.05 percent of his net worth, toward a prize for “best carbon capture technology.”

More details about Musk’s donation arrived on Monday via the XPrize Foundation, an organization that holds competitions to drive innovation and will be launching a new contest for Musk’s prize money on Earth Day. As it turns out, Musk is not backing the best carbon capture technology, but the best carbon dioxide removal technology.

That may sound semantic, but it’s a distinction that brings a different set of contenders to the table. Carbon capture typically refers to technology that can suck up CO2 directly from an emitting source, like the flue of a steel plant or the smokestack of an oil refinery, and prevent it from entering the atmosphere. Instead, Musk is funding a competition for the best solution to draw carbon dioxide out of the atmosphere or the ocean, where it has already accumulated, and safely contain it for at least 100 years.

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Musk’s initial announcement on Twitter was met with cynical takes like “ever heard of trees?” But trees can fall prey to disease, not to mention wildfires, and journalists have repeatedly found that forestry programs don’t always deliver the carbon benefits they advertise. The XPrize contest is looking for solutions that can scale up to remove gigatons of carbon from the atmosphere, and relying on trees alone would cost the world a lot of land.

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Entrants to the contest are allowed to pursue nature-based solutions on land or in the ocean, such as sequestering carbon in soil, plants, or algae. Alternatively, they can work on “direct air capture,” which already exists in the form of shipping container–sized machines that suck carbon out of the ether. There are also methods that involve grinding up minerals to enhance their ability to absorb carbon from the air.

All of these methods have problems right now. They’re too expensive, their results are hard to verify, they require too much energy, or they require too much land. Musks’s sponsorship of the XPrize contest is the latest development in a steady stream of corporate interest in fixing these problems. Microsoft, Amazon, Stripe, and Shopify have all established multimillion dollar funds or programs dedicated, at least in part, to helping nascent carbon removal startups scale up and lower their costs.

As these corporate titans try to go green, they are realizing how hard it could be to cut their emissions in line with the Paris Agreement’s goals — which scientists say require that we stop allowing carbon to accumulate in the atmosphere by 2050. There may not be a way to transport goods around the world or manufacture the materials that make up our laptops and smartphones without emitting some carbon by then, so these companies want to be able to purchase tons of carbon removal to make up for continued emissions. They may also be placing a bet that paying for carbon removal could turn out to be cheaper than sustainable shipping and manufacturing.

Though he hasn’t said so explicitly, Musk’s new interest in carbon removal may be motivated by the same factors. In the past, Musk has lamented the fact that his SpaceX rockets can’t be powered by electricity and hinted that he wants to make “zero net carbon” rocket fuel.

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But carbon removal is not simply a self-serving scheme for corporations. The Intergovernmental Panel on Climate Change, a group of the world’s leading scientists and economists, has found that the most cost-effective path to stabilizing the climate will likely involve removing carbon from the atmosphere in addition to cutting emissions. Some activists and scientists argue that global warming has already become dangerous, and carbon removal offers a potential way to reverse some of its effects. Researchers debate how much carbon removal may truly be necessary, but many argue that it’s past time to invest in solutions. Taking their advice, Congress also ordered the Department of Energy to host its own competition for the best direct air capture technology in last December’s omnibus bill.

The XPrize Foundation is asking contestants to demonstrate a working prototype for a carbon removal solution that can verifiably remove 1 ton of CO2 per day, and to prove it has the potential to scale up to remove 1 billion times that amount. Winners will be chosen in 2025. The top prize is $50 million, with smaller amounts going to runners-up.

This is the second time the XPrize has waded into the problem of managing carbon dioxide. An earlier contest, which began in 2015, invited contestants to create a market for captured CO2 by turning it into consumer products. Winners will be announced later this year.