It may be hard to remember now — when Elon Musk is making headlines for attempting a hostile takeover of Twitter — but a year ago, the Tesla CEO made waves in the climate world when he announced that he was donating $100 million to the XPRIZE Foundation to run a competition for the best carbon removal solution. The grand prize won’t be awarded until 2025, but on Friday, the contest doled out $15 million of the prize purse to reward the 15 most promising contestants so far.

Though Musk’s motivation for donating the money isn’t entirely transparent, the point of the contest is to catalyze innovation. The world has waited so long to cut greenhouse gas emissions that scientists now say we’ll need to phase out fossil fuels and actively remove carbon from the atmosphere to stabilize the climate at a relatively safe temperature. But the carbon removal techniques that exist today are extremely limited, small scale, and costly, and their effectiveness is difficult to measure. 

Musk’s original announcement on Twitter was met with snarky replies like “ever heard of trees?” But while trees can remove carbon from the air, they are subject to fire and disease, and they compete with other essential uses for land, like housing and agriculture. Other possible solutions include machines that pull carbon directly out of the air; methods to enhance soils’, plants’, and the ocean’s ability to store carbon; and schemes to speed the uptake of CO2 by certain types of rock. 

“Every solution has a weakness,” said Marcius Extavour, chief scientist and vice president of climate and environment at XPRIZE. “The winner of this competition is not just the team that can maximize its strengths, but the team that can minimize its weaknesses,” he said.

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Ultimately, the winning teams will have to demonstrate that they can remove 1,000 metric tons of CO2 from the atmosphere per year, that the carbon will stay out for at least 100 years, and that they have a pathway to scaling up to 1 million metric tons removed per year. (The most conservative estimates say the world will eventually need to remove 1.5 to 3 billion metric tons of carbon per year.)

But the 15 “milestone award” winners announced on Friday simply had to have a “great idea,” said Extavour, as well as be able to demonstrate some key component of it and provide cost estimates and life-cycle emissions data. More than 1,100 teams have applied to the contest so far, and 287 met the criteria for the milestone award. (XPRIZE is continuing to accept applications to compete for the grand prize.) XPRIZE asked expert reviewers from academia, nonprofits, and industry to narrow down the list to 60. Then the contest’s 12 official judges picked the winners.

The winners are working on a variety of approaches across four categories: air, land, ocean, and rocks. Six teams are developing technologies to separate carbon dioxide directly from the air. Four plan to produce biochar, a charcoal-like substance made from plants that is extremely stable. Scientists believe that turning certain plants into biochar and burying it in soil can sequester carbon for hundreds to thousands of years. Another team, Global Algae Innovations, has pitched an idea to grow algae, which captures atmospheric carbon via photosynthesis, and then turn it into a nonbiodegradable form of plastic that can be used to make surfboards, shoes, and cars. Three others are working on ocean-based solutions that lower the concentration of CO2 in surface waters, which in turn can help the ocean absorb more CO2 from the atmosphere. 

A few teams might be familiar to anyone who’s been following carbon removal news. Heirloom, a company working on capturing CO2 directly from the air, recently made headlines after raising $53 million in venture capital. For the XPRIZE, Heirloom teamed up with a company called Carbfix, which specializes in turning CO2 into stone. Carbfix is already demonstrating its technology at a working carbon removal facility in Iceland.

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The Heirloom and Carbfix project has a full cradle-to-grave plan to remove carbon from the atmosphere and store it permanently underground, but for some of the winners, the plans advertised in publicly available materials are currently vague. Only half of the teams in the “air” category seem to specify how and where the carbon will be stored after it is captured. 

In other cases, the ambiguity may be a symptom of how nascent the science is. Particularly for ocean- and land-based solutions, which involve complex biogeochemical dynamics, there aren’t yet established methods for measuring, reporting, and verifying how much carbon is being removed, raising questions about how the contest will ultimately weigh these projects. 

For example, the teams working on ocean-based solutions plan to either alter the chemistry of surface waters or stimulate seaweed growth, both of which could lower the concentration of CO2 in the water. But David Ho, a professor of oceanography at the University of Hawaiʻi at Mānoa, and one of the XPRIZE’s expert reviewers, said it’s not enough to know how much CO2 a project pulls out of the water. You also have to measure how much CO2 the ocean then absorbs from the atmosphere to replace it.

“I think a lot of people assume that it’s 100 percent, or they don’t even think about that part,” he said, “but that number is likely a lot lower.”

Ho said he’s not sure anyone has a good process for measuring that yet, or for assessing how permanent these forms of carbon removal are, or how they affect ocean ecosystems. He’s actually in the process of raising money to work on these issues himself.

When asked if he thinks it will be possible to answer these questions by the time XPRIZE gives out its grand prize in 2025, Ho said it will be tight. We might be able to measure the flux of CO2 for various ocean carbon removal approaches on a large scale, he said, but it will be a lot harder to do at the scale of one project.

Extavour was more optimistic about XPRIZE’s ability to verify contestants’ results. “It is possible today to do verification on any individual carbon removal project. It might be painful, but you can do it. At minimum, it’s a challenge that we at XPRIZE have to solve,” he said. But he admitted that it will not be easy to scale up verification to the point required for these teams to turn their carbon removal solutions into legitimate businesses. “Just because you can do it once doesn’t mean it’s a helpful or scalable or cost effective validation method.”