As the Earth continues to heat up, so have calls to dramatically reduce carbon dioxide emissions to avoid catastrophic climate change. But many experts say that even if all emissions stopped tomorrow, the planet would continue to warm and seas would continue to rise.

A key issue is that atmosphere would still be clogged with 200 years’ worth of human-produced carbon dioxide. “The question is, what do we do with all this excess CO2 in the atmosphere?” said Noah Deich, executive director and co-founder of the nonprofit Center for Carbon Removal.

A new application of old technology may be the answer. “Direct air capture” that removes the gas from ambient air has possible since the 1940s, but — at a cost estimated in 2011 to be as much as $1,000 per metric ton of CO₂ — it has long been viewed as too expensive to be practical.

David Keith, the Gordon McKay Professor of Applied Physics at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and professor of public policy at the Harvard Kennedy School, thinks it can be done for a lot less. He and his colleagues estimate that their company, Carbon Engineering, could capture CO₂ for between $94 and $232 per metric ton. In the journal Joule, the team outlined the material and engineering costs of their system — the first time the costs of a commercial direct-air-capture process have been published.

The paper could have major ramifications across the industry.

“Until now, basically no one in the industry has published an open-book number that will give credibility that direct air capture costs less than the $500 to $1,000 per metric ton that has been estimated,” Deich said.

CO₂ molecules make up only .04 percent of the air — that’s one in 2,500 molecules. Nonetheless, “We need enormous volumes of CO₂ removal and to achieve that, we need accurate economic analysis and hard engineering data,” said Julio Friedmann, CEO of Carbon Wrangler LLC and senior advisor at The Global Carbon Capture and Storage Institute. “This paper provides that transparency.”

Keith co-founded Carbon Engineering in 2009, when direct air capture was still on the fringes of industrial climate solutions. Carbon Engineering’s goal is to use direct air capture to produce carbon-neutral fuels and converting carbon-free energy into high-energy fuels for vehicles such as planes and barges, which are difficult to electrify.

The Carbon Engineering team’s approach differs from their few competitors in the field.

“We’re not developing a fundamentally new product or unit operation,” said Keith. “That’s the design choice we made. We’re making something that’s never been done before — commercial large-scale air capture — but we’re doing it on a basis of technology that already exists.”