Despite the efforts of delegates at this month’s climate summit in Glasgow, the world is still careening toward potentially catastrophic levels of global warming. Now, some countries and corporations are turning to new technologies to pull carbon out of the air.

Today, the US Department of Energy (DOE) announced a bold new plan to make those technologies, called carbon dioxide removal (CDR) technologies, cost-effective and scalable with the launch of a new “Carbon Negative Shot” initiative. Through this initiative, the agency seeks to bring the cost of CDR down dramatically this decade — to less than $100 a ton — so that it can be deployed at a big enough scale to remove “gigatons,” or billions of tons, of carbon dioxide from the atmosphere.

That is a hell of a lot of CO2 pollution. Sequestering one gigaton of carbon dioxide would amount to removing the pollution of about 250 million vehicles — the US’s entire light-duty fleet — in one year, according to the DOE. With CDR technologies still in pretty early stages of development, there are significant hurdles to overcome before the DOE can do so.

CDR is a suite of strategies aimed at drawing down CO2 to keep it from trapping heat in the atmosphere. Nature can do some of that for us — trees and plants pull CO2 out of the air. There’s also “direct air capture” technology that mimics that process using carbon-sucking machines, but it has yet to be deployed at a large scale.

To draw down enough heat-trapping pollution, the US will likely need large-scale direct air capture plants. The largest direct air capture plant came online in Iceland earlier this year, and it’s only able to pull out 4,000 tons of carbon dioxide annually. That’s roughly equivalent to the emissions from 790 passenger vehicles in a year. To date, there are only 19 direct air capture plants around the world, according to the International Energy Agency, and they only have the capacity to capture a tiny fraction of what the DOE’s aims are.

Cost is one big reason why the tech hasn’t advanced further. Companies like Microsoft pay about $600 for each ton of CO2 the Iceland plant captures. The company pumped out the equivalent of 11,164,000 metric tons of carbon dioxide in its 2020 fiscal year. At $600 a ton, Microsoft would need to pay almost $6.7 billion to remove just one year of its pollution.

But cost isn’t the only challenge. Direct air capture plants trap CO2 using filters or chemical solutions. To release the trapped CO2 so that it can be safely stored, the filter or chemical solution needs to be heated up to very high temperatures — between 100 and 900 degrees Celsius. That takes a lot of energy. In a catch-22, the machines that pull carbon out of the air could wind up using as much as a quarter of the global energy supply by 2100, according to a 2019 study published in the journal Nature Communications. If that energy comes from burning fossil fuels, it could contribute to the problem it’s trying to solve. (And it’s still technically difficult to use purely renewable energy to reach the extremely high temperatures required for the chemical solution method of direct air capture.) That’s likely why the DOE says in its announcement today that it wants to ensure that “emissions created when running and building the removal technology are accounted for.”

Lastly, the DOE is aiming to secure places to store CO2 where it can be monitored for at least 100 years. It ideally needs to stay sequestered for much longer to keep humanity from falling deeper into climate crisis. At the Iceland plant, CO2 is pumped underground, where the companies behind the project say it can be stored in rock formations for thousands of years. Volcanically active Iceland has relatively young and porous basalt rock that’s ideal for this kind of storage.

The US will not only need to find similarly well-suited locations — it’ll need to transport it there via new pipelines. The Biden administration’s infrastructure bill that’s inching closer to a final vote includes billions of dollars for new pipelines, and $3.5 billion for four direct air capture “hubs.” That already has some environmental groups concerned about pipeline ruptures, like one that sickened residents of a small, majority-Black community in Mississippi last year. At high concentrations, carbon dioxide is a dangerous asphyxiant.

Despite all those challenges, leading climate scientists working with the United Nations have included carbon removal in roadmaps for limiting the climate crisis to somewhat manageable levels. That’s gotten criticism from some progressive activists who see carbon removal as a distraction from transitioning from fossil fuels to renewable energy. And even experts optimistic about the future of the technology caution that it’s meant to be a side dish and not the main course in any plan to combat climate change.

“It is at most a supplement that can help us reduce climate change,” David Morrow, director of research at the Institute for Carbon Removal Law and Policy at American University, told The Verge in September when the Iceland plant came online. “But it can’t take the place of cutting emissions.”

The US, the world’s second-biggest CO2 polluter, still needs to focus primarily on finding alternatives to fossil fuels so that it can prevent greenhouse gas emissions in the first place.