When we remove carbon dioxide from the atmosphere, how long does it stay locked away?

In theory, we can remove CO₂ from the air for thousands or even millions of years, which in terms of addressing climate change is effectively permanent. But carbon removal can also fail.

November 24, 2025

Since humanity started burning fossil fuels for energy, we have added roughly one trillion tons of climate-warming carbon dioxide (CO₂) to our atmosphere. One could imagine a very literal way to deal with the climate change that has resulted: take the CO₂ back out. For many reasons, though, this kind of “carbon removal” is a very tall order.

Among those reasons is that sucking out the CO₂ is only the first step.

“You want to keep the CO₂ out of the atmosphere for at least, let’s say, hundreds of years,” says Ruben Juanes, a geophysicist and MIT professor of civil and environmental engineering.¹ Shorter-term storage, on the scale of decades, may usefully slow the pace of warming and buy time for more permanent solutions, but ultimately still leaves us to deal with the same CO₂ in the midst of ongoing climate change.

Why might carbon removal fail to keep CO₂ out of the air over centuries? It depends on how that CO₂ is stored.

Today, the large majority of carbon removal relies on “natural climate solutions.”² These use trees and other plants to draw CO₂ out of the atmosphere, storing the carbon in their tissues and the soils below. If these living storage vessels are lost, says Evan Fricke, an ecologist and research scientist in the MIT Department of Civil and Environmental Engineering, the carbon is destined to return to the air.

“Some of the prominent failures of forest-based natural climate solutions include attempting to plant trees in ecosystems where they don't naturally occur,” he says. “And the trees simply die or burn up.”

So in the worst case, natural carbon removal may last only years or decades. But it can also be effectively permanent. Earth, after all, is home to forests that are tens of millions of years old.

Successful carbon removal, Fricke says, takes good planning and continued care. That can mean planting the right vegetation in the right ecosystems for long-term survival. It can be a matter of conservation, making sure a new or restored forest is not later cut down for timber or development. And it can also be quite active. Certain farming practices, for example, can help soils store extra carbon. But it is widely assumed that, to prevent that carbon from leaking back out, farmers must keep up these practices year after year.³

Still, Fricke says a well-managed project can handle some reversals. Even if wildfire razes a forest, a healthy ecosystem can regrow. The key is to pursue large-scale projects in diverse ecosystems, and prepare to manage them for the long term. Planners can also hedge against risks with “buffer pools” of land beyond what they need to meet their carbon storage goals.

“A leaf will fall and decompose,” says Fricke. “A tree might burn. But a forest landscape can be resilient for millennia. And so, as we implement these projects at scale, we should expect reversals in certain areas, but if we're getting our science right, the numbers for the entire portfolio should be quite predictable.”

Other approaches to carbon removal keep their carbon out of the elements. In “carbon capture and storage” (CCS), for example, CO₂ is pumped underground. A nearly-pure stream of CO₂ is pressurized for easier transportation, then injected into carefully chosen geological formations.⁴

This, says Juanes, is not so different from the way nature stores carbon underground, as oil and natural gas. “These hydrocarbons have been underground for millions of years, over geologic time,” he says. Pressurized CO₂ is very similar, chemically and physically—so “in principle, it can be sequestered essentially indefinitely.”

The trick is in the execution. Hydrocarbons, Juanes points out, take millions of years to form, and the surrounding rock has ample time to react. CO₂ injection applies pressure a lot faster. “So it is really crucial to understand how that overpressure can affect the safety or security of storage over the long term,” he says. The wrong choice of geology, or a too-aggressive injection, could let captured CO₂ escape.

The world’s first CCS project began off the coast of Norway in 1996. Over the 30 years since, MIT Climate was unable to find any documentation of a CCS project losing a significant amount of CO₂ to the atmosphere,⁵ but small leaks have been detected.⁶

There have also been several false starts and warning signs. “Very little of what happens underground happens exactly as we expect,” Juanes says, “from prospecting for minerals, to storage of CO₂, to production of hydrocarbons.” CCS operators try to map the subsurface as well as possible before injection starts, but we cannot know everything about the rock in which CO₂ will be stored.

As a result, Juanes says, “some projects have experienced unexpected behavior underground, and have had to either ramp down or shut down.” In 2011, for example, the In Salah project in Algeria halted early after researchers found signs that CO₂ injection was cracking the rock and deforming the surface.⁷ As critics have noted, this is part of a pattern of projects underperforming.⁸

One could see this as a good sign for the permanence of CCS. Operators have spotted problems in advance, and adjusted or shut down without major leaks. But the challenges of dealing with pressurized CO₂ underground add to the risks and costs. Are there more stable options?

A few types of carbon removal “mineralize” CO₂ into limestone and other solids. This can be done by injecting CO₂ into the right geologic formations, where it reacts with the underground rock. Or it can be done aboveground through “enhanced rock weathering,” speeding up chemical reactions between certain rocks and the CO₂ in the air.

“In principle, if we can turn the CO₂ into a solid, that's the most permanent form of storage,” says Juanes. This carbon is almost surely safe over tens of millions of years, until the slow forces of volcanism vent it back into the air.

But, he adds, all kinds of carbon removal can be effectively “permanent” if done right. As we choose which ones to pursue, we’ll have to weigh much more than their duration. Natural climate solutions are relatively cheap and ready to deploy. CCS can in principle handle large volumes of CO₂ quickly. And all forms of carbon removal must compete with not emitting CO₂ in the first place, by far the most “permanent” solution of all.

“Each of these has its trade-offs related to risks of reversals, the scale angle, the cost angle,” says Fricke. “Anyone who says that one of these is the silver bullet is probably lying to you. If there was a clear winner that we could implement at scale, without costs, we would have done it already.”

Thank you to Donald Yates of Perth, Australia, for the question.

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