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Is there a place in the atmosphere where carbon dioxide is concentrated, and if so, can we remove it?
Carbon dioxide mixes evenly through the atmosphere. But the atmosphere as a whole is densest near the ground, so a cubic foot of air at ground level will contain more carbon dioxide molecules than a cubic foot of air high up in the sky.
If the carbon dioxide (CO2) in Earth’s atmosphere were clustered in certain spots, then it might be easier to remove CO2 from the air by building machines that capture CO2 in those places. But this is not the case, says Jesse Kroll, MIT professor of civil and environmental engineering and director of the Ralph M. Parsons Laboratory for Environmental Science and Engineering. Kroll says the CO2 emitted by cars, factories, and other sources mixes thoroughly into the atmosphere, so the amount of CO2 in the air is more or less consistent around the world.
“There will be some [variations] depending on how close you are to sources or sinks,” he says. (A sink is a place where CO2 is absorbed out of the atmosphere: for example, over a forest, where trees suck in CO2 to grow.) But those variations are small, Kroll says. There is no place with especially high levels of carbon dioxide to grab. There are more CO2 molecules at the Earth’s surface compared to miles up in the sky, but that is because the atmosphere is densest down low and slowly gets thinner as you get higher.
Plucking carbon from the air
It is possible to use machines to pull CO2 out of the air. Submarines and spacecraft do this so the people inside don’t run out of oxygen to breathe. The idea of trapping CO2 from the atmosphere at large is called direct air capture (DAC), says Howard Herzog, a Senior Research Engineer in the MIT Energy Initiative and author of a recent book chapter on the subject. “DAC invokes images of machines sucking carbon dioxide out of the air, allowing the world to continue business as usual, emitting CO2 from the chimneys of our houses, the tailpipes of our cars, and the smokestacks of our industries,” Herzog writes. “No wonder DAC is such a seductive concept.”
DAC projects and plans are underway, and a few startup companies are trying to turn direct air capture into a cost-effective climate change solution. However, Herzog says, using DAC to combat climate change would require the technology to capture billions of tons of CO2 annually. That is about a million times more than what the few currently operational pilot projects can capture. The technology must become cheaper and more efficient if we are to build the big projects that would allow DAC to make a major climate impact.
Because the level of CO2 in the atmosphere is the same around the world, there is no shortcut to making direct air capture more effective. There is, however, one way to put carbon absorbers in places with extra CO2, and that’s to focus on the exhaust coming from power and industrial plants that burn fossil fuels. This kind of carbon capture is much cheaper and more efficient than DAC. But it’s also philosophically different: instead of trying to draw down the CO2 we’ve already added to the atmosphere, it aims to prevent new CO2 from entering the atmosphere.
One bright spot for DAC: it works best near the Earth’s surface—in other words, where we live—because that’s where the atmosphere is thickest, Herzog says. Direct air capture installations are made of huge, hulking machines, and if it were indeed true that most CO2 lived high in the atmosphere, then engineers might face a double challenge of building tall structures to loft CO2-absorbing materials way up in the air.
Thank you to William Solomon of Calderdale, West Yorkshire, U.K., for the question. You can submit your own question to Ask MIT Climate here.