The Suddenly Urgent Quest to Remove Carbon Dioxide from the Air

“I’m skeptical there is a technology that will cheaply capture CO2 at 400 parts per million when it’s expensive to do at 400,000 parts per million in a smokestack,” says Rob Jackson, a professor of Earth sciences at Stanford University who has researched negative emissions.  “It’s tougher thermodynamically. Carbon dioxide in air is a thousand times less abundant.”

Jackson and other researchers have studied what is perhaps the leading current idea for how negative carbon emissions are supposed to happen, and it’s very different. It’s called bioenergy with carbon capture and sequestration, or “BECCS.”

The idea here is that you would pair two existing technologies:  Growing corn, trees or other forms of biomass to burn in power plants, and then storing the burned-off carbon in the ground instead of letting it escape into the air.  When the plants regrow , they would pull carbon dioxide out of the air again, and the net result would be a removal of carbon dioxide from the atmosphere.
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BECCS may have an Achilles heel.  Namely, the amount of land required to grow enough trees or corn to remove carbon at a planetary scale would be enormous.

One recent study, for instance, found that in order to offset about a third of current global carbon emissions, you would need to cover the entire lower 48 states in BECCS projects.

Yet another negative emissions idea, simply planting huge amounts of trees where they currently do not exist, faces a similar hurdle.  There’s no doubt more trees means less carbon dioxide in the atmosphere.  Yet once again, vast areas could be required — and in the future, people will need even more land to grow food than at present.

Some researchers advocate a completely different approach, called “enhanced weathering.”

Scientists have long known that over vast time periods, carbon is removed from the atmosphere by becoming embedded in rocks.  Here’s how it works:  Rainwater contains some carbon from the atmosphere in the form of carbonic acid.  As it falls on certain types of rocks, they break down or “weather,” and the interaction eventually leads to the formation of a carbonate — such as limestone — with the carbon locked inside.
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The idea is to speed up this process by crushing silicate rocks like olivine, and spreading them over landscape surfaces to get rained upon.  Sounds simple, but it takes a lot of money and energy to mine and then crush rocks, and the fine dusts that result could also prove a major health hazard if people breathe them in.

It’s also expensive.  A recent study estimated a price of between $60 trillion and $600 trillion to remove 50 parts per million of carbon dioxide from the planet’s atmosphere in this manner.

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