Planet Earth itself will be just fine. The carbon dioxide we’ve been pumping out over the past few centuries is causing unprecedented warming right now but there are natural processes that will take care of the gas quite nicely. Carbon captured in seashells will sink to the seafloor, where tectonic action will fold it deep within the planet. Other carbon from dead plants and animals will get trapped in mud and get compressed into new deposits of coal, oil and gas.
The catch: this will all take a few hundred million years.
Now some researchers and start-ups believe that we can accelerate these and other processes to lock away carbon on a timeframe that might help humanity, and the fragile ecosystems we rely on, in the blink of a geological eye.
Trees, oceans, and algae have proven to be able warriors in the fight against global warming but rocks are on the move. Which will get carbon out of the atmosphere faster and for good: geology or biology?
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Rock Solid Carbon Sinks
1. A massive vacuum cleaner for carbon. Startup Heirloom Carbon might appear to be making our carbon problem worse by heating limestone to release the eons-old carbon dioxide within. But it’s all part of a longer game. The ancient CO2 will be pumped underground (see below), leaving the leftover calcium hydroxide hungry for more. That material is then spread across large trays in a direct air capture system, where it absorbs CO2 from air flowing across it in a matter of hours, the company claims. The rocks are then heated to release the CO2 for storage, and the process begins anew. Microsoft recently signed a long-term contract with Heirloom for up to 315,000 tons of CO2 removal over a multi-year period, a deal worth at least $200 million, according to the Wall Street Journal.
2. Fracking in reverse. Carbon dioxide is a natural escape artist. Almost anywhere it is stored in nature, CO2 is vulnerable to being released back into the atmosphere through fire, sea-level change, or microbial action. Dispose of it deep underground, though, and it could be there for a long, long time. There are several ideas of how to do this. Some scientists are pretty sure that CO2 could be stored for millennia in stable subterranean formations, called pore spaces, located in permeable rocks beneath solid caprock. Ironically, this could leverage not only the technology developed for gas fracking—but potentially the very same wells. Another option, developed by Icelandic start-up Carbfix, is dissolving CO2 in water and pumping it underground. There it reacts with basalt to form geologically stable rock within about two years, says the company.
3. A carbon crush. Grinding up rocks naturally accelerates their chemical reactions. One low-tech way to harness that is to sprinkle ground-up rocks on agricultural land, where they would react with slightly acidic rain and irrigation water to slowly absorb carbon and boost crop yields. Another option is to go straight to the ocean. A start-up called Vesta plans to take naturally alkaline rocks like olivine and reduce them almost to dust. The attractive green sand would be deposited on seashores, where wave action will further speed up the absorption—and reduce ocean acidification at the same time.
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Biology Owns the Carbon Fast Lane
1. Life is swift, geology is glacial. In the race to strip CO2 from the atmosphere, nature is the fleet-footed hare to geology’s stone-cold tortoise. The so-called fast carbon cycle of photosynthesis, microbial action and air-sea interactions moves about a thousand times quicker than slow geological processes, according to NASA. The problem is, trees do burn.
2. Biology is free. . .or at least cheaper. Most trees and nature-based solutions don’t require any drilling, mining, grinding or heating to lock carbon away. Speeding up natural geologic processes, on the other hand, is only as efficient as the energy you add to make it happen. And storage to date has often been tied to enhanced oil recovery—extracting even more fossil fuels. Obviously, the heavy infrastructure needed for extraction and processing will have to be powered by renewable energy to avoid simply undoing all the good effects. But even then you’re using electrons that could be decarbonizing other sectors of society. The math for the carbon storage payoff is complex and, as yet, far from certain.
3. Biology has win-wins. Geology has question marks. Saving a forest for its carbon has wide knock-on benefits for biodiversity, weather mitigation, soil health, and water and air quality. Locking up carbon in underground vaults results in none of these, and opens the door to potential problems if things go wrong. This in-depth story at Energy Wire details several occasions when moving and storage carbon dioxide already hasn’t gone as planned, including risks to groundwater and human health. There could also be as-yet unknown impacts to, say, marine organisms from adding alkaline rock en masse to seawater or other ecosystems.
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What To Keep an Eye On
1. Billionaires. Last week, Bill Gates said, “I don’t plant trees,” calling the idea that climate change could be addressed by just planting enough trees “complete nonsense.” And he’s not the only one. Patrick Collinson, founder of Stripe, is putting his company’s money behind a number of mineralization, weathering and storage efforts, while a non-profit funded by Larry Page has funded research into ocean geo-engineering. Even a fracking billionaire, Harold Hamm, is getting in on the action.
2. Scale. Whether you’re pro-tree or pro-rock, there’s little doubt that we’ll need a huge increase in carbon sequestration infrastructure to reduce CO2 in the atmosphere to safe levels. A trillion trees sounds appealing (if you’re not Bill Gates) but will be tricky to plant given the lack of a seedling supply chain at anything like the scale required. Meanwhile, in the geology camp, there have been only two mineralization storage projects to date, in Iceland and the US. One idea, from Canadian start-up Arca, is to focus on magnesium-rich mine tailings already on the surface, and that it thinks could helpfully store about 40 gigatons of carbon dioxide.
3. Artificial rock stars. It isn’t a natural geological process but making concrete provides another potential route for sequestering carbon in rock-like structures. Adding CO2 to cement can mineralize the gas, giving a double whammy of locking away the greenhouse gas and reducing use of other energy-intensive materials. A German study from 2021 estimates that using CO2 mineralization could reduce the carbon footprint of the cement industry by between 44 and 85 percent.