One of the many possible tools to stabilize Earth’s climate by reducing atmospheric carbon dioxide had its potential better defined this week, with a new study shedding light on the effectiveness of enhanced rock weathering as a way to mitigate human-caused global warming.
As certain types of rocks break down, they react with water and the atmosphere to capture carbon dioxide and lock it up in a solid form. Natural weathering of minerals has always been part of the planet’s carbon cycle, acting as a key regulator of greenhouse gases before fossil fuel emissions disrupted the climate. Recently, there have been proposals to try and enhance mineral absorption of carbon dioxide by grinding rocks rich in silicates and spreading them across huge swaths of farmland to draw CO2 out of the air.
Up to now, scientists have mainly measured how well that could work using lab experiments. The new research, published today in Science, helps establish how effective it could be in the real world, on a global scale. The findings suggest that rock weathering removes more CO2 in a warmer, wetter climate, but also shows that it’s not a silver bullet to stop global warming.
Scientists have long had a basic understanding of rock weathering, said the study’s lead author, Sue Brantley, a Penn State geochemist, but there have been big question marks, including how global temperatures affect the rate at which the process works.
“One of the big problems with predictions for enhanced weathering is that we have never been able to take lab rates and accurately predict field rates,” she said. “This may seem surprising, but there are just so many processes coupled together … and they respond nonlinearly. This is a step toward making better models of how the Earth responds to perturbations like the big one we are causing right now with burning fossil fuels.”
A related Perspective article in Science explained that the new research pulled together all laboratory observations about how temperature affects the rate of rock weathering, and combined those assessments with data from global soil profiles and river catchments, showing that the rate of weathering increases with rising temperatures.
“This can be explained by an array of processes that happen in landscapes, including physical mechanisms such as fracturing, the formation of new minerals during weathering, and the role of microorganisms,” University of Oxford researcher Bob Hilton wrote.
“I know that in the U.K., the enhanced weathering idea has recently had some funding to try and move it from concept, something people have done in labs or over small areas, to try and push it further,” he said.
As with other types of climate mitigation, he said there is “increasing recognition of whatever you do, you want to make sure that you don’t impact other key functions. You don’t want to impact biodiversity. We don’t want to end up with pollution impacts that we weren’t expecting from other sources.”
Most current proposals to use enhanced rock weathering to draw down CO2 are for agricultural lands, and he said that’s where the focus should be.
“I think people proposing to do it on non-agricultural land is quite outrageous, really,” he said. “I don’t see the justification for that.” The idea makes the most sense where the land is already being used for farming, with the equipment to move and spread the minerals, he added.
“For instance, we already crush up a lot of limestone in some areas, and we apply that to agricultural fields already,” he said. “So we could change this to silicate minerals, and then we get some CO2 drawdown.”
The new findings support other recent estimates that about half the world’s cropland might be needed to sequester 2 gigatons of CO2 per year, Brantley said.
“And of course we would have to convince society to allow lots of mining, maybe two or three times the amount of coal mined per year,” she said.
It’s important to also consider the impacts of the trucks used to transport millions of tons of rock dust, she said.
“And we’d have to convince farmers to spread the dust, and find ways for them to do it without breathing it,” she added. ”It probably has a place in our quiver of answers for CO2 and CO2 sequestration. But it’s not, in my opinion, a panacea.”
Not all of the mineral dust would need to come from new mining operations. Some of it could be sourced from existing industrial activities like construction and road building. And recent research also suggests there are co-benefits, with the rock powder helping to enrich soils to reduce the use of fossil fuel-based fertilizers.
Some ongoing experiments show that the technique works in the real world, Brantley said. Researchers with the Hubbard Brook Ecosystem Study project have been spreading calcium silicate across a small watershed near Woodstock, New Hampshire for about 15 to 20 years.
“They’ve got data for 15 to 20 years, and they can see that there has been more carbon sequestration,” she said. “They can see that it’s happening over a long timescale.”
But she also warned that spreading rock dust could have direct human health implications without proper precautions, because the silicate dust harms lungs if it’s inhaled, and there are also trace metals that could pose health threats if they are released to the environment during the process.
Other options for drawing CO2 out of the atmosphere include significantly expanding forest areas around the world, fertilizing parts of the ocean with minerals to boost plankton production, growing crops that capture carbon and can then be used to produce low-emission energy, as well as direct air capture, which uses industrial machinery to remove CO2 from the air with a chemical process.
A 2020 study in Nature concluded that China, India, the United States and Brazil have great potential for using rock weathering to help achieve global goals of removing between 0.5 to 2 gigatons of CO2 from the atmosphere annually, at a cost of between $8 to $180 per ton. By comparison, the cost of direct air capture was recently estimated at about $600 per ton, although new technologies could bring that down to less than $100. In 2021, global CO2 emissions totaled 37 gigatons, with the biggest share coming from China (11.47 gigatons) and the United States (5 gigatons).
According to the latest climate science assessments by the Intergovernmental Panel on Climate Change, carbon dioxide removal will be needed on a large scale in the second half of the century to reach the goal of limiting warming to around 1.5 degrees Celsius unless current emissions are halved by 2030.
Despite the challenges of implementing enhanced rock weathering at a meaningful scale, there are already numerous startups working on deploying projects with an eye toward selling credits in the growing global carbon market. Just a few days ago, a company called InPlanet said it wants to spread 50,000 tons of rock powder in South America this year, which could remove 10,000 tons of carbon dioxide from the air.
But considering the scale of the climate problem, enhanced rock weathering should probably be studied more as a potential part of the solution, Brantley said, adding that natural rock weathering will help rebalance the climate system over many millennia.
“We’re poking the bear right now by putting so much CO2 out there,” she said. “We’re really making a big perturbation to the climate system that it is going to take a long time for the Earth to recover.”