Study: Crushed rock added to soil can pull CO2 out of atmosphere to mitigate climate change

Study: Crushed rock added to soil can pull CO2 out of atmosphere to mitigate climate change

By Steve Valk

For several years, scientists have warned that it will take more than reducing greenhouse gas emissions to stay within safe limits of global warming, which many experts now say is 1.5 degrees Celsius. To keep the planet from overheating, it will also be necessary to pull carbon dioxide out of the atmosphere.

The problem is that nascent technologies to extract CO2 are prohibitively expensive and difficult to scale to the level that’s needed.

A study released Monday by the University of Sheffield in England, however, finds hope in a promising agricultural technique: Spreading rock dust on farm fields. In particular, the study found that adding crushed silicate rock like basalt—the residue of ancient volcanic eruptions—can act as a carbon sink. When these fine rock grains dissolve chemically in the soil, carbon dioxide is absorbed and essential nutrients are released for plants.

“Human societies have long known that volcanic plains are fertile, ideal places for growing crops without adverse human health effects, but until now there has been little consideration for how adding further rocks to soils might capture carbon,” Professor David Beerling, lead author of the research, said in a press release. “This study could transform how we think about managing our croplands for climate, food and soil security. It helps move the debate forward for an under-researched strategy of CO2 removal from the atmosphere—enhanced rock weathering—and highlights supplementary benefits for food and soils.”

Those supplementary benefits include reducing the use of fertilizers and pesticides, thereby increasing the profitability of farms.

Dr. Danny Richter

“This approach checks all the boxes in terms of feasibility and potential,” said Dr. Danny Richter, CCL’s Vice President of Legislation and Research who also holds a Ph.D. in oceanography. “The bedrock of the entire ocean floor is basalt, as are larger swaths of land in many countries, so this is not a limited resource. Further, geologists and paleoclimatologists understand silicate rock weathering to be a primary moderator of CO2 in the atmosphere on long timescales, absent human activity. This is also the same rock type in which a liquid CO2-injection experiment reported in 2016 achieved results so complete and so quick, they thought their equipment was breaking down.”

How much CO2 can the world potentially draw down by spreading crushed rock on crop fields? That will require further study.

Dr. James Hansen

CCL Advisory Board member Dr. James Hansen, one of the study’s co-authors, said in a press release, “Strategies for taking CO2 out of the atmosphere are now on the research agenda and we need realistic assessment of these strategies, what they might be able to deliver, and what the challenges are.”

Dr. Hansen elaborated in an email, “I can verify that there are other feasible CO2 capturing techniques. Some will prove to be very minor. This is one that just might be large. But the co-benefits have to be demonstrated in large scale tests, and the possibility of unintended effects must be assessed. Farmers already have equipment to spread such fine-grained material [like] lime. If the co-benefits from appropriate rock dust, including increased soil productivity, reduced pests, reduced need for fertilizer, work out as we are suggesting, this could be the biggest of the essentially ‘natural’ interventions.”

Time being of the essence, Dr. Hansen says “fairly large-scale” implementation of the crushed rock technique is needed now “to show that REALLY large scale, near-global makes sense.”

At the same time, the transition to clean energy must accelerate. “This does not alter the fact that phase-out of fossil fuel emissions needs to begin soon, or the whole problem runs out of control,” said Dr. Hansen.

The potential impact of this breakthrough is hard to overstate, considering the high cost of other technologies currently in development. One process for capturing CO2, created by the Swiss company Climeworks, employs a battery of fans and filters to capture CO2, which it sells to a nearby greenhouse for $600 a ton. While economies of scale could eventually bring down the price, even at $100 per ton, the cost of sucking hundreds of billions of tons of CO2 out of the air would be staggering.

Steve Valk
Steve Valk is Communications Coordinator for Citizens' Climate Lobby. Steve joined the CCL staff in 2009 after a 30-year career with the Atlanta Journal-Constitution.