Eventually (ideally sooner rather than later), efforts to reduce greenhouse gas emissions are going to have to be joined by a technology that actively removes CO₂ from the atmosphere. There are a number of options—from re-growing forests to burning biofuels in power plants that capture the emitted CO₂—and we’ll probably need several of them to get us to net zero emissions. Some of these options involve agriculture, and a new feasibility study suggests that one of them—spreading crushed rock on farm fields—deserves serious consideration.

The study was led by the University of Sheffield’s David Beerling; it estimates both the potential for this method of carbon capture in each country and the cost required to do so.

Carbon crush

Using crushed rocks isn’t a new idea. Some common minerals react with water and CO₂ as they weather, converting CO₂ from the air into bicarbonate dissolved in water. That bicarbonate (along with some calcium and magnesium) may hang out in groundwater or make its way into the ocean. And along the way, it can also turn into solid carbonate. Whatever route it takes, it’s no longer a greenhouse gas in the air.

Over hundreds of thousands or millions of years, this process has an important stabilizing influence on Earth’s climate. Warmer climates encourage more weathering, pulling greenhouse gas out of the atmosphere. What we need to do now is speed this process up so it has a meaningful effect in a human lifespan.

One way to accelerate weathering is to grind up that rock into small particles. Just as powdered sugar dissolves in water much more quickly than a large solid candy would, these small particles will weather much faster. Spreading that crushed rock over farm fields not only nicely exposes it to the elements but can also be beneficial for the soil, replenishing nutrients and counteracting pH changes in heavily farmed soils.

To estimate each country’s potential for using this technique, the researchers built gridded maps of croplands and climate conditions. A simple chemical model estimated the rate of weathering for the crushed rock based on the local soil conditions. They also calculated energy requirements based on distance from likely rock sources, as well as accounting for the energy mix available to run everything. (The more fossil fuel burned to carry out the work, the less CO₂ removed from the atmosphere in the final accounting.)

Small rocks, big difference

Globally, the researchers estimate that this process could be used to capture 500 million to 2 billion tons of CO₂ per year in 2050. For comparison, scenarios that limit global warming to 2°C generally involve capturing something like 2 to 10 billion tons per year in a few decades from now.

The majority of this capture potential comes from the US, China, India, and Brazil. Many other countries could offset a meaningful portion of their emissions this way, but these four countries have the most suitable farmland. For the US and China, it could offset up to 5 to 10 percent of emissions in 2030. India could offset as much as 40 percent, and Brazil could fully offset its emissions—though these numbers represent technical capacity, which would require a very strong commitment to this scheme.

After all, this isn’t free. Estimated costs vary depending on a country’s rock resources, labor costs, and transportation needs. It’s not as though an entirely new industry needs to be created, though. Mining and crushing for aggregate is hardly a cutting-edge or niche industry, and crushed limestone is already used as a soil amendment in some cases.

In the US, EU, and Canada, the researchers estimate that all this would cost about $160 to 190 per ton of CO₂ captured, while China, India, and Brazil could do it for $55 to 120 per ton. That’s in the same ballpark as other some options for atmospheric CO₂ removal. Biofuels burned in carbon capture plants, or using industrial plants to capture CO­₂ from ambient air, would both cost over $100 per ton. Other methods for sequestering carbon in agricultural soils might be at least a little less expensive, but multiple methods can be used together. It’s possible they might even synergize with each other.

Paying the costs

Still, costs are greater than zero, so something has to pay for these processes. A tax on carbon emissions of $100 per ton is capable of making a lot of techniques attractive, but current carbon pricing policies (where they exist) come in well south of that number.

Obviously, increased mining to meet this demand for crushed rock would have separate environmental consequences. But interestingly, the researchers identify some unused resources that could minimize or even eliminate the need for mining. Many rock crushing operations have leftover stockpiles of unwanted powder-sized grains that could be perfect. Slag from steelmaking could also work, as could recycled cement and masonry.

All these mineral sources need to undergo trials in different areas, the researchers say. It could be that some specific materials work better than others or that some release unwanted metals or contaminants. But studies could demonstrate that this technique—possibly combined with other additives like charcoal from untreated organic matter—has truly attractive benefits for soil health.

As researchers Johannes Lehmann and Angela Possinger—who were not involved in the study—write in an article accompanying the paper in the journal Nature, “The main lesson here might be that several of the major potential technologies for removing atmospheric CO₂ could generate substantial benefits for food production, and are centered around managing soils. Farmers must be fully behind such a global effort or it will fail… Such an approach of financially supporting soil health and crop production could emerge as our best near-term solution to the problem of removing CO₂ from the atmosphere.”

Nature, 2020. DOI: 10.1038/s41586-020-2448-9, 10.1038/d41586-020-01965-7 (About DOIs).

https://arstechnica.com/?p=1691273