Rocks and the natural carbon cycle
Enhanced rock weathering builds on a natural part of our climate system. As rocks are worn away by rain (or “weathered”), they release elements like calcium and magnesium. Meanwhile, CO2 in the air goes through other natural reactions to become carbonic acid (found in rain) or bicarbonate (found in the ocean). When these different compounds meet, they join to form new rocks like calcium carbonate, better known as limestone.
In this way, rocks help draw CO2 out of the atmosphere.
Natural rock weathering takes place over many millions of years: far too slow to help against today’s human-caused climate change. But beginning in the 1990s, scientists concerned about the buildup of CO2 in our atmosphere started to suggest ways to speed rock weathering up (1). This “enhanced” weathering would aim to take hundreds of millions or billions of tons of carbon out of the air every year.
Types of enhanced rock weathering
Scientists have come up with several ideas to make rocks combine with carbon faster.
The simplest is to grind the rocks up, making a fine gravel or dust that reacts more easily with the air or water. Olivine, for instance, is a very common rock below the Earth’s crust, but a rare one on the surface because it weathers so quickly. In theory, olivine dust spread on beaches or in the ocean would break down in a matter of years, locking up carbon as it dissolves. (This might also help address ocean acidification, by taking up some of the excess carbon that is making the seas more acidic.)
Basalt is another candidate for this kind of enhanced weathering. In some experiments, farmland treated with ground basalt not only captured carbon, but also grew more crops as the basalt helped the soil hold onto needed nutrients (2).
Other ideas for enhanced weathering try to speed up the chemical reactions involved. This might be done by adding chemical catalysts, or living things like bacteria or lichens—anything that eases the path for carbon to bind with elements in the rock.
Mineral trapping
“Mineral trapping” or “carbon mineralization” is a related form of carbon removal that also takes advantage of natural reactions between carbon and rock. Mineral trapping is paired with carbon capture, in which machines take pure CO2 out of industrial waste streams or the air itself.
In most carbon capture operations, the captured CO2 is stored underground as a fluid, not unlike oil. But with mineral trapping, the CO2 is instead dissolved in water and injected into basalt deep underground. Here, naturally high pressures and temperatures speed up the reactions between CO2 and rock, causing the carbon to quickly become part of the stone. At a working site in Iceland, the company Carbfix reports that 95% of the carbon it injects underground turns into the mineral calcite within two years (3).
Difficult and expensive
Geologists and climate scientists agree that the basic idea of enhanced rock weathering is sound. The challenge is making this a practical solution to climate change.
Mining, grinding and transporting rock takes a lot of energy, most of it from climate-polluting fossil fuels. Enhanced weathering needs to be careful to remove more CO2 than it creates. Most enhanced weathering ideas also need large tracts of land, which means that many farms or beaches will need to participate before they take a meaningful amount of carbon out of the air.
It’s also unclear how well large-scale enhanced weathering will work in practice. The chemical reactions of ground rock in natural soils or seawater are complex and hard to predict. Some studies have suggested that olivine in seawater may quickly stop combining with CO2 in certain environments (4), or that using the wrong type of olivine may actually add CO2 to the atmosphere through secondary reactions involving iron (5). Some byproducts of mining, grinding and applying rock may harm natural ecosystems or human health (6). These details need to be worked out before enhanced weathering is used at scale.
But the biggest challenge, as with many ideas for carbon removal, is cost. Enhanced weathering is simply more expensive than more direct ways of dealing with our carbon emissions—like building clean energy so we can shut down climate-polluting coal and gas. And unlike a wind or solar farm, enhanced rock weathering can’t make a profit without some form of government support. This makes it hard for enhanced weathering operations to get off the ground.
The idea is still worth exploring. Experts agree that, to meet the world’s targets for limiting climate change, some amount of carbon removal will be needed in the coming years (7). Enhanced weathering is one possible path to taking CO2 out of the air at a scale that will make a real difference for our future climate.
Published November 9, 2023
FOOTNOTES
1 Seifritz, W., "CO2 disposal by means of silicates." Nature, Volume 345, 1990, doi:10.1038/345486b0. See also Lackner, Klaus, et al., "Carbon dioxide disposal in carbonate minerals." Energy, Volume 20, Issue 11, 1995, doi:10.1016/0360-5442(95)00071-N.
2 See for example Kantola, Ilsa, et al., "Improved net carbon budgets in the US Midwest through direct measured impacts of enhanced weathering." Global Change Biology, August 2023, doi:10.1111/gcb.16903; Kelland, Mike, et al., "Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil." Global Change Biology, April 2020, doi:10.1111/gcb.15089; and Vienne, Arthur, et al., "Enhanced Weathering Using Basalt Rock Powder: Carbon Sequestration, Co-benefits and Risks in a Mesocosm Study With Solanum tuberosum." Frontiers in Climate, Volume 4, 2022, doi:10.3389/fclim.2022.869456.
3 Matter, Juerg M., et al., "Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions." Science, Volume 352, 2016, doi:10.1126/science.aad8132.
4 Montserrat, Francesc, et al., "Olivine Dissolution in Seawater: Implications for CO2 Sequestration through Enhanced Weathering in Coastal Environments." Environmental Science & Technology, Volume 51, Issue 7, 2017, doi:10.1021/acs.est.6b05942.
5 Griffioen, Jasper, "Enhanced weathering of olivine in seawater: The efficiency as revealed by thermodynamic scenario analysis." Science of the Total Environment, Volume 575, 2017, doi:10.1016/j.scitotenv.2016.09.008.
6 Suhrhoff, Tim Jesper, "Phytoprevention of Heavy Metal Contamination From Terrestrial Enhanced Weathering: Can Plants Save the Day?" Frontiers in Climate, Volume 3, 2021, doi:10.3389/fclim.2021.820204.
7 Intergovernmental Panel on Climate Change: "Mitigation pathways
compatible with long-term goals." In IPCC, 2022: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. doi:10.1017/9781009157926.005.