How rock weathering could fight back against climate change

Enhanced rock weathering takes advantage of Earth’s natural carbon cycle, which keeps carbon dioxide in perpetual rotation between air, water and soil. Volcanic rock naturally traps atmospheric CO2 in a process that transforms the gas into a solid form. Like a thermostat, rock weathering can slowly moderate Earth’s temperatures over geologic time by keeping atmospheric levels of CO2, a greenhouse gas, in check.

For the past twenty years, researchers globally have studied methods to accelerate weathering as a potential climate solution. Grinding volcanic rock into a fine powder increases the surface area for capturing CO2. Although this rock dust can be distributed anywhere, supporters usually focus on cropland due to its easy access and various crop benefits.

Enhanced rock weathering is among several carbon dioxide removal technologies designed to reduce atmospheric CO2 levels. The Intergovernmental Panel on Climate Change (IPCC) suggests that strategies to sequester CO2 may be crucial for achieving climate targets. According to the IPCC, to limit global warming to 1.5°C above preindustrial levels, as outlined in the 2015 Paris Agreement, it is necessary to remove between 100 billion and 1,000 billion metric tons of carbon dioxide by the century's end.

A team of researchers in the United States reported last year in Earth’s Future that enhanced rock weathering has the potential to significantly reduce that amount. According to their computer simulations, applying this method to all arable land worldwide could sequester up to 215 billion tons of carbon over the next seventy-five years. This figure represents one-fifth of the IPCC's highest estimated requirement for carbon removal.

“Crushing rocks — it’s almost too dumb an idea. But it actually works,” says Adam Wolf, Co-Founder and Chief Innovation Officer of Eion.

Enhanced rock weathering appears promising, but it faces several hurdles, including concerns about environmental contamination and the difficulties in measuring and verifying carbon removal. Additionally, there is worry that funding might be diverted from established priorities, such as forest preservation and their carbon-absorbing trees.

Critics are also concerned that some carbon dioxide removal methods, supported by the fossil fuel industry, could enable continued emissions from oil and gas producers instead of a transition to clean energy.

Despite these challenges, the world is significantly behind on emission-reduction goals and running out of time. The IPCC stresses that both emission reductions and carbon dioxide removal are essential now. Gabrielle Walker, an author and scientist who has founded multiple carbon dioxide removal initiatives, argues that we must stop the infighting and adopt all climate solutions, including new ones, immediately.

“If we don’t build climate removals to gigaton scale by 2030, we cannot achieve our climate goals,” says Walker.

How the method works

Enhanced rock weathering harnesses an ancient process, predating even the dinosaurs: the natural breakdown and formation of rock. In the atmosphere, water droplets mix with carbon dioxide, creating carbonic acid. This weak acid falls as rain onto volcanic rock, dissolving its silicate minerals. This weathering releases essential nutrients like calcium and magnesium for plants and produces stable bicarbonate ions, which trap atmospheric carbon.

These bicarbonate ions move from groundwater into streams and eventually reach the ocean. There, marine organisms such as corals and clams use bicarbonate to form their shells and skeletons. When these creatures die, they sink to the ocean floor, and their carbon is eventually drawn into Earth's interior at subduction zones, where one tectonic plate dives beneath another. This carbon remains trapped within the planet for tens of thousands of years until volcanic eruptions release CO2 and silicate-rich lava, renewing the cycle.

Natural rock weathering sequesters about 1.1 billion tons of CO2 annually. In 1990, German physicist Walter Seifritz proposed that weathering could help mitigate climate change. However, significant progress was not made until 2006, when Dutch geochemist Olaf Schuiling suggested spreading highly reactive olivine, a mineral found in volcanic rock, on land to absorb excess atmospheric CO2.

Since then, research in enhanced rock weathering has surged. The Leverhulme Centre for Climate Change Mitigation at the University of Sheffield in England has led many studies, conducting both laboratory and field experiments across various continents.

Encouraged by promising results, numerous companies focused on enhanced rock weathering have emerged in recent years. Some already spread volcanic rock like basalt, some are in trial phases, and others are in the early stages of development. Most of these companies target cropland, sourcing basalt from mines or quarries, grinding it, transporting it to farms, and spreading about 25 tons per hectare. In contrast, the Irish company Silicate uses leftover concrete, rich in silicate, for their soil amendments. These companies offer the soil amendment to farmers for free, aiming to generate revenue by selling carbon credits to large corporations such as Microsoft, which seek to offset their carbon emissions.

The Netherlands-based company greenSand employs a different strategy, using olivine gravel for pathways and edging, driveways, decorative stones, and as bedding material to keep artificial turf resilient.

Other companies are investigating the feasibility of spreading rock dust on beaches and ocean waters, which could not only sequester carbon but also help counter ocean acidification.

The perks of enhanced rock weathering methods

Advocates argue that enhanced rock weathering offers several advantages over other carbon dioxide removal methods. It leverages a natural process, which may enhance its credibility, and builds on existing industrial processes, such as mining and agriculture, allowing for rapid scale-up and reasonable costs.

All carbon dioxide removal methods require energy, land, and water. In these areas, enhanced rock weathering holds a competitive advantage over more technologically advanced methods like direct air capture and bioenergy with carbon capture and storage (BECCS), a technique popular in Europe. BECCS involves growing crops such as sugar cane to absorb CO2, then burning the plants, capturing the released carbon for underground storage, and using the generated heat for energy.

The energy requirements for rock weathering, primarily related to the grinding and transport of rock, vary widely depending on factors such as distance to a farm and the fineness of the powder. Using rock dust left over from mines can help save energy. Overall, enhanced rock weathering typically requires only half the energy of direct air capture, according to David Beerling, director of the Leverhulme Centre, and his colleagues, who reported their findings in 2022 in Communications Earth & Environment. Their study examined carbon dioxide removal techniques across various countries.

A direct air capture plant uses massive fans and filters to extract CO2 from the air. By one estimate, removing 1 billion tons of CO2 using this method would require about 1,200 terawatt-hours of electricity, which is roughly three times the amount generated by the entire U.S. renewable sector in 2019.

In addition to energy savings, enhanced rock weathering can complement existing land uses, unlike some other carbon dioxide removal methods such as BECCS. To achieve significant carbon removal, BECCS would need a total land area larger than India. A study published in February in Science concluded that scaling up BECCS could “threaten food security and human rights…with potentially irreversible consequences.”

Enhanced rock weathering also requires only a tenth or a hundredth of the water needed for direct air capture, BECCS, and some other carbon dioxide removal strategies, according to Beerling and his colleagues.

Beyond climate benefits, enhanced rock weathering could benefit farmers. Modern agricultural practices, such as tilling soil, removing dead plant matter, monocropping, and applying pesticides, herbicides, and synthetic fertilisers, have damaged soil structure, eroded fields, and acidified soil. Acidic soil can lose essential nutrients and harm microbes that help plants grow. Overuse of nitrogen fertilisers can lead to the formation of nitrous oxide, a potent greenhouse gas.

Farmers already use crushed limestone to raise soil pH. Volcanic rock powder has a similar effect and can remove more atmospheric carbon due to its chemical composition. It also releases essential plant nutrients, adds silicon (which may make plants more resistant to pathogens), and increases crop productivity. Additionally, it appears to reduce nitrous oxide emissions. Many of these benefits are also seen with regenerative agriculture techniques, such as no-till farming and the use of cover crops, which aim to improve soil health. Enhanced rock weathering is compatible with these practices.

Concerns with rock weathering

Although the mechanics of enhanced rock weathering are well understood, the potential long-term risks remain uncertain. The most pressing concern is the possibility of heavy metal accumulation in soils.

This issue was highlighted at last year’s United Nations Climate Change Conference, COP28. Geochemist Gideon Henderson, chief scientific adviser at the U.K. Department for Environment, Food and Rural Affairs, cautioned against hastily adopting enhanced rock weathering techniques involving pure olivine. “Olivine has high trace-metal content, including some toxins, and in the long term may cause significant negative environmental consequences,” Henderson said.

Heavy metals in olivine, such as nickel, can pose health risks if absorbed by plants in large quantities and ingested by humans (small amounts of nickel are essential for plant growth). Concerns about heavy metals stem from the fact that enhanced rock weathering is most effective with repeated applications, which can increase metal levels over time.

The study showing a 215-billion-ton reduction in atmospheric CO2 over a 75-year period assumes annual application of 10 tons of basalt per hectare. However, another study, which assumed a more common application rate of 40 tons of basalt per hectare, suggests that safety limits for heavy metal accumulation may be exceeded after only ten years of repeated application.

Eion, which has projects in the southeastern United States, uses olivine. Wolf, the company’s cofounder and chief innovation officer, notes that soil nickel levels in that region are so low that nickel fertiliser is actually recommended. Additionally, plants absorb nickel more readily in acidic soil, which olivine corrects. “No paper I know of has ever shown an actual toxicity impact of olivine,” Wolf says, including one by Henderson.

Despite these reassurances, at least one start-up is taking precautions. Metalplant, an offshoot of the climate think tank Climitigation, cultivates a nickel-absorbing crop on fields treated with volcanic rock powder. The company then extracts the nickel from the plants for sale.

There is also concern about how adding volcanic rock powder or crushed concrete will impact soil microbes. Geochemist Frank McDermott of University College Dublin, the scientific lead at Silicate (the start-up that uses concrete powder), notes that while the microbial community typically responds well to an increase in soil pH from agricultural lime, basalt and concrete have different mineral and chemical compositions. Despite extensive research, scientists still do not fully understand how adding these powders in larger quantities than lime will affect soil microbes' ability to break down organic matter, which is crucial for soil health.

“Soils have a big store of organic carbon, and the goal is to protect and increase it over time. But if you add material that’s external to the system, and the microbes there have never seen basalt or concrete before, how will they react?” McDermott asks.

Another unknown is the potential impact of these rock powders on the flora and fauna of adjacent lands, lakes, or ponds, and on marine life if the rock powder is applied to the ocean.

What’s next?

Research on enhanced rock weathering continues to evolve. In a 2020 study published in Nature, Beerling and colleagues identified the United States, China, India, and Brazil as the primary agricultural nations most suited for the immediate application of enhanced rock weathering. This approach could help the United States and China offset up to 10% of their emission-reduction targets. For India, the potential is 40%, and for Brazil, it exceeds 100%.

Integrating enhanced rock weathering with other strategies could amplify its impact. Using rock powder alongside bioenergy with carbon capture and storage (BECCS) could enhance the health of feedstock crops. Moreover, combining it with other nature-based methods could transform agricultural land into a more effective carbon sink.

So far, most government funding for carbon removal in the United States and Europe has favoured high-tech solutions like direct air capture. However, this trend may be beginning to shift. In May, the U.S. Department of Energy awarded $50,000 each to five enhanced rock weathering projects, including Eion, to help scale up the technology. Additionally, the European Union has recently opened up the possibility for enhanced weathering advocates to apply for recognition as a viable removal strategy in Europe.

In this critical time, collaboration and open-mindedness are essential to mitigating the worst effects of climate change.