Using rocks in farmlands to remove carbon dioxide from the atmosphere

We should not be fooled into believing that global warming will cease to be a problem in the coming years if we reduce emission of carbon dioxide into the atmosphere. Even zero emission followed by switching to "green" sources of energy is not going to alleviate the detrimental effects of global warming substantially. That is because carbon dioxide that we have dumped into the air since the beginning of the Industrial Revolution is going to remain in the climate system and impact the well-being of all forms of terrestrial life for what can be thousands of years.

So, what should be done to avoid a catastrophic global warming? The answer is simple: at the least, remove some carbon dioxide from the atmosphere before the climate breaches the threshold of runaway greenhouse effect due to positive feedback loops of the greenhouse gases.

We have an expanding portfolio of actions available to us for removing and sequestering carbon dioxide. One such action, albeit shockingly simple and low-tech, is to do what nature has been doing since time immemorial—sucking carbon dioxide out of the atmosphere using rocks, particularly the types created by volcanic activity. These rocks react with the heat-trapping carbon dioxide to form stable carbonate minerals which are eventually washed away to be locked into the soil or in the ocean beds. This weathering process, called "mineral carbonation," is one of the mechanisms our planet uses to remove carbon dioxide from the atmosphere across geological time scales.

Indeed, scientists recently discovered that Peridotite, a silicate containing rock pushed up from Earth's upper mantle long ago in what is now the country of Oman, is removing roughly 100,000 tons of carbon dioxide from the atmosphere each year. That is enough to soak up carbon dioxide emissions from burning more than ten million gallons of gasoline.

The process of mineral carbonation is extremely slow—far too slow to offset global warming from human activities. However, the rocks will work their magic most efficiently in the shortest possible time if they are crushed into a fine powder. Known as "enhanced rock weathering" (ERW), the powdered rock increases the surface area available for chemical reactions and thereby speed up the removal of carbon dioxide from the air.

According to a research paper published in July 2020 in the journal Nature, crushed silicate-rich volcanic rocks, such as Olivine and Basalt, have the potential to draw down billions of tonnes of carbon dioxide from the air. Consequently, among a handful of negative-emission technologies, ERW is gaining traction, although the approach is still in its infancy.

Scientists believe that for good measure, we can use the technique of ERW in the agriculture industry. By spreading crushed rocks all over the farmland, the roots of crops and fungus in the soil will hasten the chemical and physical breakdown of the rocks, while carbon dioxide will be pulled from the air into the soil as part of the weathering process.

The concept of enhanced rock weathering is not new, though. It basically replaces crushed limestone that farmers in many countries use to improve crop yields with volcanic rocks rich in silicate and magnesium. Researchers are now taking the concept out of the labs and testing ERW in real farmlands to see how it fits–practically and economically–in the wider portfolio of options for slowing down the steady rise of carbon dioxide in the atmosphere. They are conducting an experiment on a farmland in California in association with farmers, ranchers, government, the mining industry and Native American tribes.

The benefits of ERW are not just limited to the climate. In addition to capturing carbon dioxide, the weathered rocks will release valuable nutrients such as phosphorus and potassium into the soil, which will boost production of healthy crops. The rocks, preferably Basalt, will provide plants with silica as well, which will help them build stronger cells to better fend off pests. As a matter of fact, one compelling outcome of the research in California is that in controlled-environment studies involving Basalt and Wollastonite, a calcium silicate mineral, corn yield increased by nearly 12 percent.

Application of enhanced rock weathering technique in farming will create a protocol for farmers to make money from the carbon dioxide they farm into the soil. More importantly, ERW reduces soil acidity, which already affects about one-fifth of arable fields around the world. Moreover, unlike other negative emission techniques, such as Afforestation and Reforestation or Direct Air Capture, ERW does not compete for land used to grow food.

Meeting the demand for crushed rock to undertake large-scale removal of atmospheric carbon dioxide can be achieved by using existing stockpiles of Basalt–one of the most common rocks on Earth–and other silicate rock dust left over as byproducts from the mining industry. This would remove the need to grind the rocks into fine particles, which is energy intensive. Furthermore, calcium-rich silicate byproducts of iron and steel manufacturing plants, along with waste cement from construction and demolition industries, can also be processed and used for ERW. This "green recycling" of waste rocks will help in improving the sustainability of these industries alongside achieving their carbon neutrality goals.

It should be noted that enhanced rock weathering is not a cheap, easy fix for mitigating global warming. It comes with a price tag—mainly from mining, crushing and transporting rocks to farms, as well as emissions of greenhouse gases from these activities. The cost will of course be highly dependent on where the rocks are mined, and where and how they are crushed and spread. If machines used for ERW are powered by renewable energy, then scientists believe that the net amount of carbon dioxide removed from the air will be on the positive side.

Mineral carbonation in general and enhanced rock weathering in particular is not the last word to fight climate change. Nevertheless, it is an essential adjunct to other ways of ameliorating the brutal repercussions of anthropogenic climate change. If successfully applied, this natural process will ensure that some carbon dioxide will be permanently sucked out of the atmosphere. But the big challenge to implement this is the political will of our leaders who are beholden to powerful vested quarters.

Quamrul Haider is a Professor of Physics at Fordham University, New York.