We can remove CO2 by spreading crushed rock on fields

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• Carbon dioxide (CO2) levels in the atmosphere have approximately doubled since the start of the Industrial Revolution. Carbon dioxide is a greenhouse gas that warms Earth.
• Crushed rock can remove CO2 from the atmosphere. Ground-up rock like basalt spread over fields allows CO2 to react with the rocks, converting CO2 into bicarbonate and trapping the carbon in solution.
• But all soils don’t react the same with CO2. The method has a huge potential to remove billions of tonnes of CO2 a year, but it needs more study.

By Paul Nelson, James Cook University, and Wolfram Buss, Australian National University

Crushed rock can remove CO2

Carbon dioxide (CO2) is naturally removed from the air when it reacts with certain types of rock. We can accelerate this process by crushing suitable rocks and spreading them over agricultural fields. This simple method, known as enhanced rock weathering, could vastly increase the rate of CO2 removal from the atmosphere.

Modeling studies suggest we could remove billions of tonnes of CO2 annually if we applied crushed rocks to croplands globally. (A tonne equals 1.1 ton). The current energy-related emissions are at 37 billion tonnes (40 billion tons) a year. So this means enhanced weathering could contribute in a big way to reaching net zero emissions. A new industry is rapidly emerging to do just that.

But before everyone gets too carried away, it’s crucial to be able to measure how much CO2 this technique captures. Both industry and governments will need accurate measurements for effective policy, regulation and reporting.

In our new research, we measured CO2 capture in a tropical agricultural soil. We found the CO2 capture rate was low in this soil, despite significant weathering. This means people will need to carefully consider soil type when estimating CO2 capture rate.

What is rock weathering?

Research has shown that we can crush rock rich in calcium and/or magnesium, such as basalt, and spread it over the soil to capture CO2. We can mine the rock for this purpose or obtain it as a byproduct from other mines or from making gravel.

In soil, the CO2 dissolves to form carbonic acid, which is what makes drinks fizzy. This acid can react with the rocks, converting CO2 into bicarbonate, which traps the carbon in solution.

Bicarbonate can be further converted in the soil and stored as solid carbonate (limestone). Or it can leach into groundwater, then rivers to the sea, where it is stored long-term as dissolved bicarbonate or as carbonate rock.

Rock weathering happens naturally on geological time scales. But for the technology to work in tackling climate change, we need to speed it up.

So we can use existing mining equipment to grind the rock and agricultural spreaders to distribute the crushed rock over the land.

Adding crushed rock in this way can improve soil health and crop production because it neutralizes soil acidity, supplies nutrients such as magnesium, calcium and phosphorus, and can help increase soil organic content.

Remove CO2 to cool the planet

In the United States Midwest, there are estimated potential CO2 removal rates of up to 2.6 tonnes of CO2 per hectare per year over a four-year period. (A hectare is about 2.5 acres.) This was based on applying 50 tonnes of finely crushed basalt per hectare every year. That rate is close to the highest natural rate reported globally, in Java, Indonesia (2.8 tonnes of CO2 per hectare per year).

But when we applied crushed basalt to a sugarcane field in Australia, over five years at the same rate, we had very different results. Our measurements of bicarbonate and carbonate in soil and water showed very low rates of CO2 removal.

Overall, the results from laboratory and field trials vary greatly from one study to the next. CO2 removal estimates range from 0.02 to more than 10 tonnes of CO2 per hectare. This variation could be due to the nature of the crushed rock and the way it was applied, the climate, soil type, cropping system and the duration of the trial. The figure also depends on the measurement method used.

Are we measuring it right?

We have now shown there can be a large gap between directly measured carbon capture and the amount estimated in other ways. Direct measurement of CO2 removal through conversion to bicarbonate and carbonate in soil and water can be difficult and costly. So far, this is mainly done in research trials, as we did, in our studies.

Researchers are developing other techniques to estimate in-field CO2 removal in a way that is easier for monitoring, reporting and verifying large-scale projects. These estimates use a combination of modeling and estimates of weathering rate.

We have shown in a glasshouse study that the discrepancy between measured and estimated CO2 capture varies a lot between soils. We found this discrepancy is largely driven by soil acidity.

In our recent field trial in an acidic soil, weathering was mostly due to acids that are stronger than carbonic acid. The added rock preferentially reacts with these stronger acids rather than carbonic acid, so rock weathering takes place, but without capturing much CO2.

Great potential

It’s becoming increasingly clear we will have to remove greenhouse gases from the atmosphere if we are to avoid dangerous climate change. We must cut carbon emissions, but these cuts won’t be deep enough to achieve net zero emissions by 2050. So we will need carbon removals to equal remaining emissions.

Enhanced rock weathering has great potential for CO2 removal, but we don’t yet have robust ways of measuring its effectiveness. We need to better understand the reactions in soil that affect CO2 removal across different soil types and under different management. We may also need to continue directly measuring carbon capture until we have confidence in more convenient estimates.

Paul Nelson, James Cook University, and Wolfram Buss, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: Scientists said that crushed rock spread on farm fields can remove CO2 to help regulate our warming world. But removal rates aren’t the same in all conditions.

Read more: Younger trees excel at capturing – and storing – carbon