Crushed basalt is spread in a field trial of enhanced rock weathering for carbon dioxide removal in Queensland, Australia
Paul Nelson
Spreading crushed silicate rocks such as basalt on fields could remove up to 1.1 billion tonnes of carbon dioxide from the atmosphere annually, while increasing crop yields, according to an analysis of the method’s global potential. But some researchers question whether this number is really achievable.
Known as enhanced rock weathering, this technique accelerates the breakdown of rocks by rainwater, a natural process that over millions of years has transferred CO2 from the atmosphere to the ocean and helped cool the planet during greenhouse-earth periods. Farmers have spread ground limestone on fields for centuries to improve nutrient uptake in crops.
“The biggest benefit is through a kind of solution of atmospheric CO2 through chemical reactions,” says Chuan Liao of Cornell University in New York. “And there are also some side benefits, like adding … magnesium, potentially calcium, to supplement soil nutrients.”
As emissions continue to rise, the UN’s climate agency has said humanity will require carbon removal to limit global warming to 1.5°C above pre-industrial levels. Countries such as Brazil have encouraged increased rock weathering to cut both emissions and fertilizer costs. Last year, an improved weathering startup in India called Mati Carbon won the $50 million grand prize in Elon Musk’s XPRIZE competition for large-scale carbon removal potential.
Atmospheric CO2 dissolves in rain and forms carbonic acid. In silicate rocks, this reacts with silicon dioxide and metals to lock the CO2 away in bicarbonate ions. The bicarbonate washes into rivers and oceans, where it can remain dissolved for millennia or become incorporated into the calcium carbonate exoskeletons of clams, corals and sea urchins. Crushing the rocks exposes more surface to rain, which increases this CO2 removal.
Based on how much rock can fit on farm fields, studies have estimated that increased rock weathering could draw down 5 billion tonnes of CO2 a year this century. Liao and his colleagues did a “reality check” on these estimates by including how quickly other innovations such as irrigation have been embraced by farmers and how effective weathering can be in different regions.
They modeled scenarios with both limited and widespread use of enhanced weathering and found that the technique could remove 350 million to 750 million tons of CO2 per year by 2050 and 700 million to 1.1 billion tons per year by 2100. By comparison, global CO2 emissions from fossil fuels in 2028 billion tons were around 2038 billion tons.
While Europe and North America would initially do most of this removal, they would be overtaken by Asia, Latin America and sub-Saharan Africa as silicate rock supply chains were established and costs fell. Higher temperatures and precipitation increase weathering in these regions, potentially allowing farmers there to sell more carbon removal credits per ton of rock spread.
“(For) farmers in the Global South, there will be less barriers for them to do so decades from now,” says Liao.
However, Marcus Schiedung of the Thünen Institute of Climate-Smart Agriculture in Germany and his colleagues argue in a recent paper that estimates like this hide large uncertainties about enhanced rock weathering. For example, if it does not rain and the soil remains dry, carbon removal can be up to 25 times slower. The estimate of 1.1 billion tonnes of carbon removal will probably be inflated, says Schiedung.
In soils with a high pH, precipitation can weather carbonates in the ground instead of the crushed rock. They will eventually be converted back to carbonates in the ocean, releasing CO2 and resulting in no net carbon removal, he says. In soils with a low pH, naturally occurring acids can react with the crushed rock and carbon will not be removed from the precipitation. When soil acidity decreases, CO2 emissions from microbes increase.
In addition, mining and hauling rock to the farm can in some cases release more carbon than is removed, says Schiedung.
“I’m a skeptic,” he says. “We have to be sure that CO2 is taken up. Otherwise we risk that we measure something (removing carbon), but somewhere else it is released again, which is likely to happen in this geochemically complex system.”
Some also fear that increased rock weathering could introduce toxins into the food supply. Olivine, the rock on which Liao’s estimate is based, contains heavy metals such as nickel and chromium.
The tailings at most existing mines are also contaminated with metals, according to David Manning of Newcastle University, UK. Countries would likely have to open massive amounts of basalt quarries instead, which would take time and money.
“One gigatonne of CO2 removed per year requires 5 gigatonnes of rock per year, and that’s a problem, because nobody knows where the rock comes from,” says Manning. “It’s a big barrier to growth.”
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