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Is Farming the Key to Carbon Capture?

Researchers from Northwestern and the Chicago Botanic Garden will study the carbon capture effects of soil additives on agricultural fields.

A photo of a crop field with many uniform rows of unidentified green plants growing. The crop field extends far into the distance, where some mountains are visible against a horizon and a setting sun. The sky is slightly cloudy.
Image: Adobe Stock

By Sean Hargadon
Spring 2024

For the world to meet the global average temperature goals set forth in the 2015 Paris Agreement, we need to actively decarbonize. One way to decrease carbon emissions, suggests Earth and planetary sciences professor Andy Jacobson, is to capture atmospheric carbon dioxide (CO2) and lock it away for a few thousand years. Earth science could help with that. 

The chemical weathering of rocks is a natural process that converts atmospheric CO2 into a stable mineral. Carbon dioxide dissolves in water to form carbonic acid, which breaks down rocks. Chemical weathering liberates calcium and other elements from silicate rocks while transforming CO2 into bicarbonate. Over millions of years, bicarbonate combines with calcium to form calcium carbonate, the building blocks for coral reefs and limestone. This process sequesters carbon in solid form and ultimately serves as Earth’s natural, long-term climate stabilizing mechanism.  

“Over much shorter human timescales, bicarbonate is a sink for atmospheric CO2,” says Jacobson. “Can we accelerate the weathering process and actively remove carbon from the atmosphere” at rates fast enough to help mitigate modern climate change?  

Enhanced rock weathering was proposed as a decarbonization strategy decades ago but only recently gained interest. A 2020 Nature article estimated that enhanced weathering could remove up to 2 billion tons of CO2 annually from the atmosphere by 2050. 

Jacobson is leading an interdisciplinary team of researchers at Northwestern and the Chicago Botanic Garden to investigate this “negative emissions technology” with a two-year demonstration grant from Northwestern’s Paula M. Trienens Institute for Sustainability and Energy 

In a series of mesocosms (experimental systems that simulate natural conditions) at the garden, researchers will test the effects from adding different types of crushed rock to soils used to grow various crops. The soil additives will include basalt, a volcanic rock that chemically weathers faster than other rocks and minerals, potentially expediting CO2 capture and storage.  

“Many geologic processes are slow, even ones that are enhanced.” — Andy Jacobson

“In a greenhouse environment, we can measure the bicarbonate coming out of the mesocosms. And we can trace how much of that bicarbonate is coming from the weathering of silicate minerals,” says Allegra Tashjian, a doctoral student in Earth and planetary sciences who is leading the initial experiments. Basalt also could help improve the soil and boost crop yields. The long-term goal is to move the experiment into a field test.   

“Farmers already have much of the infrastructure needed to implement enhanced weathering as a CO2 removal strategy,” says Brad Sageman, professor and director of undergraduate studies in Earth and planetary sciences. “Crush up the rocks, put them on the field and wait. That said, the idea itself is so new that we don’t yet have sufficient empirical data to know the best way to approach it. How will soil amendments affect the soil microbiome and plants, and how much carbon will be converted? Our goal is to answer those questions.” 

The market for enhanced rock weathering is skyrocketing, with startups already partnering with farmers on carbon offsets. “It’ll be awesome if enhanced rock weathering works well at scale,” adds Tashjian, “but we also want to understand any limitations to ensure that carbon offsets from this technology are accurate.” 

The project begins in May, but don’t expect results overnight, says Jacobson. “Enhanced weathering doesn’t mean instantaneous,” he says. “Many geologic processes are slow, even ones that are enhanced.” 

For Sageman, a geologist who has spent his career “studying events that happened millions of years ago,” this project is a perfect capstone. “Studies of climate change through Earth’s history provided the foundation for realizing the potential of enhanced weathering,” he says. “Building on that work, we now have an opportunity to make the world a better place for our grandchildren.” 

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