Could a pile of rocks help to solve the climate crisis? Scientists at California’s Stanford University say they could.
The chemists have developed a practical and low-cost method to permanently remove atmospheric carbon dioxide: the greenhouse gas responsible for most of the Earth’s warming.
“The Earth has an inexhaustible supply of minerals that are capable of removing CO2 from the atmosphere, but they just don’t react fast enough on their own to counteract human greenhouse gas emissions,” Matthew Kanan, a professor of chemistry at Stanford, said in a statement. “Our work solves this problem in a way that we think is uniquely scalable.”
Kanan is the senior author of the research, which was published Wednesday in the journal Nature.
So, what is this miracle method?
They put rocks in a kiln, like the thermally insulated ovens used to make cement. The heat transforms minerals, naturally occurring elements or compounds in the rocks, into materials that pull carbon from the atmosphere.
It’s a process akin to weathering: the breakdown or dissolution of rocks and minerals on the surface of Earth. It’s caused by chemical and physical interactions with water, the air, and living organisms. But, that can take time. Sometimes, thousands to millions of years. Weathering can also form new minerals.
For example, common minerals called silicates — which contain silicon and oxygen and are found primarily in the Earth's crust — react with water and atmospheric carbon dioxide to form stable bicarbonate ions and solid carbonate minerals.
Inspired by a centuries-old method for making cement, the authors have developed a technique to convert slow-to-weather silicates into more reactive minerals that capture and store atmospheric carbon quickly.
To produce cement, limestone is converted into calcium oxide in a kiln that’s heated to about 1,400 degrees Celsius, or 2,552 degrees Fahrenheit. That chemical compound is then mixed with sand to produce a key ingredient in cement.
Instead of using sand, the Stanford researchers combined calcium oxide with another mineral containing magnesium and silicate ions. When heated, the minerals transformed into magnesium oxide and calcium silicate, which react quickly with acidic carbon dioxide in the air.
To test this approach, the produced calcium silicate and magnesium oxide were exposed to water and pure carbon dioxide at room temperature. Within two hours, both materials transformed into new carbonate minerals with carbon from carbon dioxide trapped inside. Wet samples of the materials were also exposed to air, which has a much lower concentration of carbon dioxide than pure carbon dioxide. In that test, the carbonation process took weeks to months, but it was still faster than natural weathering.
“You can imagine spreading magnesium oxide and calcium silicate over large land areas to remove carbon dioxide from ambient air,” Kanan said. “One exciting application that we’re testing now is adding them to agricultural soil. As they weather, the minerals transform into bicarbonates that can move through the soil and end up permanently stored in the ocean.”
The ocean is already the planet’s largest carbon sink, taking in 25 percent of all carbon dioxide emissions and capturing 90 percent of the excess heat generated by these emissions.
In 2023, the global average atmospheric carbon dioxide was at a record high. During the previous year, data from the Environmental Protection Agency showed U.S. greenhouse gas emissions had totaled over 6,300 million metric tons of carbon dioxide equivalents. The global amount of carbon dioxide emissions produced annually is in excess of 40 billion tons.
Climate scientists say the only real way to slow the devastating effects of the climate crisis is by slashing these emissions. The impact of carbon capture methods has been a topic of debate. One Stanford researcher has said current approaches could increase air pollution and others have said it’s too costly a strategy.
Kanan said that this process requires less than half the energy used by leading direct air capture technologies. He and associate professor Jonathan Fan are trying to develop kilns that run on electricity. Although trapping carbon dioxide on the scale necessary to affect global temperatures would require the yearly production of millions of tons of magnesium oxide and calcium silicate.
Still, this could potentially be carried out using olivine or serpentine rocks that are found in California, the Balkans, and other places. They are often leftover materials from mining.
They said that each ton of reactive material could remove a ton of carbon dioxide from the atmosphere.
“It’s estimated that there are more than 100,000 gigatons of olivine and serpentine reserves on Earth, enough to permanently remove far more carbon dioxide than humans have ever emitted,” said lead author and postdoctoral scholar Yuxuan Chen.
