Study challenges CO2-removal potential of enhanced weathering

Spreading finely crushed basalt across farmland has been promoted as a relatively simple way to remove large quantities of carbon dioxide from the atmosphere. But a new study led by Cornell researchers suggests the real-world potential of this method, known as enhanced weathering, may be far smaller than widely cited estimates.

The study, "Critical Zone Processes Limit Alkalinity Export from Natural Basaltic Systems," published Aug. 26 in Nature, draws on observations of natural volcanic landscapes to examine what happens after basalt begins to weather. The researchers found that chemical reactions and limited water movement can substantially reduce the amount of weathering-generated alkalinity that leaves soils, moves through rivers and, ultimately, reaches the ocean. That alkalinity helps convert carbon dioxide into forms that can remain stored for long periods.

The study challenges a central assumption underlying many projections for enhanced weathering: that carbon captured during the initial dissolution of basalt in soil can largely be counted as durable carbon removal.

"You obviously can buffer some CO2 emissions with this kind of process, but much less than people have hoped," said lead author Louis Derry, professor in the Department of Earth and Atmospheric Sciences in the Cornell Duffield College of Engineering. "The idea that we're going to get gigaton levels of CO2 reduction is not going to happen."

Co-authors are professors Kate Maher of Stanford University and Oliver Chadwick of the University of California, Santa Barbara.

Natural volcanic watersheds composed largely or entirely of basalt generally export carbon at rates far below those projected for agricultural enhanced weathering, according to the study. Median estimates compiled by the researchers range from about 0.22 metric tons of CO2 per hectare annually to roughly 1 metric ton in highly active volcanic regions of the Philippines. Those rates are substantially below projections for some enhanced weathering deployments, even though agricultural amendments typically add only a small percentage of basalt to existing soils.

The difference, the researchers argue, comes partly from focusing too narrowly on what happens immediately after basalt is added to soil.

Many enhanced weathering assessments estimate CO2 removal from the disappearance of calcium, magnesium and other elements from crushed basalt in the upper 10 to 30 centimeters of soil. But dissolving the rock is only the first step.

As water carries those weathering products downward through soil, groundwater and streams, it passes through the "critical zone" - the reactive layer where rock, soil, water, air and living organisms interact.

Along the way, newly formed clays, oxides and carbonate minerals can capture calcium and magnesium or generate acidity that consumes some of the alkalinity created by weathering. In some Icelandic environments, previous research indicates that secondary minerals retain 30% to 67% of calcium and 45% to 81% of magnesium entering the critical zone from silicate weathering.

"The net result is that only a modest fraction of dissolution products of weathering reactions - natural or engineered - is actually exported through the critical zone," Derry said.

The study also questions the assumption that grinding basalt into smaller particles will necessarily cause it to react dramatically faster than rock in natural settings. The researchers found only a weak relationship between particle size and commonly measured surface area except at very fine grain sizes. Among basalt samples proposed or used for enhanced weathering, the paper found no notable correlation between grain size and measured surface area.

Water poses another major constraint. The highest natural basalt-weathering rates occur in very wet tropical environments, where runoff can reach 2 to 3 meters per year. Major agricultural regions targeted for enhanced weathering can be much drier.

In the upper Mississippi basin, for example, effective hydrologic recharge is 0.25 meters per year or less. The researchers calculate that transporting the equivalent of 2.5 metric tons of CO2 per hectare annually through such a system would require average soil-water alkalinity outside plausible ranges.

Enhanced weathering may still provide useful local benefits, including buffering soil acidity, supplying nutrients or modestly offsetting emissions in some settings. But Derry argues that with limited resources available to address climate change, expectations for its global carbon-removal role should be grounded in how real landscapes behave.

"We've got a lot of data from natural systems across a range of conditions," Derry said. "We consistently see values that are much, much lower than the model studies. If you have finite resources to address a problem, you want to put them where they're most likely to make a difference."

Chris Dawson is a communication coordinator for Duffield Engineering.