‘Enhanced weathering’ to trap CO2 may harm peat soils

Climate interventions known as geoengineering typically conjure up images of the skies, with sophisticated machines to deflect the sun’s rays or scour carbon emissions from the air. But some ideas, like ‘enhanced weathering,’ are more earthbound and they, too, are fraught with challenges.

New research from Universität Bremen suggests that enhanced weathering, meant to accelerate a natural process that traps carbon dioxide (CO2) in water and rock, may actually fail to remove as much CO2 as it causes. The effect was demonstrated by Dr. Alexandra Klemme and her colleagues, based on measurements taken across 10 years in Indonesian and Malaysian rivers, as well as coastal Sumatra’s ocean waters.

The results of their study were published this month in the journal Nature Communications.

Weathering removes CO2 from the atmosphere slowly, on chemical and geological timelines that extend across thousands of years. The enhanced weathering idea involves intentionally crushing rocks into powder so there’s more surface area for the CO2 to bind to rock, forcing the chemical process that traps CO2 to speed up.

“Spreading pulverized rock over land – for example agricultural land – is being discussed as a possible method for reducing atmospheric CO2,” Klemme explains. Long-term projects at the University of Sheffield’s Leverhulme Centre in the UK, as well as in the United States, Australia, and Borneo, are exploring the possibilities.

It’s considered a promising strategy, especially in the world’s peatlands. Tropical heat and humidity also accelerate the natural weathering process so, in theory, peatlands may be optimal zones for trying to make enhanced weathering work. That’s especially true in Southeast Asia, where peatlands already protect against sea level rise and absorb significant amounts of carbon, but the land use loss to agriculture is changing the equation.

Enhanced weathering increases the pH value in the soil and peatlands are naturally acidic. It’s one of the reasons why researchers think the practice of adding basalt rock might improve crop yields while sequestering carbon too. But raising the pH level in peat soils also destabilizes them, Klemme says, and estimates of how much CO2 might be removed from the atmosphere aren’t accurately taking the impacts of that destabilization into account.

Klemme’s team found the technique caused more carbon to leach from peat soils into rivers, actually raising CO2 emissions from the rivers and coastal waters themselves.

“We estimate that the potential carbon uptake associated with enhanced weathering is reduced by 18–60% by land-based re-emission of CO₂ and is potentially offset completely by emissions from coastal waters,” the paper said. “Our findings suggest that in contrast to the desired impact, enhanced weathering may destabilize the natural carbon cycle in tropical peatlands that act as important carbon sinks and protect against coastal erosion.”

Klemme and her colleagues haven’t ruled out the benefits of enhanced weathering entirely, but the researchers conclude that more study of different soil types is needed to fully understand how they respond to added basalt.