Using crushed rock dust to speed up the rate at which soils absorb carbon dioxide could also affect the climate by making Earth鈥檚 surface reflect more or less of the sun鈥檚 radiation.
Enhanced rock weathering (ERW), as it is known, is rapidly growing around the world as scientists and businesses hunt for ways to capture carbon from the atmosphere.
It involves sprinkling crushed volcanic rock, such as basalt, onto soils to accelerate the natural weathering process by which CO2 in the atmosphere is converted to stable minerals.
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But if the practice is scaled up to cover large swathes of land around the world, it could alter the planet鈥檚 reflectivity, or albedo, according to at Brown University in Rhode Island.
鈥淲hen I realised that we were talking about millions of square kilometres at least, I realised that even slight changes in albedo could have a big effect,鈥� he says.
Albedo is a measure of how much light something reflects, on a scale from 0 to 1. Whiter areas of the planet, such as the polar ice caps, bounce more sunlight back into space, providing a cooling effect, while darker areas absorb more heat and speed up warming.
If the rock dust spread on fields is paler than the existing soil, Earth鈥檚 albedo could increase, says Marston, boosting the cooling effect from ERW. However, if the dust is darker than the soil, it could reduce albedo and have the opposite effect.
Based on mathematical modelling, Martson and his colleague , also at Brown University, estimate that if 1 million square kilometres of agricultural land was spread with crushed rock, it could remove about 1 gigatonne of CO2 per year from the atmosphere. Yet the impact of the change in albedo could equal or even exceed the climate impact of the carbon removed by ERW, at least in the short term, they conclude. An increase of the albedo by just 0.1 on the 48 million square kilometres of land used worldwide for agriculture would result in Earth cooling by roughly 1掳C, Marston and Ibarra estimate.
The crucial thing is to make sure ERW uses rock dust that is paler than the soil it will be applied to. If ERW had a positive impact on Earth鈥檚 albedo, it could provide a double climate benefit, says Marston, offering long-term cooling thanks to the soil鈥檚 increased CO2 absorption and short-term cooling from the albedo change.
鈥淵ou could imagine deliberate strategies to maximise reflectance at the same time you are deploying crushed rock,鈥� he says, such as leaving soils untilled so the paler rocks sit on the surface for longer. The same principle could apply to marine carbon dioxide removal, which could involve adding alkaline minerals to seawater, says Marston.
But other researchers aren鈥檛 convinced ERW could prompt an overall change in planetary albedo. at Yale University says most farmers try to minimise the amount of time that soil is exposed. 鈥淔or most of the year, in a healthily managed field, you have crop that is totally covering the soil, or you have the dead remains of a cover crop,鈥� he says. That means Marston and Ibarra may have overestimated the albedo impact of ERW, he says.
at the University of Sheffield, UK, doubts the rock dust would change the soil colour for long enough to have an impact. 鈥淚n our experience, the rock dust is very quickly incorporated into the soil, either through biological processes or physical processes,鈥� he says. 鈥淓ven if there was a negative effect on albedo, it would be gone before it had any measurable radiative forcing effect.鈥�
Marston says he would welcome further research to confirm whether large-scale ERW would affect planetary albedo. 鈥淎s far as I can tell, this hasn鈥檛 been considered before, and I think it really needs to be.鈥�
at UK enhanced weathering firm UNDO Carbon invites Marston and Ibarra to measure albedo changes at the company鈥檚 field trial sites. 鈥淯NDO trials cover a wide range of ERW feedstock across four continents and all soil types, including major global crops,鈥� he says. 鈥淯NDO would welcome the authors to conduct experimental measurements across all of its large-scale field trials to test their hypothesis.鈥�
Arxiv
