Weather disasters, dire predictions from climate modelers and polarizing language from politicians continue to fuel concerns about our global carbon footprint, as well they should.
The linkage between increased levels of carbon dioxide in our atmosphere and increased global temperature and weather instability is obvious and well accepted by science. Burning of fossil fuels in the 1960s contributed 3.1 billion tonnes (metric tons; a tonne is equal to 2,200 pounds) of carbon annually; we now add more than three times that much each year. Although U.S. carbon emissions may have recently leveled off — mainly due to a switch from coal to natural gas — emissions from China and India are growing exponentially.
Even if we stopped all fossil fuel use immediately, global temperature would continue to increase and atmospheric carbon concentrations would only decrease very slowly. The planet’s oceans and lands currently absorb about half of our annual carbon emissions. This leaves about 5 billion tonnes per year to mix into the Earth’s atmosphere, increasing the concentration of CO2 by over 2 parts per million (ppm) per year. Even with no additional inputs from fossil fuel burning, atmospheric CO2 would fall only by about 1 ppm per year, requiring over 100 years to return to pre-1900 levels, and most likely much longer.
Enhanced rock weatheringWe may need to do more than just drastically cut our fossil fuel use to avoid catastrophic warming. We need to find ecologically and economically viable ways to reabsorb the CO2 released by human activity since the beginning of the Industrial Age, using so-called negative emission technologies.
And these technologies would have to work on such a scale that carbon removal exceeded fossil fuel release — well above 10 billion tonnes per year. Is this even possible?
One process, called enhanced rock weathering, has recently received a lot of attention and research. By spreading tons of finely ground silicate rocks over farm fields and forests, we can speed up the natural weathering process, which is known to remove and capture CO2. When CO2 is dissolved in rainwater, the water becomes acidic. In contact with alkaline silicate minerals in rocks such as basalt, the water is neutralized, changing the CO2 into a dissolved form called bicarbonate.
The bicarbonate is either washed into lakes and oceans where it is converted into very stable calcium compounds, or it forms stable calcium carbonates in deep soils. Either way, the carbon dioxide will not return to the atmosphere for hundreds, if not thousands, of years.
Enhanced rock weathering is essentially the same process as a long-standing practice in organic agriculture, namely “remineralizing” soils with applied rock dust to improve plant health and the nutritional value of crops. Rock dust was found to increase yields of sugar cane in Mauritius and has become part of agricultural policy in Brazil to improve heavily weathered acidic tropical soils.
Different rock dusts, applied in different amounts to different kinds of soils, have different effects. Olivine, often used in enhanced rock weathering studies, is a very basic igneous rock and has high neutralization potential, but it releases toxic amounts of nickel and chromium. So the choice and source of rock dust matters.
Not all farms and forests will be near sources of basalt. Locally, we have several sources of basalt rock dust, left over from mining and crushing trap rock for construction. The dark-colored Holyoke basalt has iron, magnesium and calcium oxides (roughly 12%, 6% and 9%, respectively) and very low nickel and chromium, so it’s ideal for enhanced weathering. Applications of this rock dust are also likely to neutralize the soil, raising the pH. On the Holyoke Range, calcium-loving plants can be found on basalt-derived soils of the Metacomet Range.
How well might enhanced rock weathering work? A study at the University of Sheffield in England concluded that if we amend about a billion hectares (2.5 billion acres) of the most productive farmland over the earth with 10-30 tonnes of basalt powders per hectare per year we could be removing up to 2 billion tonnes of carbon per year by the year 2050. Another analysis concluded that basalt applications of 10 to 50 tonnes per hectare per year to the 70 million hectares of the annual crops of corn and soybeans in the North American corn belt could sequester up to 1.1 billion tonnes of carbon, equal to 13% of the global annual agricultural emissions of CO2.
However, a more complex and comprehensive study in Austria, which included the climate impacts of mining and transportation, recently concluded that application of 100 tonnes of basalt dust per hectare per year over 1.8 million hectares could draw down only about 2% of the country’s annual greenhouse gas emissions. The huge energy demand related to the grinding of this amount of rock, based on an application cycle of 10 years, would require up to 5 % of Austria’s total annual power generation.
To date, all the studies on this subject have mainly been on small plots or in laboratories, followed by massive extrapolation. Numbers do not all add up. Even so, I find it hopeful that an agricultural process that may improve soil health and productivity — something sorely needed — may also help reduce atmospheric carbon and reduce the impacts of climate change.
We are stuck in a colossal dilemma of our own making where we may find ourselves choosing between the impacts of technologies we use to survive, and the very survival of whole ecosystems. This technique may prove to be a useful survival tool in the future.
Lawrence J. Winship is emeritus professor of Botany at Hampshire College and a former board member of the Hitchcock Center for the Environment.