|Fig. 1: A piece of olivine, the most common mineral proposed for enhanced weathering. (Source: Wikimedia Commons)|
The biggest contributor to the natural cycle of removing carbon dioxide from the atmosphere is the chemical weathering of certain types of rocks. Certain types of rocks, especially silicates (see Fig. 1), are particularly prone to absorbing carbon dioxide from the atmosphere. However, this rate of absorption is limited by the surface area of exposed silicates, and so is naturally quite slow. As rising carbon dioxide levels become an ever more pressing problem and researchers look for ways to remove carbon dioxide from the atmosphere to create negative emissions, speeding up this process of silicate weathering (referred to as enhanced weathering) has been one of the possible techniques explored. The concept is still in its infancy and has not yet been implemented in a large scale manner, but typical enhanced weathering schemes involve the distribution of large amounts of crushed silicate materials over areas of open land. Like many negative emissions techniques, enhanced weathering has both large amounts of potential and possible risks.
Enhanced weathering has the potential to remove large amounts of carbon dioxide from the atmosphere, especially if implemented in the correct climate. One group of researchers who analyzed the potential effects of spreading olivine over nearly one third of earth's tropical land could reduce atmospheric carbon dioxide concentration by between 30ppm and 300ppm by 2100.  This reduction would result from an application of between 1kg and 5kg of olivine per square meter of land area per year - a stupendously large total amount.  The proposed plan also had the added benefit of reducing ocean acidification.
As with many negative emissions plans, cost and practicality is a major drawback from the large-scale implementation of enhanced weathering. In order for the technique to be implemented, large amounts of heavy rock must be collected, crushed into a fine powder, and transported before it can be distributed. All of these factors combined make the scheme described above have an estimated cost of up to 600 trillion US dollars, giving it a steeper price tag than direct air capture.  Additionally, an enormous amount of rock would have to be used to make any significant impact.  To get a sense of the scales of rock mass involved, in the extreme case if you wanted to remove all the carbon from the atmosphere (ignoring the obvious limitations of the reaction as carbon levels reach zero) you would need to spread a layer of olivine 0.4cm thick over all of the land of the world. 
In addition to monetary concerns, enhanced weathering potentially also have detrimental effects on ecosystems. Enhanced weathering techniques tend to increase the pH of the soil in the areas in which they are implemented, sometimes quite significantly. This certainly put strain on the existing ecosystems in a way that is not fully understood yet. Some have suggested using this pH effect as a benefit by replacing the common agricultural practice of liming of soil with distribution of olivine.  It would be easy to achieve identical pH effect on the soil, and the substitution of olivine would have the added effect of removing carbon dioxide from the atmosphere. However, pH management using olivine would be slightly more expensive and so would be difficult to implement in an industry driven by profits.  Additionally, the scale would be too small to make a substantial dent in global carbon dioxide levels.
Overall, enhanced weathering is a technique that shows a fair amount of promise. Despite its costs and still unknown environmental risks, it could very well end up being a crucial part of a carbon stable future.
© William Marshall. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 L. L. Taylor et al., "Enhanced Weathering Strategies for Stabilizing Climate and Averting Ocean Acidification," Nat. Clim. Change 6, 402 (2016).
 R. D. Schulling and P. Krijgsman, "Enhanced Weathering: An Effective and Cheap Tool to Sequester CO2," Climate Change 74, 349 (2006).