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Enhanced Rock Weathering is a fascinating subject with lots of different dimensions to dig into. Here’s a solid sample of resources to help you get started on your Carbon Gardener learning journey.
Enhanced Rock Weathering is a fascinating subject with lots of different dimensions to dig into. Here’s a solid sample of resources to help you get started on your Carbon Gardener learning journey.
BBC, May 12, 2023.
Energy Monitor, March 27, 2023.
The Guardian, July 8, 2020.
Frontiers in Climate, December 2022.
Nature Plants, March 2018.
Some excellent backgrounders on ERW on Youtube.
Science in Motion: Can enhanced weathering help slow climate change?
Carbon Gardener co-founder Dr. Garret Boudinot’s Introduction to ERW (aka “Rock Dust”)
Carbon Dioxide Removal: Enhanced Weathering As a Billion Ton Solution?
Enhanced Weathering in the Global South with Shantanu Agarwal, CEO, Mati.
Rock weathering happens naturally when rocks are broken down through chemical, physical, and biological processes. For the CDR pathway explored in this project, we will focus on chemical rock weathering, a process through which rock is broken down, exposing reactive minerals to dissolved CO2. Methods to accelerate this process, referred to as Enhanced Rock Weathering (ERW)1, include grinding up the rock to make “rock dust.” This increases the amount of exposed surface area, and consequently, can speed up the reaction rate.
Many parameters affect weathering rates including rock type, temperature, precipitation, particle size, and mixing depth. Natural rock weathering is estimated to remove on the order of a gigaton (billion tons) of carbon per year. 2,3 Studies project that ERW could remove 2-4 billion tons of carbon by midcentury and could possibly scale to the order of 20 gigatons by 2100.4 One study found that enhanced weathering could lower atmospheric CO2 by 30–300 ppm by 2100.5
The speed at which rock dust can remove carbon varies widely depending on factors including size of particles, climate, and dissolution time of a given mineral. For example, a 1mm sphere of calcite can dissolve in months while a 1mm sphere of quartz would take on the order of tens of millions of years.6
ERW is considered a relatively durable carbon removal pathway as it can store carbon either as carbonate minerals in soils, which are stable for thousands to millions of years, or bicarbonate in soil waters, which flows through groundwater and rivers into the oceans, where it is stable for ~10,000-100,000 years.7 The exact fate of carbonate and bicarbonate over long timescales, as soils change and water flows, is still an active area of study.
MRV methods are still under development, but see here for an example of a verification framework. The purpose of this project is to help accelerate the development of a useful MRV approach to measure the rate and amount of carbon removal via ERW.
The most reactive rocks for ERW include basalts, periodites, or serpentinites. These particular rocks contain alkaline cations (for example, Magnesium or Calcium) that react with CO2 in acidic aqueous solutions to form carbonates.8 ERW could also work well with certain alkaline industrial waste products such as mine tailings.
While more research is needed, evidence indicates that ERW could improve soil health and increase agricultural productivity.9 Given that ERW increases soil pH, it could serve as a potential substitute for lime – a source of significant agricultural emissions.10 Over time, the alkaline runoff from soil waters to the ocean at scale could provide a co-benefit of reducing ocean acidification.11
Potential risks and harms include emissions from grinding and transport, leaking of heavy metals into ecosystems and water sources from certain materials, increased mining, and adverse effects on soil ecology.12 Some of these risks can be mitigated easily by, for example, sourcing rocks from nearby-locations to reduce transport emissions, or using materials that have low or no trace metal contents. Other risks require more research to assess their probability, like adverse impacts on soil ecology such as increased respiration ((which releases more CO2), or harm to soil microbiology.
The footprint of ERW depends strongly on the energy sources used in grinding and transport – one study found that they could account for up to 94% of emissions from the ERW process.13 These can be reduced by using pre-ground material (i.e., biproducts of existing mining operations), deploying near material sources, and using low-energy methods for transport
Academic research on the efficacy of ERW for carbon removal is relatively nascent. Existing studies, from models, to potted plant studies, to field trials, show a wide range of impacts, which depend on the type and amount of rocks used, soil and climate conditions of the study, plants introduced to the soils, and measurement techniques (see for example table 1) . Overall, research indicates that ERW *could* be a useful method for CDR at scale, but much more research is required to ensure it is deployed safely and effectively.
Understanding the efficacy of ERW in different soil types is an active area of research and a major motivation behind this project. Overall, soils with lower pH (acidic) and more moisture will more rapidly dissolve rock dust and convert CO2 to bicarbonate. Temperature and plant type may also control the weathering rate and efficacy.14 One modeling study indicated that India, Brazil, South-East Asia, and China hold 75% of the global ERW potential.15
Rock dust is estimated to cost $24-578 per ton of CO2 removed.
