Enhanced rock weathering: a robust or crumbly foundation?

Many geoengineering techniques, such as sulphate aerosol spraying (SRM), are criticised for not addressing ocean acidification. I thought it would be befitting therefore to assess the case for enhanced rock weathering: a CDR technique that does combat ocean acidification whilst mitigating climate change (Taylor et al, 2015).

Rock weathering

Weathering is defined as the wearing away of rocks via contact with the Earth's atmosphere, e.g. rainwater, winds and extreme temperatures, and biological organisms. Rock weathering is a primary component of the carbon cycle (Hartmann et al, 2013) as the process consists of a chemical reaction which consumes approximately 0.1 Pg C per year of atmospheric CO2 (~1% of current anthropogenic CO2 emissions) (Caldeira et al, 2013). Although natural weathering processes cause a net removal of CO2 from the atmosphere, they can take up to 1,000-10,000 years. Humid, damp and warm climates provide optimum conditions for weathering.

Figure 1: The rock weathering cycle

The chemical reaction for weathering of minerals are as follows:

1) Carbonate minerals

CaCO3 + H2O + CO2 --> Ca2+ + 2HCO3-

(calcium carbonate + water + carbon dioxide --> calcium + bicarbonate)

2) Silicate minerals

CaSiO3 + 2CO2 + H2O --> Ca2+ + 2HCO3- + SiO2

(calcium silicate + carbon dioxide + water --> calcium + bicarbonate + silicon dioxide)

Enhanced rock weathering (ERW)

These reactions can be accelerated by mining, crushing and depositing silicate and carbonate minerals on terrestrial surfaces. This will increase their surface area, atmospheric exposure and thus reaction rate (Cressey, 2014). The goal is to consume and store carbon as a dissolved bicarbonate in the oceans, or produce solid carbonate minerals. Atmospheric CO2 can be sequestered over shorter decadal-centennial timescales (Taylor et al, 2015). Furthermore, the transportation of alkaline products, e.g. Ca2+ and 2HCO3, could offset ocean acidification by increasing pH (Caldeira et al, 2013).

Olivine, a magnesium silicate mineral, has been proposed for industrial mining and distribution. Its natural abundance and reactivity with atmospheric CO2 has made it an attractive prospect (Cressey, 2014). Countries with vast deposits include China, India, Brazil, Canada and Indonesia. Some argue that this will encourage international efforts to reduce CO2 emissions as developing countries will benefit economically from mining (Schuiling and Tickell).

It would need to be ground as a fine power to optimise carbon sequestration efficiency (Kohler et al, 2013). It has been estimated that finely ground olivine, distributed in the humid tropics, could potentially sequester 1 Pg C yr-1 (Kohler et al, 2010). In terms of concentration, atmospheric CO2 could be reduced by as much as 300ppm by 2100 (Taylor et al, 2015); 3/4 of our current CO2 atmospheric concentrations.

Figure 2: Olivine in rock and powdered form

Safer geoengineering?

The Leverhulme Centre for Climate Change Mitigation at the University of Sheffield presents ERW as a safer geoengineering technique as it uses natural chemical reactions that have stabilised the Earth’s climate for millennia. Furthermore, it is proposed as practical due to its scalability, as well as mitigating ocean acidification simultaneously with climate change.

Practical limitations

- Distributing crushed olivine/carbonate minerals in densely vegetated tropics, where the weathering would be optimal, is difficult.

- The process of mining, crushing and distributing may consume a lot of energy, which could outweigh the carbon sequestration efficiency of the minerals

- The process can restrict land use for agriculture, infrastructure etc (Taylor et al, 2015).

Unintended consequences

- Some studies have equated the effects of enhanced rock weathering on aquatic ecology with ocean fertilisation. Silicic acid, a byproduct of olivine dissolution in oceans, is for some diatoms a limiting nutrient (as silica is used in shell formation). Changes in water pH and chemical composition could cause shifts in phytoplankton communities and alter the biological carbon pump (Kohler et al, 2013). Some see this as positive and propose 'diatom farming' on an industrial scale to capture atmospheric CO2 (Schuiling and Tickell).

- Crushing rocks will produce dust which can cause respiratory problems and mortality (Cressey, 2014). An example is silicosis - a lung disease caused by exposure to siliceous dusts (Bang et al, 2015). Finer dusts have the potential to be transported long-distances, thus affecting more people and ecosystems.

- Olivine can contain heavy metals such as nickel (Cressey, 2014) that are highly toxic to humans due to their carcinogenic influence, and wildlife due to birth defects (Tchounwou et al, 2012). 

-     

The health and social implications of dust storms in the Middle East (Source: Euronews)

Conclusion

As the concept has gained traction relatively recently, I think more research needs to be conducted. Compared to other techniques, the chemical reactions that take place in ERW are not completely alien to us: so it may be deemed less risky. The possible detrimental health & ecosystem consequences, however, do cause concern. Coupled with practical limitations & rising populations requiring space and food, there are a lot of unanswered questions surrounding ERW.