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). 

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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.

Geoengineering clouds: a hazy proposition? (Part 2)

Marine cloud brightening (MCB)

By increasing the reflectivity, or ‘brightness’, of marine stratocumulus clouds, the aim is to reduce incoming solar energy and prevent further warming. By enhancing the cloud condensation nuclei (CCN), the surface area of droplets is increased which results in bigger clouds to deflect solar energy (Caldeira et al, 2013).

Figure 1: Solar radiative potential of different cloud types (Source: Earth Gauge).

Essentially, MCB is similar to the sulphate aerosol technique in that an aerosol is sprayed into the atmosphere. CCN is enhanced by spraying a salty seawater mist into the marine atmosphere. Marine stratocumulus clouds are particularly proposed for MCB techniques as they are low-lying, thicker and more sensitive to aerosol additions – thus their albedo effect is more efficient than higher, thinner clouds (Latham et al, 2014). The video below explains the technique in more depth and scientific research on MCB.

Cloud albedo feedbacks strongly control atmospheric circulation and climate, as well as mediating radiation, water and heat transfer. For example, by mediating water and heat transfer, clouds in the tropics play a necessary role in tropical circulations. The primary tropical circulation is also known as the Inter Tropical Convergence Zone: a rain belt that shifts from the northern to the southern tropics (Bony et al, 2015). Furthermore, the liquid water content of clouds strongly regulates rainfall intensity during the Indian Monsoon (Maitra et al, 2014).

Climatic impacts

Many climatic modelling studies assume that enhancing cloud CCN will automatically increase albedo and thus cause cooling – however changes in cloud microphysical properties and how this will implicate albedo and cooling is difficult to to predict (Caldeira et al, 2013).

Some studies suggest radiative forcing, from a doubling of CCN, has the potential to offset the warming of a doubling of atmospheric CO₂ concentrations (approx. 440 ppm) as well as retain polar sea-ice extent (Latham et al, 2012). Enhancing cloud CCN would further influence the Earth’s hydrological system. Assuming a CCN enhancement of all marine clouds, global mean precipitation precipitation would decrease by 1.3%, however terrestrial runoff would increase by 7.5% (Bala et al, 2011). However, there is no certainty in regards to the spatial distribution of this runoff and its local implications.

With the frequency of droughts expected to increase in a warming world, extra rainfall may be required in arid regions. A marine brightening simulation has proposed a reduction in terrestrial dry spell frequency (Aswathy et al, 2015). Moreover, MCB can potentially reduce crop failure frequency in water stressed regions (Parkes et al, 2015).

In polar regions experiencing amplified global warming due to sea-ice albedo feedbacks, MCB techniques have the potential to reduce sea-ice loss and dampen feedback effects. This would be achieved by increasing sea-ice thickness and reducing sea-ice loss during summer months (Latham et al, 2012), by causing localised ocean surface cooling (Latham et al, 2014). Subsequently, this would help to reduce methane release from subsea permafrost in the East Siberian Arctic Shelf - a potent positive feedback for climate warming (Shakhova et al, 2015). Relative to other geoengineering methods, therefore, MCB techniques can be more efficiently targeted in polar regions (Parkes et al, 2012).

Ecological impacts

Although MCB techniques can slightly decrease net primary productivity in some light-limited oceans e.g. off the Peruvian coast, the regional effect of this is more pronounced (Partanen et al, 2016). This would result in a decrease in phytoplankton carbon uptake. Though this may seem minor relative to the radiative offsetting of MCB, phytoplankton play a large role in regulating the global carbon cycle (Litchman et al, 2015). 

Figure 2: Phytoplankton carbon capture via photosynthesis, using sunlight, across the sea-air interface (Source: Najjar, 2009).

Parallel to sulphate aerosols, there is a potential for salt damage to terrestrial ecosystems due to long-distance transport of marine salt aerosols. A recent study suggests that MCB would result in a decrease in gross primary productivity in tropical rainforests, with some models proposing a dieback of the Amazon rainforest (Muri et al, 2015). Thus, changing the balance of limited nutrients in ecosystems makes it difficult to predict future ecological impacts.

On the other hand, MCB techniques could potentially reduce coral bleaching episodes in the Great Barrier Reef, Caribbean and Polynesia. This would be due to a reduction in sea surface temperature (Latham et al, 2013). Currently, coral bleaching is a pressing concern as the worst destruction of coral was recorded this year (BBC News, 2016).

Figure 3: The use of MCB to prevent coral bleaching (Source: Anderson, 2013).

In conclusion, the MCB technique shares similarities with sulphate aerosol methods: in both climatic impacts and ecological concerns. The abundance of sea salt, coupled with potential global impacts would make it a relatively cheap, efficient and straightforward technique to implement. However, current models are contradictory and it is difficult to predict the spatial heterogeneity of impacts. Although MCB techniques could create localised ocean cooling, the ocean is essentially a 'global conveyor belt' that needs to be thought of in a holistic sense: as localised changes can have unpredictable global consequences. I feel more research needs to be undertaken to understand the implications of MCB techniques on all spatial scales, coupled with potential localised experiments, due to uncertainty with modelling.