Enhanced Weathering: Time to mine?

In my previous blogs I have explored land and ocean based geoengineering methods. Therefore, next on the list is the method which affects both; Enhanced Weathering. This method proposes to remove CO₂ from the atmosphere and reduce ocean acidification by enhancing the natural chemical weathering process. In nature, carbon dioxide which is dissolved in rainwater (carbonic acid) is neutralized by rocks, which produces a bicarbonate solution as a result. If you are mathematically minded, this is described by the first equation:

(Dupre et al., 2003)

The second equation describes the process where the bicarbonate solution eventually enters the ocean and forms carbonate sediments. Overall, chemical weathering acts to remove carbon from the atmosphere and store it in ocean sediments. These are a significant carbon sink, storing 1,500 times the amount of CO₂ than the total amount found in the atmosphere, biomass and dissolved in the oceans.

The geological carbon cycle (Anders, 2016)

This process forms part of the carbon cycle which operates over geological timescales, and has regulated the climate of the Earth over billions of years. The impact of reduced weathering is commonly cited as one of several reasons for the Snowball Earth event around 635 to 720 million years ago, highlighting the importance of this process to the Earth’s climate.

The 'Snowball Earth' when there was a global freeze (BBC, 2010)

The Process

Clearly the rate of natural weathering cannot keep up with the huge amounts of CO₂ we have emitted. Therefore, the Royal Society (2009) list several different ways we could rapidly speed up this process:

1. Crushing rocks abundant in silicate minerals and mixing them with soils. Olivine weathers the fastest, but even so, one tonne of olivine spread over one hectare of soil in a temperate climate will last 30 years.
2. Reacting carbonate rocks with carbon at the source (e.g. power plants) and releasing the dissolved solution into the oceans. Carbonate rocks do not store as much carbon per molecule as silicate rocks, but they react a lot quicker.
3. Directly releasing ‘slaked’ lime into the oceans. The lime production process produces the equivalent amount of carbon dioxide being offset, so these emissions would have to be stored for any net effect.   

The rock types referred to above: Olivine (University of Arizona, 2013), Limestone (University of Auckland, 2005), Limestone process (bioLime, 2016)

These methods reduce ocean acidification because the products of weathering (bicarbonates, HCO₃^-) which are not taken up by ecosystems will enter the sea and reduce the acidity of the oceans. This is generally seen as a positive impact because it could compensate for the amount of ocean acidification arising from increased carbon dioxide in the oceans. On the flipside, Schuiling and Tickell (2010) highlight the potential damaging impact on environments if these products end up in other areas, such as rivers or peatlands where habitats are adapted to more acidic environments. Even without leakage to the wider environment the direct implications on the soil pH on vegetation is a large consideration.

Feasibility

The first method is commonly cited because it is the most cost-effective, with a price-tag around a tenth of the price of carbon capture and storage. However, huge amounts of silicate rock, equivalent to twice the amount of current global coal mining, would be needed to offset the yearly global emissions. This huge amount is required because it takes a long time for weathering to get going and start reducing carbon dioxide to the point where global temperatures slow or stop increasing.

The UK

Renforth (2012) investigated the feasibility of enhanced weathering in the UK. The paper looked at the amount of silicate rocks which could be mined, finding the total amount of available rocks have the potential to remove 430 gigatonnes (Gt) of CO₂, if all the sources were used. Putting this into perspective, the UK emitted approximately 0.75Gt of CO₂ in 2013, highlighting a huge source at our fingertips. However, Renforth (2012) note that this theoretical maximum would be lower in reality, because of associated CO₂ release from the transport of mined materials and the fact that not all the rocks could be mined. For instance, to avoid environmental health disasters, rocks containing asbestos, mercury or heavy metals cannot be used, although these are quite rare. Furthermore, costs for enhanced weathering in the UK could vary between £15 and £361 per tonne of CO₂, therefore some areas would not be economically viable.



Silicate rocks in the UK (Renforth, 2012)

Although the Renforth (2012) paper primarily focuses on extraction and availability, there is also the question of application. Application of olivine to beaches has been investigated and ruled out as on option by Hangx and Spiers (2009) because they suggest the scale required is not feasible. However, the best locations for applying crushed rocks to soils are dependent on the temperature, rainfall and exposure. This indicates the humid tropics would be the best regions for optimal weathering rates, although these regions may not be the most accessible. As a result, Schuiling and Tickell (2010) suggest a combination of approaches, including applications to regions such as British beaches should be used globally to reduce emissions.

Industrialising Nations

One of the benefits of this geoengineering method over the others I have investigated is the applicability to a range of countries. India, Brazil and China all have significant olivine resources, which can be mined. Therefore, Schuiling and Tickell (2010) highlight that these countries could meet their emissions targets by offsetting their current emissions and providing jobs, rather than cutting emissions which could hamper economic development. This is particularly pertinent to India, where over 300 million people cannot access electricity.

To summarise, I feel that enhanced weathering could be useful in the future. As with all geoengineering methods there needs to be sufficient assessments of the wider impact in regions where it is proposed. Nevertheless, the increasing amount of research and experiments may mean this could be implemented more easily than other geoengineering methods.