Enhanced Weathering: A solution to climate change?

This week’s blog assesses a CDR process known as Enhanced Weathering. The Earth’s atmosphere and surface is shaped by biogeochemical cycles, where natural chemical weathering takes place (Renforth et al 2015). Natural chemical weathering reacts with precipitation and the atmosphere and erodes the surface where the reaction takes place. Carbon dioxide is absorbed by the chemical reaction caused by the weathering of various rocks (Caldeira et al 2013).  Therefore, the idea of Enhanced Weathering is created by the ability of artificially accelerating the natural geochemical weathering of rocks to absorb large amounts of CO₂ (Renforth et al 2015).  If this is undertaken in large quantities, it decreases global CO₂ levels in the atmosphere and decreases global temperatures.  There are two basic minerals that interact with precipitation to absorb CO₂ through chemical weathering, silicate and carbonate (Hartmann et al 2013).  However, carbonate weathering may release CO₂ and can be less effective than silicate weathering.  Hence, silicate weathering is considered for the Enhanced Weathering geoengineering process.  Equation 1 illustrates the various chemical weathering interactions.

Equation 1: The dissolution of carbonate and silicate rocks by different solutions, representing Enhanced Weathering
Source: Hartmann et al 2013

Natural Chemical weathering is relatively slow and may take thousands of years to reduce CO₂; hence a process is needed to speed up CO₂ absorption (Renforth et al 2015).  There are 7 ways that artificial enhanced weathering can take place (Hartmann et al 2013):

1. Increasing the surface area of rock being processed (e.g. crushing/ grinding)
2. Changing the solution’s pH
3. Temperature increase
4. Pressure increase
5. Choosing appropriate rocks which dissolve faster
6. Taking advantage of certain biological species that speed up weathering
7. Changing the flow regime

Representation of Enhanced Weathering on Hill Slopes
Source: GeoEngineering Watch 

Effectiveness and Costs of Enhanced Weathering

Enhanced Weathering is best effective in humid, tropical regions, where the atmosphere is relatively warm and pressure is high due to humidity (Hartmann et al 2013). Furthermore, mafic and ultramafic rocks are the most effective for enhanced weathering to take place. Therefore, for enhanced weathering, the geology of the area needs to be highly efficient and high transport costs may be essential to move minerals to specific areas to increase the rate of reaction (Hartmann et al 2013).  Furthermore, high amounts of energy may be required for the crushing of rock to the desired particle size. Another way to increase the rate of reaction, is by spraying the pH solution, by aeroplane, in the weathering region; the disadvantage is the high costs (Hartmann et al 2013). Hence the Enhanced Weathering price may range from 88 to 2120 US$t^-1 of C (Hartmann et al 2013). 

Enhanced Weathering Side Effects

Enhanced weathering of silicate rocks may be beneficial for ecosystems as well as absorbing CO₂.  A problem that may be resolved is ocean acidification caused by high CO₂ levels.  The enhanced weathering process allows silicon to dissolve in riverine and coastal systems that can reach the oceans.  The silicon solution has an alkali nature and reduces or neutralises ocean acidification (Hartmann et al 2013).  However, the effectiveness is relative to the amount of total alkalinity spread across the ocean.  A small volume of total alkalinity spread across the ocean is less effective (Caldeira et al 2013).  Oceanic models need advancement to create clarity of the degree of effectiveness (Hartmann et al 2013).

The Enhanced Weathering process releases silicon, which is very beneficial for plant growth and health (Hartmann et al 2013).  Silicon distribution to plants increases water efficiency and promotes certain plants to be resilient to droughts.  Some plant species may increase their water use efficiency by up to 35% (Hartmann et al 2013).  Moreover, it may improve poor nutrient soil and help the ecosystem advance. The continuous cultivation of crops requires more silicon, due to soil deprivation from continuous use (Renforth et al 2015).  Hence, Enhanced Weathering can help crop growth and increase agricultural productivity. 

Additionally, silicates can combine with potassium to create fertilisers (Hartmann et al 2013).  This decreases the high costs of potassium fertilisers and accelerates plant growth and crop yields.  However, for this fertiliser to be effective a combination of the correct plant species, type of soil minerals and climatic conditions are essential (Hartmann et al 2013).  Thus being an economic benefit for farmers, as they will have cheaper fertilisers and increase crop yields (Hartmann et al 2013). However, a change in fertiliser industry may be disadvantageous for the phosphate and ammonia industry and they may endure economic loses (Hartmann et al 2013).

Moreover, when soil weathers (e.g. olivine soils) it may also release magnesium and iron.  These metals are important nutrients for plant growth.   Similarly, ultramafic rocks may release iron, manganese phosphate and potassium (Renforth et al 2015).  The release of these nutrients is beneficial for plant growth and health.  A store of these metals help the continuous supply of nutrients to ecosystems (Hartmann et al 2013).  Conversely, a change in nutrient balance can have negative impacts on ecosystems.  Large amounts of iron, nickel zinc or other metals may poison some plants and animals as they may be less tolerant to high concentrations of specific metals compared to other plant species and the ecosystem balance may change (Renforth et al 2015).  Nonetheless, the amount of nutrients released in a specific area is highly dependent on the type of rock weathering and impacts may vary across various regions.

Additionally, silica release that occurs during enhanced weathering can act as an ocean fertilisation method where, algal bloom is produced in oceans and absorbs CO₂ (Caldeira et al 2013). More detail about ocean fertilisation as a geoengineering function can be found in my previous blog.  Although ocean fertilisation is a useful process and may enhance CO₂ absorption, ocean fertilisation can have negative side effects on oceanic species, such as changing species biodiversity and forcing them to migrate, or increase mortality rates (Caldeira et al 2013).  Hence questioning if silica release would be truly beneficial if it enhances algal bloom in the ocean.  Controversially, an increase in dissolved silicon may increase the growth of diatoms to enhance algal blooms to sink in the deep ocean at faster rates, reducing ocean fertilisation negative impacts (Hartmann et al 2013).

A substantial amount of soil and rock will be weathered and large amounts of mining of mountains will be required for enhanced weathering to take place (Hartmann et al 2013).  This may destroy habitats and have a substantial degradation on ecosystems (Hartmann et al 2013). Additionally the mining and dissolution of rocks can create large dust plumes in the regions where the process is taking place, creating unattractive landscapes, visual hazards and health issues (Hartmann et al 2013).

Conclusion

Overall, Enhanced Weathering seems to have high potential, as it reduces CO₂, tackles ocean acidification and enhances plant growth and health.  However, Enhanced Weathering may also be problematic, as mining and too many minerals in the soil may degrade species health and shift species population.  I believe that Enhanced Weathering has a lot of potential.  However, more research is essential to assess to what degree Enhanced Weathering can have an impact on the environment.  What is your opinion on Enhanced Weathering?