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?

Ocean Fertilisation: an enhancement of ocean productivity

1. Ocean Fertilisation Function

This blog aims to assess a CDR geoengineering process, known as ocean fertilisation.  The ocean captures CO₂ naturally from the atmosphere in two ways.  The first way involves the ‘solubility pump’, where CO₂ is dissolved in seawater due to ocean circulation (Lampit et al 2008).  The second way is the ‘biological pump’, where the production of a phytoplankton plant absorbs CO₂ through photosynthesis (Rayfuse et al 2008).  This process will be the focus of this study as the solubility pump is unable to undergo artificial enhancement.

Martin discovered, that adding iron to seawater produces massive algal blooms, which increases CO₂ absorption by photosynthesis (Factor 2015).  George (Factor 2015) suggested that enhancement of the biological pump could absorb more CO₂ from the atmosphere, as it is an increasing climate change problem.  Hence, the artificial adding of iron in the ocean is known as ocean fertilisation.  Factor (2015) gives a historical context of the trials undertaken for ocean fertilisation, ranging from small laboratory scale processes to large-scale 10,000km² of ocean experimentation.

Once iron is added to the ocean, an algal bloom is formed, making the ocean a pea green colour.  This algal bloom absorbs CO₂ and sinks to the bottom of the sea, below the ocean circulations, where there is no interaction between CO₂ and the atmosphere (Lampit et al 2008).  It may take millions of years for this CO₂ to resurface (Figure 1). This is a relatively cheap process, Stinger suggests it would cost $2.50 per ton of carbon, whereas, Markerls and Barbier suggest $2 per ton of carbon (Factor 2015).  Hence it may be conceived as a relatively effective process.  However there are many negative side effects that may occur.

Figure 1: Ocean Fertilisation Representation
Source:  MotherJones

2. Side Effects

2.1 Anoxia and Eutrophication

As there is an increase in algal bloom, this creates an excess of iron macronutrients, hence causing eutrophication.  This causes the algal bloom to expand and grow and decreases the biological diversity of the fertilised oceans (Lampit et al 2008).  This is a great concern as eutrophication causes a lack of oxygen ventilation in the ocean and causes anoxia and mortality of marine species (Lampit et al 2008).

2.2 Ocean Acidification

Ocean pH was relatively stable at pH levels of around 8 to 8.3 prior to the industrial revolution.  However, since then the oceans have become more acidic due to the increased levels of CO₂ in the atmosphere (Lampit et al 2008).  An increase in algal bloom, will absorb more CO₂ and has the potential to make the oceans more acidic.  This occurs due to higher concentrations of CO₂ collected in the oceans.  This increases species mortality or forces populations to shift to less acidic oceans (Lampit et al 2008).

2.3 Global macronutrient Balance

Another concern is the increasing input of iron macronutrients to enhance CO₂ absorption.  This may redistribute other essential macronutrients in the ocean.  Hence, a lack of nutrients in the oceans may lead to fishery degradation, leading to economic concerns.  Additionally, the marine biological productivity of an area will degrade, as some areas may not have enough nutrients (Lampit et al 2008).

2.4 Modification of global iron balance

It is suggested that iron cannot be applied to surface waters for long periods of time and lasts in oceans for only a few months.  Hence, a, continuous addition of iron micronutrients is required (Lampit et al 2008).  Therefore, questioning the degree of effectiveness that ocean fertilisation will provide in reducing CO₂ emissions. 

2.5 Other climate-relevant gases

Moreover, global warming is not only caused by an increase in CO₂ levels.  There are various gases that have an impact on global warming, including methane, nitrous oxide and ozone (Lampit et al 2008).  Hence, if algal blooms absorb CO₂, they unfortunately have the potential to increase methane levels due to a decrease of oxygen ventilation in the oceans (Table 1).  Hence, this questions to what degree this will reduce the radiative forcing of climate change impacts on Earth.

Table 1: Gases and aerosols affecting the radiative force of the Earth and their potential changes with ocean fertilisation
Source: Lampit et al 2008

2.6 Decreasing economic and social sustenance

Changes in the ocean ecosystem can cause a decline in fisheries. Fisheries provide food and economic sustenance for approximately 1.3 billion people.  Hence having social and economic impacts (Lampit et al 2008).  Controversially, ocean fertilisation supporters will argue that an enrichment of CO₂ will help fisheries to increase yields.  This may occur as an increase in algal bloom may provide more food for fish species and hence increase their populations.  However, there is great uncertainty on what may happen to fisheries, as computational models have been relatively poor in predicting changes in the community (Lampit et al 2008).  Furthermore, indirect impacts on fisheries need to be assessed as there is a potential of fish communities shifting.

2.7 Benthic Biota

Lastly, it is suggested that only 4% of CO₂ absorbed by the oceans is able to sink to the deep ocean and stay there for millions of years.   For this process to be successful, benthic biota are essential (Lampit et al 2008).  Benthic biota, are the organisms that sink to the bottom of the ocean to form sediments. However, with ocean fertilisation, it is probable that benthic biota will decrease in abundance as it is related to the organic matter of the euphoric zone (Lampit et al 2008). Furthermore, most oceanic regions consist of medium to low benthic matter, hence questioning the degree of productivity that ocean fertilisation can provide without the availability of benthic matter (Lampit et al 2008).  Therefore, it is questionable to what degree ocean fertilization will be successful due to its reliance on benthic matter to sink CO₂ in the deep ocean without any environmental interactions.

3. Conclusions

I believe when assessing the side effects of oceanic fertilisation, it underlines many probable risks.  Ocean fertilization decreases social and economic benefits and can degrade marine ecosystems.  Furthermore its effectiveness is questionable.  Therefore, I believe ocean fertilisation should not be implemented as it will not be an effective geoegnineering process.  What do you think?

A Porous Liquid and Artificial Trees: Examples of CDC processes (Part 2)

This blog is a continuation of last week’s blog, hence for a greater understanding of CDC, please read Part 1.  

Today, I will refer to two recent CDC processes under research that may be effective and are close to being implemented.  Firstly, the porous liquid is a relatively new type of CDC, which creates many inquiries due to being such a new process.  Secondly, the artificial trees, is a process Lackner has been working on for a very long time, where solid matter is used to absorb CO₂.  Lackner’s proposal may be very close to being implemented in real life. 

The Porous Liquid

The porous liquid was bought to my attention when talking to a colleague (please feel free to visit his blog on extinction of species).  This porous liquid is the first liquid to have ‘holes’.  These ‘holes’ (micropores) are very small, invisible to the naked eye and can be used to capture CO₂ and also methane (another gas increasing global temperatures) (Zhang et al 2015).  Hence, it may be very beneficial, as the capture of CO₂ and methane will reduce global temperatures and decrease global air pollution (Lepisto 2015).  However, when reading through Zhang et al, there is not enough detail concerning how it may be used and to what scale.  A fundamental question that remained unanswered, is what happens once all the pores of the liquid are filled with CO₂.  Additionally, there are no suggestions where this liquid can be placed.  If this liquid is placed on lakes and dams, this questions if oxidation will be able to take place and if it will impact water species due to oxygen deprivation.

Nonetheless, provided that there are no negative environmental impacts and it is a relatively cheap process, this could also be a viable process, reducing climate change impacts and remaining in the desired 2^oC threshold (Luntz 2015).  However, further clarification and research is essential for this process to take place and be effective in the real world.

Artificial Trees

Another feasible process to capture CO₂ is through artificial trees.  These artificial trees are designed by Lackner, and are composed of a resin that binds CO₂ and forms a bicarbonate (Lackner 2009).  This resin is found in a ‘pale beige polypropylene plastic embedded with 25-micrometer particles’ (Figure 1) (Biello 2013).   This process can be used to create artificial trees that will have the ability to absorb more CO₂ than plants.  Hence, this may be an effective process in stabilising CO₂ levels to 400ppm, or it may even gradually reduce CO₂ levels below this threshold  (Biello 2013).  Once the CO₂ is captured in this resin it can be stored by being pumped deep underground or inputted in oil or gas to be re-used.

Figure 1a: Sample of Artificial tree
Source: Schiffman 2013

Figure 1b: Chamber Absorbing Carbon dioxide using Artificial tree sample
Source: Biello 2013

Problems Induced by Artificial Trees

 Although Lackner’s process is relatively cheap, there are three main problems with the artificial trees.  Firstly, this process accounts for 700kg of carbon capture in 24 hours (around 13 people breathing for 24 hours).  Hence, a large amount of these artificial trees will be required to reduce CO₂ emissions, needing large amounts of space and money.  Nonetheless, Lackner et al 2011 proposes a solution, by suggesting highly emitting industries should pay for these artificial trees.  Furthermore, the government needs to determine a viable process of paying for these artificial trees through either implementing a carbon tax or applying a carbon footprint process.

Additionally, it is suggested that high amounts of water are essential for the CO₂ capture to take place (Lackner 2009).  Hence, questioning if these high amounts of water are available.  With increasing population rates, water is essential for agricultural, domestic and sanitary use.  Thus, the artificial trees create an opportunity cost for water, divided between the everyday use of people and artificial trees.  Therefore, water use must be sustainable to ensure water security.  This may be achieved with some difficulty, by increasing technological advancement or recycling water.

Lastly, once the CO₂ is captured, it needs to be disposed of or re-used.  This process can be relatively expensive, hence it may be very difficult to achieve this.  The high economic costs may be significant and need large amounts of energy, which will not reduce CO₂ levels to the aspired amount (Lackner 2009).  Hence, it is questionable to what degree this process will be effective.

Nonetheless, provided that this process becomes cheaper and more energy efficient, it could be a highly successful process to reduce or stabilise CO₂ levels (although this may take a few decades) (Schiffman 2013).  There are minimal or no environmental risks when undertaking this process, suggesting a high level of safety for habitats and avoiding pollution of freshwater ecosystems.

Thoughts:

I believe these processes may be highly effective, provided they work efficiently and have no risks in regards to degrading the environment.  I believe the artificial trees method has a higher potential of being effective compared to the porous liquid.  This belief is determined as the porous liquid has a higher potential of being unsafe towards the environment and more research is essential.  Therefore, the artificial trees may become a new way of decreasing CO₂ levels.  I also think I would be more comfortable using this process compared to SRM processes, as there are less negative impacts.  Nonetheless the problem with artificial trees is the high water requirements, which needs further investigation.  Furthermore this process may be a disincentive for industries and governments to reduce their carbon emissions.  Hence, questioning the ethical and moral issues surrounding this topic.  What do you think? Do you believe CDC is a feasible process?