Should we use geoengineering techniques?

Engineering the climate is clearly a risky business. Some geoengineering methods such as, putting reflective particles in the stratosphere or ‘mirrors’ outside the atmosphere, aim to reduce the incoming solar radiation to cool the planet. These methods will reduce the warming but temperature changes are not the only aspect which drives climate change (Hegerl and Solomon, 2009). For example, the 1991 eruption of Mount Pinatubo led to increases in the incidences of drought (Hegerl and Solomon, 2009). Volcanic eruptions can cause a decrease in rainfall because with less incoming shortwave radiation, the surface is cooler, and therefore there is less energy for evaporation (Hegerl and Solomon, 2009). Therefore methods which use temperature as the sole proxy for its effects are inappropriate and produce too many risks (Hegerl and Solomon, 2009).

The previous blog posts have shown some promising ideas, however all of them have risks attached. I think afforestation and a combination of CCS and enhanced carbonation of rocks should be seriously considered as methods to start mitigating climate change. Some of the issues with these are economic. For example, the cost of installing CCS will be expensive and realistically may not be implemented in developing countries. There are also the political implications of geoengineering methods. For example, the storage of the carbon would occur in particular countries which could cause an over reliance on other countries. For the geoengineering solutions to be implemented in the first place there would need to be a solid international community agreement which is unlikely to happen. Another issue is the spatial capacity to implement geoengineering solutions. For example, afforestation requires space in fertile areas which are mostly required for agriculture.

None of these options are likely to work in the long term. The only solution to mitigate global warming is to reduce our CO₂ emissions. For this to succeed local governments need to work with national governments on small scale plans which will produce small changes but right across a country over time. Otherwise geoengineering may be focusing on how to keep humanity alive rather than how to mitigate climate change. Vincent Callebaut has been generating plans for future living. The example shown below shows the Lilypad. It is a self-sufficient floating island and would be able to house 50,000 residents. The increased flooding caused by global warming would make these structures ideal but they would also be exclusively for the very rich. 

Hegerl, G. C., S. Solomon (2009) 'Risks of Climate Engineering' Science, 325, pp.955-956

DOI: 10.1126/science.1178530

www.vincent-callebaut.org

Uptake of carbon dioxide by rocks!

In 2008, Kelemen and Matter concluded that the rate of natural carbonation of peridotite, a type of rock commonly found in the Earth’s mantle, can be enhanced to produce a significant CO₂ sink. They found that >1 billion tons of CO₂ per year could be consumed by the rock reacting with the CO₂ in the atmosphere to form solid carbonate minerals.

Mantle peridotite is usually found 6km below the seafloor or 40km below the land surface (Kelemen and Matter, 2008). However, sometimes the rock can be exposed when tectonic plates collide and push the rock to the surface. The exposed mantle is called ophiolite. It is composed mainly of the minerals olivine and pyroxene which react with water and CO₂ to make hydrous silicates, iron oxide, and carbonates (Kelemen and Matter, 2008).

Kelemen and Matter’s research was on the Samail ophiolite in Oman. They observed that the peridotite was crisscrossed by carbonate veins. These veins were previously believed to have been the same age as the rock. Kelemen and Matter (2008) found that the veins had a 14C age of approximately 26,000 years. They used the ages along with the volume of the carbonate veins to calculate the rate of CO₂ uptake. Therefore they estimated that the Samail ophiolite naturally absorbs 10,000 to 100,000 tons of CO₂ per year.

This image shows the white carbonate veins weathering out in peridotite north of the village Batin, Oman with a pencil for scale (Kelemen and Matter, 2008)

If this rate can be increased by 100,000 times then there is a potential for around 4 billion tons of CO₂ per year to be absorbed through carbonation of the Samail ophiolite (Kelemen and Matter, 2008). In their paper, Kelemen and Matter (2008) explain the rate of carbonation can be increased 100,000 times by increasing the depth of the weathering horizon from 15m to 3km in the peridotite by drilling and hydraulic fracture. Another method for increasing the rate of carbonation is to increase the temperature of the peridotite and inject CO₂ rich fluids. This would jump start a chain reaction which would naturally produce heat after it was started and therefore needs minimal energy input.

I think this manipulation of a natural process could work very well in absorbing the CO₂ from the atmosphere. The study by Kelemen and Matter (2008) shows that a substantial amount of CO₂ could be absorbed from just one area. If other large ophiolites, such as the ones in Papua New Guinea and New Caledonia, were manipulated in the same way then a lot of atmospheric CO₂ could be stored. This method could also be used in connection with the CCS method mentioned in a previous post because the liquid CO₂ could be injected into the peridotite and used to jump start the chain reaction. As always caution is required because of the unknown effects of enhancing carbonation. For example, the formation of solids beneath the ground could cause earthquakes but these may not be felt on the surface. Also the ophiolites may have a limit to the extent of carbonation and therefore cannot substitute a reduction in CO₂ emissions.

Kelemen, P.B, and J. Matter (2008) ‘In situ carbonationof peridotite for CO₂ storage’ PNAS, 105, 45, pp.17295-17300

www.pnas.org/cgi/doi/10.1073/pnas.0805794105

doi: 10.1073/pnas.0805794105

Enhancing Stratospheric Aerosols

During large volcanic eruptions sulphur is ejected into the stratosphere. The resulting aerosols reflect some of the shortwave radiation which therefore cools the Earth’s surface. Unfortunately it is not only the planetary albedo that is affected by enhanced aerosol loading in the atmosphere. The chemical reactions and the radiative heating budget of the aerosol layer are also affected. For example, it has been observed that during volcanic eruptions that ozone (O₃) is depleted (Heckendorn et al, 2009).

It is thought that a process of artificially enhancing aerosols could potentially mitigate global warming (Heckendorn et al, 2009). Past computer models of globally injecting sulphate into the stratosphere will result in the equator being over cooled and the poles will be under cooled (Ban-Weiss and Calderia, 2010). There would also be less precipitation. Ban-Weiss and Calderia (2010) ran a global climate model with varying sulphate concentrations at different latitudes. They found that with more sulphate aerosol loading at the poles than the equator region, temperatures would be like those of a low CO₂ climate. However, effects on the water cycle were diminished with a more equal distribution of sulphate loading (Ban-Weiss and Calderia, 2010).

One problem with sulphate aerosol loading is that the warming from CO₂ is being balanced by the cooling from stratospheric aerosols. However the two forces have very different climate response times because stratospheric aerosols have a lifetime in the atmosphere of a few years whereas CO₂ can remain in the atmosphere for centuries to millennia (Goes et al, 2011). Therefore continuous loading of the sulphate into the atmosphere needs to take place. If the aerosol loading is not maintained (for example, in the case of war or discovery of negative effects of aerosol forcing) there would be rapid warming which society would struggle to cope with (Goes et al, 2011).

Other issues with enhanced aerosol loading are the resulting polar ozone depletion which would damage ecosystems and potentially human health (Goes et al, 2011). Also aerosol geoengineering does not stop ocean acidification. Ocean acidification is the reaction of CO₂ with sea water and can negatively impact coral reefs and the populations that depend on coral (Goes et al, 2011). The aerosol loading into the stratosphere does not affect the concentration of CO₂ in the atmosphere and therefore does not reduce ocean acidification. The variations in the concentrations of stratospheric aerosol affects the properties of El Niño, precipitation and temperature, and the Asian and African summer monsoon (Goes et al, 2011). 

Enhancing stratospheric aerosols is considered a viable option because of its cost effectiveness (Goes et al, 2011). However, I think this option is extremely risky both economically and environmentally. It cannot be considered as a main method for mitigating global warming.

Heckendorn, P., D. Weisenstein, S. Fueglistaler, B. P. Luo, E. Rozanov, M. Schraner, L. W. Thomason, and T. Peter (2009) ‘The impact of geoengineering aerosols on stratospheric temperature and ozone’ Environmental Research Letters, 4, pp.1-12

doi:10.1088/1748-9326/4/4/045108

Ban-Weiss, G., K. Calderia (2010) ‘Geoengineering as an optimization problem’ Environmental Research Letters, 5, 3, pp.1-9

doi:10.1088/1748-9326/5/3/034009

Goes, M., N. Tuana, K. Keller (2011) ‘The economics (or lack thereof) of aerosol geoengineering’ Climatic Change, online first

DOI: 10.1007/s10584-010-9961-z

Carbon Capture and Storage

Carbon Capture and Storage (CCS) is another possible technique for mitigating climate change. By 2015, 20 commercial sized plants will start trials of CCS to see whether this is a viable method (Haszeldine and Scott, 2011). By capturing the CO₂ produced during combustion and storing it in a suitable place, there is the opportunity to reduce CO₂ emissions by 20% (Haszeldine, 2009). Currently 3 megatons of CO₂ are being captured and stored. There are three methods to capture CO₂.

Post-Combustion

During post-combustion the CO₂ is captured from the exhaust gas. The exhaust gas is bubbled through chemicals which selectively react with CO_2. Heating the solvent releases the concentrated CO₂ (Haszeldine, 2009).

Disadvantages:

•          Equipment is large and therefore larger power stations will be required.
•         Large volumes of the solvent will be needed and can produce toxic by-products.
•          Toxic by-products need to be dealt with appropriately.

Advantages:

•          It can be applied to already constructed plants.

Pre-Combustion

Pre-combustion involves removing the CO₂ before combustion commences. To remove the CO₂ at this stage the fossil fuel is converted to syngas (carbon monoxide and hydrogen), which is then reacted with water vapour to form a mixture of CO₂ and hydrogen. The hydrogen is then burned (Haszeldine, 2009).

Disadvantages:

•          High construction costs.
•          Decreased short term flexibility.

Advantages:

•          Multiple fuels can be used, with multiple products produced.
•          Efficient technology.

Oxyfuel Combustion

This is when the fuel is burnt in pure oxygen rather than air which results in CO₂ and water. However, production of pure oxygen is a costly and energy-intensive process (Haszeldine, 2009).

Disadvantages:

•          High energy cost of producing O_2.
•         Materials that can withstand higher temperatures would be required.

Advantages:

•          Easier separation of CO₂.
•          No solvent.
•          Potential to convert existing plants.
•         Smaller physical size.

After the CO₂ has been captured it is pressurized so that it forms a liquid. It is then stored 800m below the surface within rock pores (Haszeldine, 2009).  Good storage sites will be able to store CO₂ without seepage for thousands of years (Haszeldine, 2009). However, monitoring of the sites will have to take place to ensure that there are no adverse impacts. The step which consumes the most energy is the capturing of CO₂ but as CCS becomes more widely used improvements to the efficiency of the capturing processes will surely happen.

It is extremely important that we understand as much as possible about CCS because of the lack of action taken to reduce our CO₂ emissions. The world is dependent on fossil fuels for energy and it is unrealistic to believe that we can rapidly reduce this dependency. Therefore CCS can be a viable method of assisting a transition to a carbon neutral world. 

Haszeldine, S (2009) 'Carbon Capture and Storage: How Green Can Black Be?' Science, 325, 5948, pp.1647-1652

DOI: 10.1126/science.1172246

Haszeldine, S and V. Scott (2011) 'Carbon capture: An unprecedented challenge' New Scientist, 2806

Iron Fertilization during the Last Glacial Maximum

It has been thought that changes in the biological pump, such as iron fertilization, caused atmospheric carbon dioxide drawdown of 80-100 parts per million during the Last Glacial Maximum (LGM) (Kohfeld et al, 2005). Kohfeld et al (2005) analysed sediment records to see whether there is link between atmospheric dust deposition and CO₂ concentrations. Atmospheric dust deposition in the oceans increases the nutrient content of the oceans, and therefore more phytoplankton can grow. They expected to find increased CO₂ uptake by the oceans because of the increased dust deposition during the LGM.

The figure shown above shows the changes in CO₂ uptake for (A) stage 5a-d (80,000 to 110,000 years ago) minus Late Holocene (0-5000 years ago), (B) LGM (18,000 to 22,000 years ago) minus stage 5a-d, and (C) LGM minus Late Holcene. The dark and pale blue circles represent lower or slightly lower uptake of CO₂. The dark or pale red circles represent higher or slightly higher uptake of CO₂. White circles represent no change and grey circles represent ambiguous data. The size of the circle represents the confidence interval (Kohfeld et al, 2005). Their results showed that the CO₂ uptake by the Southern Ocean was lower during the LGM, particularly during a period of increased dust deposition (Kohfeld et al, 2005). Kohfeld et al (2005) concluded that iron fertilization from dust deposition could not be the only cause of CO₂ drawdown. 

Kohfeld, K. E., C. Le Quere, S. P. Harrison, and R. F. Anderson (2005) 'Role of Marine Biology in Glacial-Interglacial CO₂ Cycles' Science, 308, 5718, pp.74-78

DOI: 10.1126/science.1105375

Iron Fertilization

What is Ocean Iron Fertilization?

Ocean Iron Fertilization (OIF) is the intentional introduction of iron into ‘high nutrient, low chlorophyll’ (HNLC) oceans to induce phytoplankton blooms. John Martin developed a two part hypothesis (Strong et al, 2009).   

1.   HNLC regions of the oceans can be explained by limited iron availability and therefore nitrates and phosphates are not depleted because additional phytoplankton growth is limited by iron (Strong et al, 2009).
2. If iron does control productivity in HNLC regions and therefore the burial of organic carbon via the biological pump, it could explain the dust deposition and atmospheric CO₂ during the Last Glacial Maximum (Strong et al, 2009). 

Martin also mentioned how this hypothesis could be important for the potential of drawing down CO₂ from the atmosphere (Martin et al, 1990). 

OIF experiments

Since Martin’s initial hypothesis the last 20 years have seen a series of experiments which attempt to understand more about the process of iron fertilization. The first two experiments called IronExI and IronExII were done in the equatorial Pacific and spread several kilometres. The Southern Ocean was the next to be experimented on because this is where the most HNLC regions are found (Strong et al, 2009).

The experiments were designed to track the fate of carbon fixed in phytoplankton blooms in the surface layer because this is where the CO₂ will be absorbed from the atmosphere and transported to the deep ocean.

The European Iron Fertilization Experiment took place in 2004 and was the longest iron fertilization experiment to date. The aim of the experiment was to investigate the carbon export response and community shifts in the Southern Ocean iron-induced bloom. The experiment showed the highest ratio of carbon export to iron added (Strong et al, 2009). 

The experiments have improved understanding considerably. They have proved Martin’s original hypothesis and there is now more information on the initial phytoplankton response to iron enrichment. However, the results have proven to be inconclusive in regard to carbon sequestration (Strong et al, 2009). There are too many varying factors that affect the blooms in the long term.

Modelling Iron Fertilization

Zahariev et al. (2008) modelled the global elimination of iron limitation in oceans. They found that 0.9Gt C yr^-1 would be taken from the atmosphere. This is equivalent to 11% of global emissions in 2004. Models also show that in order to maintain carbon sequestration continued iron fertilization of the entire Southern Ocean would be needed with enough iron to deplete the ocean of macronutrients.

Martin, J.H., R.M. Gordon, and S.E. Fitzwater (1990) ‘Iron in Antarctic waters’,   Nature, 345, 6271, pp. 156–158

Strong, A.L., J. J. Cullen, and S. W. Chisholm (2009) ‘Ocean Fertilization: Science, Policy, and Commerce’ Oceanography, 22, 3, pp. 236-261

Zahariev, K., J.R. Christian, and K.L. Denman (2008) ‘Preindustrial, historical, and fertilization simulations using a global ocean carbon model with new parameterizations of iron limitation, calcification, and N2 fixation’ Progress in Oceanography, 77(1) pp.56–82

The Biological Pump

Before I discuss the next geoengineering technique of iron fertilization, I will discuss the basic understanding of the biological pump in oceans and the relation to climate. The diagram below shows how the biological pump works. 

1. CO₂ is dissolved into surface waters.
2. Phytoplankton take up carbon through photosynthesis (CO₂ + H₂O + light ->    CH₂O + O₂)
3. Some of this organic matter can then be decomposed back to CO₂ through being consumed by other species or through respiration.
4. The rest of the organic matter will sink into the deeper water where it can either settle on ocean floor or continue the cycle by releasing the CO_2. In some places upwelling can cause nutrients to return to the surface layer.

The video below also helps to explain this process.

As mentioned in the video, the ocean holds about 50 times as much carbon as does the atmosphere, and about 20 times as much as the terrestrial biosphere. Therefore the ocean is a very important area to study and focus on. 

De La Rocha, C. L. (2007) 'The Biological Pump' Treatise on Geochemistry, 6, pp. 1-29

doi:10.1016/B0-08-043751-6/06107-7

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B782S-4CJV6M2-2T&_rdoc=4&_hierId=3021000007&_refWorkId=107&_explode=3021000007&_fmt=high&_orig=na&_docanchor=&_idxType=TC&view=c&_ct=21&_acct=C000010182&_version=1&_urlVersion=0&_userid=7237960&md5=0ce17aa7bb6fa21bd14a7f9f0d476723