As I have said last week, the National Academy of Sciences published 2 reports on Climate Intervention (more or less synonymous with geoengineering).
The technical brief (4 pages long) is almost useless because it only gives qualitative statements.
There are two reports, addressing:
- CO₂ removal/sequestration
- Albedo modifications (making the atmosphere reflect more of the incoming sunlight)
The point of view regarding climate change is that
1 clearly it is happening and
2 it is going to suck.
3 however most studies so far are about limiting emissions and other impacts from human activities
And they want to explore trying to counter those effects rather than trying to avoid those effects.
For those wanting to read those reports: massive amounts of repetitions, cover-my-back verbiage (‘governance is not a synonym for regulation’ got a chuckle out of me).
Below the cut is a shortish (~2400 words) summary/explanation of what is in the first report. If you have questions or notice a glaring mistake, please feel free to use my Ask box.
(also below the cut, a few gratuitous pictures of minerals)
CO₂ removal and sequestration
The introduction is a short summary of the recent studies on climate change‘s observable effects and predicted trends. Reaching the necessary low levels of CO₂ production does not appear to be workable so far, with slow implementations of policies. It would also probably take decades to develop the technical solutions to limit the amount of carbon released by energy production and usage (what they call 'Decarbonizing the energy system’).
Adapting to climate change is sometimes presented as something that will occur gradually in the future, and could be planned to alleviate the costs. They doubt it could be done easily (especially because of the relocation of population issues, and the large, very expensive to reproduce infrastructures). Second problem with adapting: food production. There will be a dual loss of arable land: from sea-rise and from the migration towards the poles of the optimum growth regions. Then there are the changes for which we are ill-equipped. The biggest one is acidification of the oceans, which will have large economical impacts. This is not looking good.
Now for the meat of the first report: CO₂ removal. Recently, human emissions are ~35-40 billions of tons of CO₂ per year, 16 of which staying in the atmosphere. The rest dissolves in the oceans or is trapped in terrestrial biosphere (plants). An idea would then be to increase the transfer towards the oceans or to have a lot more trees on lands. The second sounds simple, but is hard in practice, for the same reason that higher CO₂ is not necessarily going to mean bountiful harvests: it is most often not the amount of CO₂ that is limiting plant growth, but other nutrients (iron, available nitrogen, iodine, potassium, phosphorus, …). Sorry Mr Dyson.
Now for some details on the proposed solutions.
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I Land use
a) Reforestation
Basically, the idea is to plant trees. That’s all (well not quite all, it requires fertilizers to be efficient). At best it would counter a tenth of the current excess emissions. It also requires not cultivating large fertile areas, which is going to be hard to sell. Right now, the urgent step is stopping deforestation, especially in the tropics.
On large time scales*, terrestrial ecosystems also store carbon (dead wood, coal, peat, organic carbon in soils). Some of that carbon is constantly returned to the atmosphere as CO₂ (often by biological activity), and how much may depend on the temperature. Not looking good.
*there are interesting things to explore in how the difference in time scales between geological processes and human activities cause us to misunderstand what is going to have large impacts and what is going to be absorbed with little to no consequences. For another day.
b) Sequestration on cultivated lands
Cultivation leads to a transfer of carbon to the atmosphere (less carbon in the ground, plus emissions of methane). It may be possible to reverse using 'cover crops’: plants that you grow and let rot in your field, and whose carbon is going to be added to the soil. The authors also suggest to stop plowing before planting. As for forest management, at best a tenth of human emissions.
Both present three disadvantages: reduction of the yield by unit surface, limited total pool of carbon trappable by those methods (after a couple of decades, no effect), and reversibility if one stops using those methods.
Conclusion for land uses changes: not going to the silver bullet, especially with demographic pressure, but could be part of a diversified approach. Cost between $1 and $100 per ton of CO₂.
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II Reacting carbon with minerals
The aim of this approach is to produce carbonate minerals (for example calcite, the main mineral in limestone, CaCO₃). One place where this is done right now is in the oceans, but dissolving CO₂ directly in the oceans change the acidity as said above. The reaction occurring when CO₂ dissolves in the oceans is:
(calcite)
H₂O + CO₂ = HCO₃¯ + H⁺
That produces 1 H⁺ ion, hence so change in acidity. So another way is to react CO₂ with minerals on land, to produce bicarbonate (HCO₃¯) ions with no H⁺. For example,
CO₂ + CaCO₃ + H₂O = Ca²⁺ + 2 HCO₃¯
And then export that to the oceans. That can also work with silicates minerals (here an example with wollastonite)
2CO₂ + CaSiO₃ + H₂O = Ca²⁺ + 2 HCO₃¯ + SiO₂
Small problem, those reactions are very slow, the time scales are thousand of years. They can be accelerated (high temperature, high pressure, addition of catalysts), but they also require, by weight, about 2 g of rock for each g of CO₂ removed that way. That would mean mining tens of billions of tons of rock every year (the total production of coal worldwide is 8 billions of tons/year). The logistical problems make that a very difficult avenue to follow, before even considering the technical details of how to make those reaction work efficiently. Oh, and the environmental problems associated with large mining operations and localized injection of concentrated solutions to the oceans.
Another strategy to avoid having to mine is to inject the CO₂ in a place where there are silicate minerals (preferably hot and damp) and they will hopefully bond forever as carbonate minerals. I know of a few laboratories where this is studied, often with olivine.
(olivine)
Example of a relevant reaction:
2 CO₂ + Mg₂SiO₄ = 2 MgCO₃ + SiO₂
We know it occurs naturally in a few select places. The problem is finding out how to make it happen fast enough, cheaply enough, and efficiently enough.
For the authors of the report, it is clear that further investment in the study of those reactions (especially their kinetics — how fast they are and how to change that) is necessary. They also suggest incentives for testing and modeling those approaches and their consequences.
Conclusion: more work needed. Maximum rate of CO₂ trapping expected with today’s knowledge: 1 to 4 billions of tons of CO₂ per year depending on the method. Not that great. Costs between $50 and $1000 per ton of CO₂.
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III Ocean fertilization
From the title I though they were not going to talk about this, but here it is. Right now the oceans act as a CO₂ pump: the plants use the CO₂ at the surface to grow, when they die some of that carbon is stored deep into the oceans or even ends up as carbonate minerals at the bottom.
Idea: boost this! Add needed nutrients (most often nitrogen and phosphorous), ensure optimal use of the nutrients, possibly enhance sinking for more efficient trapping of carbon.
The best solution seems to be adding small amounts of iron (rather than directly N or P). Only small amounts are needed, and it can increase the use of other nutrients (there are places with abundant P and N but low productivity because of low iron concentrations). There have been some small-scale experiments, there is an increase in productivity at the surface but not necessarily field evidence of carbon trapping deeper in the ocean, which is what we want to achieve (to be fair, it is hard to observe).
Estimates of the cost and efficiency therefore depend on modeling, with a big question mark on how much matter sinks when there is iron fertilization. The result is modest, between 1 and 4 billions of tons per year, current estimates of cost $450 per ton of CO₂.
Additional costs:
- the effects on nutrients supplies may have large consequences higher in the food chains (fishes, birds, sea mammals, and the people feeding on them).
- changes in ocean geochemistry if more organic carbon sinks, the deep ocean are going to be oxygen-poor. Possible increased amounts of N₂O and CH₄ released (other greenhouse gases).
Conclusion from the report:
More studies needed, on every single point. At this stage, don’t do it, too risky.
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IV Capture from air
a) Mediated by plants
The concept of bioenergy is simple: grow plants, convert the biomass in something usable. Something usable might be heat (burning the plants) or fuels (ethanol for example). As all the CO₂ produced comes from the air, this form of energy is (theoretically) carbon-neutral. The suggestion is just to capture the CO₂ produced on top of that.
Theoretically, again, bioenergy could cover a large portion of humanity’s energy needs. The small print is that everyone will eat mostly grains as most of the arable surfaces will be needed for energy production (and even so the competition between food and energy is going to be fierce).
(pet peeve: the authors talk about the surface devoted to pasture as if one could just convert them to growing arbitrary plants. That would be great, but no)
Ignoring the problems linked to the production those plants which are not going in anyone’s belly (but still necessitate water and fertilizers), there is little reason to invest in the carbon capture part of those solutions as long as fossil fuels are widely used (CO₂ can also be captured from the burning of fossil fuels).
The costs are $60-$250 per ton of CO₂, but the resulting sinks are larger than the previous solutions: 3 to 10 billions of tons of CO₂ per year. Provided you can get the necessary arable land.
b) Direct capture
The aim is to produce concentrated CO₂, most commonly by absorbing this gas with chemicals (for example amines) that can release it and be reused. The released CO₂ can then be used or stored.
The problem is that CO₂ is not that concentrated in air, so the energy cost to separate it is high, the surfaces needed are large compared to the solutions needed to capture gas from a thermal power plant. As a result, the costs range from $60 to $1000 per ton of CO₂, not including compression of the gas and sequestration. However, this solution presents a great flexibility, because it can capture CO₂ emitted by distributed sources (not only localized power plants). An upper bound for the sink is 13 billions of tons of CO₂ per year.
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V Geological sequestration
In both of those solutions, the produced CO₂ needs to be stored (sequestrated) to prevent its return to the atmosphere. Right now oil companies are pumping CO₂ into oil reservoirs to improve the recovery of hydrocarbons. Another candidates are deep saline aquifers. The estimates on the total amount of CO₂ that could be stored vary widely, and due to practical limitations, a good estimate for the available geological storage is a few thousands of billions of tons of CO₂.
There are a few small-scale projects, their rates of sequestration are too small to make a dent in the emissions. Although those projects have so far demonstrated that little to no CO₂, large changes in scale are necessary. Injection is also likely to trigger seismic events. The chance of leakage can in many places only be checked empirically, and the reliability of the storage at the scale of centuries is uncertain. There are costs associated with pumping the CO₂ as well as characterizing and monitoring the reservoirs, which could at worst double the cost of electricity production. By ton of CO₂, estimates range from $6 to $20 (which have to be added to the cost of capturing the CO₂, of course).
Pumping CO₂ directly deep into the oceans is considered unpractical at the moment. The main concerns are environmental impacts and reversibility. If pumped deep enough, it could lead to the formation of stable CO₂ lakes at the bottom of the oceans due to the pressure and temperature conditions.
It is quite obvious that as the only solutions providing large enough sinks to matter only capture the CO₂ but do not dispose of it, geological sequestration is going to be key. The projects need to be expanded, research needs to be done on how to find the good reservoirs.
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VI The human factor
a) Legal and ethical considerations
Those are mainly regarding the use of the oceans for CO₂ capture. Current laws prohibit the dumping of wastes in the oceans. There has already been one unilateral experiment in the Northern Pacific, as well as NGOs blocking experiments. Not encouraging for the future.
b) Political, social and economic considerations
The food vs fuel dilemna is the bantha in the escape pod. However, the larger issue is who is going to pay for that, and especially how are different countries going to coordinate. The authors expect debates and dissensions similar to those about limiting fossil fuel burning.
The deployment of any of the solution at the necessary scale is also an enormous challenge economically and politically.
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Qualitative Summary
Only the land usage solutions are mature technologies (except bioenergy + CO₂ capture), but their impact will not be great even in the best conditions.
The rest is at best at the prototype stage.
At best, deployment will only be significant in a decade.
None of the solutions are cheap, and none can be expected to compensate more than 30% of the current emissions in the near future.
Most of those solutions will have large environmental impacts, possibly social impacts too.
There is little risk of deployment from unilateral actors, mainly because of the large amount of resources needed (well at least there is 1 positive point!).