What to do about global warming - 4: Sequestering Carbon Dioxide

In our previous installment we calculated the expected magnitude of CO2-induced warming over time, and showed that relying solely on controlling CO2 emissions cannot prevent exceeding the 2°C “tipping point” identified by the (IPCC, 2018).

As discussed in the first installment we accept the premise that human-generated CO2 causes unacceptable atmospheric warming. In the second installment we showed the life cycle of CO2 in the atmosphere and how it persists for centuries and millennia after emissions. In the third installment we showed how the emitted CO2 affects global temperatures and demonstrated that no amount of CO2 emission control will be sufficient to prevent reaching the 2°C “tipping point”.

In this article we examine proposed methods for actively reducing atmospheric concentrations of carbon dioxide, including planting more trees, engineered, active carbon dioxide sequestration, and the use of carbon credits. We analyze the effectiveness and relative costs as $/Gigatonne (Gt, or trillion kg) removed and the associated $/°C prevented.

How much sequestration is required? We first estimate how much sequestration is required. The simplest scenario (again, a bounding analysis) assumes that all CO2 emissions cease in 2020. Because halting all emissions is inadequate to prevent exceeding the tipping point, the question is how much additional sequestration is required to avoid exceeding the 2°C tipping point, and for how long? We can’t do anything about the past emissions, but we can make decisions about the future. The simplest scenario is depicted in Figure 1.

Figure 1. CO2 emission profile to avoid tipping point.

The profile of Figure 1 requires a net -8 Gt/yr emission for 190 years (until 2020+190=2210). This means that, in addition to ceasing all CO2 emissions, we must actively remove an additional 8 gigatonnes (trillion kg) each year for 190 years to avoid exceeding the 2K tipping point, as indicated in Figure 2.

Figure 2. Net CO2 (red line) and Net Temperature Change (blue line) to avoid exceeding 2K tipping point.

Figure 2 recalculates the net CO2 and temperature changes from the prior article with the modified CO2 emission profile of Figure 1. Other emission profiles are possible, but this one is simple: net -8 Gt/yr emissions for 190 years after 2020, followed by net zero emissions. This causes atmospheric CO2 to drop rapidly after 2020 until net emissions return to zero (pre-industrial emission levels). At year 2210, the CO2 emissions trajectory changes due to the net zero emissions, and the temperature experiences a slight increase before continuing its decay, based on the temperature vs. CO2 models of the prior article.

The dependencies of the result are that (a) net -8 Gt/yr emissions are needed to avoid the near-term exceedance of the tipping point at about year 2060; (b) the 190-year duration of this net sequestration is needed to avoid any longer-term exceedance of the tipping point (about t = 420 (year 2270) in Figure 2).

Figure 2 and its supporting calculations represent the minimum necessary changes to avoid exceeding the 2K tipping point. Again, we have only addressed the rate at which CO2 must be removed from the atmosphere; we have not yet addressed the problem of immediately halting carbon dioxide emissions. We now examine several methods for realizing these kinds of CO2 profiles.

“Plant a Tree for Your Tomorrow”. The most common method proposed for carbon dioxide sequestration is to allow nature to do it by increasing vegetation; in short, by planting trees.

Trees sequester about 2.5 ton/acre/yr, which equals 560 tonne of carbon dioxide per yr/km^2 (Urban Forestry Network, 2018). To sequester 8 Gt/yr we would need 14.3 million km^2 of trees growing, or about 1.5 times the area of the USA. These are “new trees” that need to be planted. Maximum tree growth and CO2 uptake occurs (depending on species) after about the tenth year. Since the urgency is immediate, the trees must be planted immediately!

If planting needs to be done twice to get to 190 years (assuming 100-year tree life), then, at a cost of US$80/acre (Kelly, 2019), this would have an estimated cost of US$564 billion. This works out to US$70/tonne of CO2 (and at 20 lbs. CO2 emissions/US gallon of gasoline (petrol) this equals US$0.64/gallon). Again, this assumes that all current CO2 emissions (about 40 Gt/yr) cease in 2020. If that does not occur, then the area of new trees scales proportionately to the amount of continuing emissions (up to an additional 5 times the required net reduction).

If this is effective in limiting temperature growth to 2°C instead of the 3.5°C calculated in the prior installment, this works out to US$380 billion/K in terms of temperature increase avoided, assuming the cost of the land is free, there are no ongoing maintenance costs besides one replanting, and all CO2 emissions are halted in 2020.

One $/tonne as a cost or tax works out to US$0.0091/gallon of gasoline consumed assuming 20 lb. CO2 are emitted per gallon of gasoline burned. We can use this scaling to assess the financial impact on the public of each sequestering scheme, since motor vehicle fuel taxes are directly and immediately borne by the traveling public, at least in the USA. However, we emphasize that the point of sequestering is to have a net removal of carbon dioxide from the atmosphere. Any further emissions, taxed at whatever rate, must also be removed at a cost associated with the technology used.

Money for tree planting may not be the key resource limitation. Finding suitable land (1.5x USA area) for planting productive trees would be the challenge. Growing new forests for carbon dioxide capture would likely compete directly with food production for land use, or would require significant engineering to turn something like the Sahara Desert (about 9 million km^2) into a forest by adding copious quantities of water. And this by itself would affect the global weather system, even if the countries involved would agree.

Carbon Credits

Carbon credits can be considered as a market place for offsetting carbon dioxide production with tree planting or other carbon-dioxide sequestration methods. In the current discussion this would need to be extended as a market for net reduction of carbon dioxide, not simply to offset current emissions. If the purchased credit is applied appropriately, then the cost calculated above for the effectiveness of trees (US$70/tonne of CO2) would set the appropriate market price for the credits.

The current price of US$3.30/tonne carbon dioxide for carbon credits (Energy Sage, 2019) indicates that the current market price should rise significantly to be able to directly fund the off-setting tree planting. Again, a major challenge will be finding suitable new land for tree planting that doesn’t displace food production and yields the tree growth and carbon capture assumed above. And the huge increase in demand to yield net negative carbon dioxide emission would likely cause the market price of such credits to increase significantly because of the higher demand and limited supply of potential tree-planting areas.

In any case, the price of carbon dioxide credits should be at what it costs to actually remove carbon dioxide from the environment because this creates a neutral incentive to either emit CO2 or pay the cost of removing emissions. The latest (IPCC, 2018) proposal to tax emissions at US$5500/ton equals US$6050/tonne or US$55/gallon of gasoline, well above the current market price of fuel and many times the cost of removing the emitted carbon dioxide. Any taxes or credits must be dedicated to and directly fund carbon dioxide removal in order to be effective. If they are treated as general taxes then the benefit of removing carbon dioxide is lost, and the goal of controlling atmospheric temperature below the tipping point is not achieved.

Engineered systems for carbon dioxide sequestration

Since planting trees to capture carbon dioxide requires so much land, we need to consider higher-density solutions that can remove and permanently capture the carbon dioxide.

Gas well injection

Existing technology for carbon capture and storage uses deep-well injection of carbon dioxide to sequester the gas (in liquid form) deep underground.

“It is stored in porous geological formations that are typically located several kilometres under the earth’s surface, with pressure and temperatures such that carbon dioxide will be in the liquid or ‘supercritical phase’. Suitable storage sites include former gas and oil fields, deep saline formations (porous rocks filled with very salty water), or depleting oil fields where the injected carbon dioxide may increases the amount of oil recovered. Depleted oil and gas reservoirs are more likely to be used for early projects as extensive information from geological and hydrodynamic assessments is already available. Deep saline aquifers represent the largest potential carbon dioxide storage capacity in the long term, but are currently less well understood.” (CCSA, 2019, p. Storage)

“Recent studies conclude that the first CCS [carbon capture & storage] projects in the power sector are likely to cost between €60 – 90 per tonne of carbon dioxide abated although these costs are expected to decline significantly reaching €35 – 50 in the early 2020s primarily as a result of cost reductions for carbon dioxide capture. (CCSA, 2019, p. Affordability)

Taking the midpoint of the lower cost range (€43/tonne) works out to US$47/tonne or US$0.43/gallon.

Conversion to Soda Ash

A more recent technology (Rathi, 2017) has demonstrated that it can capture and convert carbon dioxide from coal combustion into soda ash (a solid) at a cost of about $30/ton, or US$33/tonne. This cost works out to about US$0.30/gallon. The technology has focused solely on coal combustion effluents and there is no information about extracting carbon dioxide from the atmosphere.

Rock-On

A new technology, “CarbFix” (Reykjavik Energy, 2019) turns CO2 directly into subsurface rock:

“CarbFix is the industrial process to capture CO2 and other sour gases from emission sources and permanently store it as rock in the subsurface. The process can furthermore be applied in relation to direct capture of CO2 from air….the CarbFix team has demonstrated that over 95% of CO2 captured and injected was turned into rock in the subsurface in less than two years.”

The technology has been demonstrated at a scale of turning 60,000 tons CO2 into rock in two years. At a cost of $US 24.8/ton (Gunnarsson, 2018) this is equivalent to US$27.3/tonne.

Sequester Method and Cost Summary

Table 1 displays a summary of the costs for the various methods identified. The last two columns are the total costs ($/°C) assuming (c) all CO2 emissions are terminated in 2020 or (d) CO2 emissions continue at a constant rate of 40 Gt/yr for 190 years and are offset by the same sequestering method. Again, the issue of scaling is of critical importance since land for planting new trees is scarce, and none of the technologies has been demonstrated at the Gt/yr scale. The cost/gallon (b) is only included to normalize the costs at the consumer level. We emphasize that global CO2 sequestering must yield a net reduction in atmospheric carbon dioxide levels. Any new emissions, however taxed, must be offset by even more sequestering to achieve the net reduction of 8 Gt/yr.

Table 1. Summary of carbon dioxide sequestering methods and costs.

The Trees option is clearly the cheapest (assuming free land) but has high risk because of the huge amount of suitable and zero-cost land area required, which may not exist. The other engineered solutions have yet to demonstrate performance at scale. Each of these methods can be built in smaller, economically viable units, and distributed globally.

What is not stated for the engineered systems (among many other things) is the power demand and other utilities, e.g., water and waste water, to run these plants. So the net benefit may be lower. For all the engineered solutions the estimated costs are in the range of US$150-US$300 trillion over the course of 190 years (or of order US$1 trillion/yr).

Can we get there?

While the price points ($/tonne) seem achievable, the fundamental issue is one of scale in removing 8 Gt/yr for 190 years (or 48Gt/yr if carbon dioxide emissions are not terminated immediately). For all methods the infrastructure costs must be paid up-front and then amortized over the life-time. Funding mechanisms could include general taxes and user fees (taxes on energy use, for example). These would most affect the poorest for whom energy costs, like food, represent a larger proportion of their household budget compared with the affluent. And, since this is a global problem its solutions should also be distributed over all countries in order to be effective. Adding taxes or fees immediately for long-term benefit is seldom a politically palatable policy, and citizens of democracies and republics may balk at the costs if given a choice. So it’s not obvious that any of these solutions is feasible.

We emphasize again that any user taxes or fees on continuing carbon dioxide emissions can mask the requirement that net global emissions must be -8 Gt/yr to avoid the 2°C increase tipping point. Any continued emissions must be offset by additional sequestering of the emissions. It would not be sufficient for all emitters to "pay their tax" while there is no net sequestering. A risk of emphasizing carbon credits is that emitters will "feel good" about their purchases that offset their emissions while failing to actually solve the problem of too much CO2 in the atmosphere.

It is worth noting the incremental contribution to mitigating global warming from avoiding burning fossil fuels. Based on the calculations described in the prior installment, we have used 1.37°C/Tt (trillion tonnes CO2) as the scaling of temperature with carbon dioxide emissions. This means that avoiding 1 tonne (1000 kg) of carbon dioxide reduces warming by 1.37 x 10^(-12)°C/tonne, a little more than one-trillionth of a °C! So individual actions will have little effect on the state of the atmosphere in the future. A goal of controlling global warming will require more than an individual, a village, or even a country.

The large-scale impacts of planting millions of km^2 of trees may need their own global climate assessment, in addition to any required infrastructure to enable the trees to grow successfully in areas in which they aren't currently growing widely. An additional feature of planting more trees is that they may decrease earth’s average albedo (solar reflection coefficient) and absorb more solar energy than the deserts or grasslands they replace, leading to higher global warming. In the next installment we analyze the global heating balance as a prelude to examining additional methods of managing global warming.

Works Cited

CCSA. (2019, October 27). CCSA Affordability. Retrieved from Carbon Capture and Storage Association: https://www.ccsassociation.org/why-ccs/affordability/

CCSA. (2019, October 27). CCSA Storage. Retrieved from Carbon Capture & Storage Association: https://www.ccsassociation.org/what-is-ccs/storage/

Energy Sage. (2019, June 07). Costs and benefits of carbon offsets. Retrieved from https://www.energysage.com/other-clean-options/carbon-offsets/costs-and-benefits-carbon-offsets/

Gunnarsson, Ingvi et al., (2018, December). The rapid and cost-effective capture and subsurface mineral storage of carbon and sulfur at the CarbFix2 site. International Journal of Greenhouse Gas Control, 79, 117-126.

IPCC. (2018). "Global Warming of 1.5°C", Summary for Policymakers. Intergovernmental Panel on Climate Change (IPCC), 6 October 2018.

Kelly, David. (2019, October 24). How much would it cost to plant an acre forest? Retrieved from Quora: https://www.quora.com/How-much-would-it-cost-to-plant-an-acre-forest

Rathi, Akshat. (2017, January 8). Two Indian engineers have solved one of the biggest hurdles in the fight to make lower carbon-emissions targets a reality. Retrieved from Quartz India: https://qz.com/india/878674/two-indian-engineers-have-drastically-reduced-the-cost-of-capturing-carbon-dioxide-emissions/

Reykjavik Energy. (2019, October 27). What is CarbFix? Retrieved from Carbfix: https://www.carbfix.com/what-carbfix

Urban Forestry Network. (2018, August 17). Trees Improve Our Air Quality. Retrieved from Urban Forestry Network: https://urbanforestrynetwork.org/benefits/air%20quality.htm

(c) 2019. Ronald S. Carson. All rights reserved.