As of late 2012 the concentration of CO2 in the atmosphere was 392-393 ppm.1 Current annual global CO2 emissions are estimated to be between 30 GtCO2 to 34 GtCO2. Estimates place the ability of natural sinks to absorb this additional annual CO2 at anywhere from 33% - 50%. Note that these numbers involve absorption rates not exchange rates, which are much larger. Unfortunately these absorption estimates have significant range of uncertainty where it more likely that they are smaller than larger. Also note that the concentration in the first sentence is only CO2 and not CO2 equivalency (CO2eq), thus it does not take into account methane and other greenhouse gases, which also have a warming effect on surface and ocean temperatures.
The general goal of the environmental movement is to limit the maximum global surface temperature increases to 2 degrees C. Most presume that such a goal will require limiting CO2 concentrations to a 350 ppm equilibrium point. Unfortunately as noted above the current atmospheric CO2 concentration is already significantly passed that goal target. More bad news is that even if the global community could eliminate all carbon emissions within a short time period there is little reason to suggest that the 350 ppm goal could be reached through the operation of natural sinks alone.3,4 Some suggest that 400 ppm is a suitable goal, but with the negative environmental consequences already being realized at 393 ppm there is appropriate concern that natural CO2 sources like permafrost and forest death are unavoidable at prolonged exposure to a 400 ppm environment. If these natural sources start releasing CO2 then a greater level of environmental consequence will be expected.
A further problem is that the influence of CO2 on environmental conditions is currently divided into fast and slow feedback elements.5 Most climate scientists believe that only the fast feedback element has exerted its influence over the last few decades of carbon emission to this point. Thus, the amount of climate forcing that has currently been applied to the environment has not been fully compensated for through change in average surface temperature. That is to say that if all human based CO2 emissions were ceased tomorrow, the average global temperature would still increase some unknown, but believed to be significant amount. In 2008 Hansen and associates believe that at current CO2 concentrations an additional 1.4 degrees C of warming will occur no matter how fast carbon mitigation occurs.5
The principle reason natural sinks will have trouble neutralizing the required amount of atmospheric carbon is that there is little reason to expect these areas to retain their existing sink capacities.3 Overall it is probably unrealistic to expect an increase in absorption capacity percentage for natural sinks and reasonable to expect a greater than even chance that capacity will decrease over time. Some argue that sinks have increased the amount of absorbed CO2 over the years, but while true that increase is derived from the increase in human emissions and the absorption increase as a change percentage has decreased.3,4 For example in year x humans were emitting 100 tons of CO2 and sinks were absorbing 50 tons and in year x + 10 humans were emitting 150 tons of CO2 and sinks were absorbing 65 tons, higher absolute number, but lower absorption percentage. Therefore, with capacity reductions occurring it is important for humans to execute strategies to increase existing sinks or develop new sinks to avoid further detrimental environmental consequences from global warming stemming from both future emissions and the slow feedback of current emissions.
The most popular means of carbon remediation is to plant new trees and other flora (afforestation or reforestation depending on the history of the planted land) throughout the world. The major advantages to such a strategy are:
- Planting trees is a known empirically proven means of reducing atmospheric CO2 that can be implemented immediately.
- The cost of such a strategy is extremely low relative to other potential CO2 removal strategies.
- New trees can develop new terrestrial biospheres that could aid the development other flora and fauna, which would have positive secondary environmental effects.
While planting new trees and other flora would definitely increase the rate of atmospheric CO2 removal there are some concerns.
- CO2 absorption through photosynthesis is a rather slow process and would demand numerous additional plantings, which would require time and resources. In fact the slow nature of absorption may heavily handicap the entire strategy relative to reducing the detrimental consequences of global warming because it would take too long for such a process to remove enough CO2.
- The additional plantings could be problematic in finding the land area in which to plant trees. Most deforestation, both in the past and the present, is the result of creating a more suitable environment for agricultural purposes (growing food or biofuel production). These competitive elements in land use create a conflict between the ability to continue to feed a growing population who desire significant choice in consumable food, not just grains and vegetables, filling a growing misplaced demand in food-based biofuels/biomass energy and a safe haven to a forestry environment for sufficient tree growth to play a role in further CO2 removal.
- While it has not yet become a trend, recent precipitation changes have created significant concern regarding the survival probability of young newly planted trees either due to a lack of water or too much water. Reliance on rain may create a capricious survival probability and developing irrigation systems increases costs and diverts already shrinking water resources.
- Finally there is question to how increasing forests will change surface albedo in that the shading provided by trees may decrease soil albedo below them. Overall this concern is probably minor.
Another popular remediation strategy is the generation of bio-energy with associated carbon capture and storage (BECCS or BECS) of the resultant emissions. Basically feedstock is either grown or forest/crop residue is collected after harvest and burned with the resultant CO2 emissions captured and later sequestered in the ground. The advantages of such a system are:
- BECS is a carbon negative means of generating energy, although the energy used to capture the resultant carbon significantly limits this advantage.
- BECS has a general lower cost structure than other more technical options and a perceived higher absorption to cost ratio than all other options aside from planting trees. Of course BECS is still expensive on an absolute level at this moment due to the carbon sequestering aspect so governments will need to either provide incentive for scale-up or undertake this strategy themselves.
- Empirically proven and is currently in use throughout the world, thus the issue of proof of concept and convincing the public that it is a viable strategy has already been carried out.
However, there are some concerns with the further development of BECS:
- Despite the incredibly high removal potential cited by BECS proponents one must remember that these numbers are derived from theoretical scale-up aspects. Overall the reliance of small feedstock residue reduces the total potential of CO2 that can be eliminated from the atmosphere. Increasing this amount will require either scaling-up residue collection, which will create a problem with soil replenishment and wind/water erosion or will require growing new feedstock, which will create a problem similar to afforestation in that there is only so much land available and numerous ways to use it. Due to the fact that feedstock is more micromanaged than trees this competition will extend beyond land to also include irrigated water and fertilizer as well. Diverting feedstock growth to lower quality land could alleviate some of these stressors, but will also result in low quality feedstock, which may reduce the economic viability of the BECS.
- Soil quality has declined in recent decades largely due to worsening environmental conditions brought on by excess CO2 emissions and questionable farming practices. This declining soil quality may reduce the efficiency of the BECS process by shortening feedstock survival and growth rate, especially when considering the fact that numerous rotations will occur on the same land over a short growing time frame. The worsening soil also creates a competition between BECS and the soil for the nutrients in the crop/forest residue and their erosion limiting effects.
- Like with all carbon removal technologies that require sequestration there are concerns about the long-term storage rate of various target locations as well as the means to transport collected CO2 to a proper sequestration location. With BECS transport issues may be an issue in scale-up because ideal BECS plants will require constant feedstock as well as a sequestration location, which may not be widely available. Basically BECS plants need a good growing area for feedstock and a proper location for sequestration otherwise the emissions created through transport will eliminate the net CO2 removal of the process. This location issue may be a partial explanation to why BECS as a fully scaled and economic industry is only sequestering 550 kilotons of CO2 per year.
Another strategy is to use technology to remove atmospheric CO2 over natural methods. The technological removal of atmospheric CO2 from a non-point source is referred to as direct air capture (DAC) or sometimes simply called air capture. DAC operates by interacting atmospheric CO2 with a sorbent (usually an alkaline NaOH solution) to form sodium carbonate and water. The carbonate then reacts with calcium hydroxide (Ca(OH)2)) resulting in the generation of calcite (CaCO3) and reformation of the sodium hydroxide. This process of causticization transfers a vast majority of the carbonate ions (»94-95%) from the sodium to the calcium cation and the calcium carbonate precipitate is thermally decomposed to regenerate the previously absorbed gaseous CO2. The final step involves thermal decomposition of the calcite in the presence of oxygen along with the hydration of lime (CaO) to recycle the calcium hydroxide.6
DAC has two chief advantages over all other existing methods of absorption.
- It has the highest potential for speed of removal.
- There are almost no competitive questions regarding scale-up because these capture units can be constructed nearly anywhere and do not require any land resources that are utilized by other industries like food growth, biofuels, minerals, etc.
Therefore, DAC has the highest capacity for CO2 removal both in speed and total amount of CO2 removed after scale-up due to the lack of competitive constraints on its scale-up.
However, DAC also has some important concerns that must be addressed if it is to be utilized as a carbon remediation option.
- It is currently an expensive option from a dollar/ton of CO2 removed basis. Most commentators place estimates of $400-500 per ton of CO2 captured (note that this assumes a trace emission energy provider not fossil fuels), which is a difficult to manage amount.7,8 Note that this cost does not appear to include capital costs or maintenance costs for the facilities, just routine operational costs. There are some hopes that costs can be reduced to $50-$100 per ton of CO2, but at the moment these hopes are questionable, but because few have actually broken down the costs on a point-by-point (in-voice) basis perhaps such a decrease is possible.
- The energy demands required to recycle the sorbent and as well as purify the CO2 stream for sequestration. This concern is little different from most of the other methods in that most require outside sources of energy to function properly.
- Probably the most pressing concern is that scale-up will demand significant quantities of water. In the reaction scheme water acts as a pseudo-catalyst released when the NaOH interacts with the CO2 then absorbed in the interaction with lime to form calcium hydroxide.7 However, because the air capture reaction is designed in theory as a closed system it does not account for the steady stream of atmospheric air which brings the CO2 into the system and can also absorb water stripping it from the system in practice. When this water is lost new water has to be introduced for the hydration reaction to complete the close product-recycling loop. The level of water loss is somewhat controllable through regional temperature and relative humidity (lower temperature and high humidity reduce water loss).
- Like in BECS, how to manage the sequestration of hundreds of gigatons of CO2 that will be collected from these DAC units.
Another method to remove atmospheric CO2 that has received significant attention in the last decade is synthesizing bio-char from plant matter. At its core bio-char is black carbon synthesized through pyrolysis of plant biomass. The idea of bio-char utilizes the natural photosynthetic cycle in most forms of flora. Normally plants eventually release a significant amount of CO2 via respiration through the course of their life until reaching a dynamic equilibrium between absorption and respiration. Additional CO2 is commonly released after death. However, using plants as feedstock in pyrolysis seals a percentage of the absorbed CO2 in the bio-char. In the formation of bio-char slow pyrolysis is preferred because it results in a greater bio-char yield versus fast pyrolysis.9
There are two specific advantages to bio-char:
- Bio-char is believed to be a very stable means of retaining/storing carbon based on samples of Amazonian based terra preta, which have “lifespans” of over 6000 years.10 While direct testing of bio-char stability is difficult few people are genuinely concerned about the stability of carbon within bio-char for the purposes of neutralizing short-term global warming detriment. The two major questions on stability is how oxidization potential is influenced due to microbial activity and how accelerated bio-char formation through direct pyrolysis may differ from the natural formation of terra preta.
- Bio-char has demonstrated numerous positive benefits to soil, which enhances growth rates and yields for future crops. This soil improvement stems largely from an enhancement of supply and retention of nutrients.9 Another advantage may be increased water retention, which is also thought to improve soil quality, but confidence in this influence is not as high as nutrient retention due to a lack of sufficient field studies.
The potential concerns with the application of bio-char are as followed:
- The dark brown/blackish color of bio-char when reinserting it into the soil will create a negative albedo shift resulting in a greater probability of heat absorption at the surface. How problematic this albedo shift may be is unknown, but it may have detrimental effects on crops by increasing the temperature of the local region beyond the atmospheric temperature creating greater temperature stress.
- Not surprisingly there are some competition issues in that creating sufficient feedstock would result in significant land use. Some proponents argue that because typically non-consumed plants like switchgrass can be used as a feedstock low quality agricultural land that is not used for food growth can be used. While true, use of low quality land will result in slower growth rates and yields for even switchgrass reducing the efficiency of CO2 removal. However, unlike BECS there is no energy expectation creating efficient conversion potentials, thus this reduced efficiency is not as large of a negative.
- The overall speed of CO2 removal through bio-char is questionable because unlike decades old trees or BECS, energy is required in the pyrolysis process and unlike DAC the absorption rate is basically fixed based on the type of feedstock utilized. Some estimate that the initial pyrolysis will produce CO2 equal to 45-55% of the total carbon content of the feedstock.9 Therefore, bio-char may require twice the intensity as other CO2 absorption mediums. It is also difficult to estimate how quickly the required infrastructure for creating the necessary feedstock and synthesizing bio-char from that feedstock can be developed although this latter aspect should not be a significant problem.
Overall carbon sequestration potential of bio-char as a tool to limit climate change depends largely on four separate factors: stability of bio-char in a given storage medium, total rate of change in greenhouse gas emission from feedstock sources, bio-char capacity in a given storage medium and the economic and environmental requirements in the production of bio-char.
Another CO2 removal option is to accelerate natural mineral weathering. In nature magnesium-silicate minerals such as olivine (Mg2SiO4) and serpentine [Mg3Si2O5(OH)4] can react with CO2 producing magnesite (MgCO3).11 Wollastonite (CaSiO3) is also capable of reacting with CO2 to produce calcite (CaCO3).11 Unfortunately natural weathering is too slow to significantly reduce the rapidly accelerating CO2 concentrations that the environment is experiencing now.
Investigations of the ability of these minerals to interact with CO2 occur within two different methodologies, ex situ (above ground using a chemical processing plant) and in situ (below ground using little to no chemical or mechanical alteration).11 Clearly between the methods in situ has a significant energy and economic advantage due to the lack of processing facilities and alterations to the minerals, but ex situ has the potential for a significantly higher reaction rate and percent conversion advantage resulting in much faster rates of absorption. Ex situ appears to be the superior method because of superior scale-up and speed.
The chief advantages of enhanced weathering are:
- There are excess supplies of olivine and serpentine, which eliminate any significant resource issues pertaining to scale-up. This resource surplus qualifies economic viability in ex situ carbonation to the reaction rate (CO2 concentration, temperature, particle surface area and hydration water removal).
- The CO2 reaction methodology is rather straightforward requiring no exotic strategies or materials.
The chief concerns with this methodology are:
- Large amounts of energy will be required for ‘rock breaking’ in order to increase surface area by reducing particle size to increase the reaction rate because olivine and serpentine react according to the shrinking-particle model and shrinking-core model respectively.11 Unfortunately the faster the desired reaction rate the higher the costs to create the requisite particle size.
- Reaction efficiencies create significant losses decreasing speed and increasing costs. Pretreatments differ between olivine and serpentine eliminating chemically bound water (serpentine) or limiting the formation of MgCl2 as a byproduct that can bind to silica (olivine).
The idea of utilizing enhanced weathering is not limited to the method above. A more indirect method involves addressing ocean acidity, which has negative effects on biology, but also reduces oceanic CO2 sink capacity. Through the thermal decomposition of limestone into CO2 and calcium oxide and then deposition of that calcium oxide into the ocean one can facilitate a chemical reaction to sequester CO2. When dumped into the ocean the calcium oxide reacts with water forming calcium hydroxide. Finally the calcium hydroxide reacts with free dissolved CO2 in the ocean creating calcium bicarbonate.
In the interim such a system seems to be carbon negative because while 1 mole of CO2 is generated for each mole of calcium oxide created, the resultant reaction of calcium hydroxide with dissolved oceanic CO2 removes two moles of CO2 per mole of calcium oxide deposited into the ocean. The process in its purest most efficient form generates a +1 mole reduction in CO2 from the ocean per mole of processed limestone. The removal of CO2 from the ocean will increase the ability of the ocean to act as a carbon sink pulling in more CO2 from the atmosphere as long as the concentration driving force remains.
The pros of this system are:
- There are significant limestone deposits throughout the world that would be suitable for bases of operation thus lowering costs by not limiting locations for scale-up.
- Fairly low cost ranging from $50 to $150 per ton of CO2 removed from the ocean; (the cost range assumes trance emission source and varies with overall transportation medium)
However, there are some significant concerns with this system.
- The reaction rate between calcium hydroxide and CO2 is dependent on numerous factors including pH, pressure, temperature and CO2 concentration (which is rather dilute), thus the probability for reaction cannot be assumed to be 100%. Realistically probability for reaction should be estimated at 75% to 90% (1.5 – 1.8 moles of CO2 per mole of calcium hydroxide), which may place the costs of creating such a system at too high a premium for the resultant CO2 removal.11 Basically the costs will be too much not on a direct ratio, but an opportunity cost ratio (the funds would be better spent scaling-up another removal strategy).
- The processing of the limestone requires a significant amount of heat energy due to the required temperature (800-900 degrees C).11 It takes approximately 2.67 GJ (741.67 kw-h) to calcinate 1 ton of limestone.12 Unless a trace emission source of energy is utilized in this process more than likely the efficiency of CO2 removal will not be sufficient to warrant its application.
- At this time the efficiency of the program demands a localized distribution with regards to the calcium oxide deposit because transport emissions will eliminate the net negative effect; however, if too much calcium oxide is deposited in the ocean over a short period of time (which is to be expected based on the scale required for this process to be meaningful) it is highly possible that there will be detrimental consequences to the biodiversity in the deposit region.
Finally ocean fertilization has widely been regarded as the most controversial carbon remediation method. It involves seeding swatches of the ocean with iron with the presumption that iron is a limiting factor in phytoplankton growth, thus adding iron should stimulate rapid growth creating a ‘bloom’. This phytoplankton can then increase absorption rates of CO2, both from the ocean and the atmosphere, through photosynthesis. Later when these phytoplankton die they sink to the bottom of the ocean, effectively removing the CO2 from the short-term carbon cycle.
Proponents point out the following advantages:
- Believed to be the least expensive methodology when judged on a theoretical cost/CO2 absorbed ratio, especially when considering the potential environmental difficulty in bringing new forests to full maturity.
- Requiring only iron and possibly small amounts of phosphate, there are no exotic or rare materials that would prevent the continuous application of fertilization or shortages that would dramatically increase costs.
Low cost and easy application are very attractive features; however, there are some significant drawbacks to ocean fertilization.
- There is significant potential for ecological damage born from cultivating an extremely large amount of phytoplankton over a small region, most notably the creation of hypoxic areas (a.k.a. dead zones). The full consequence of this potential damage is unknown. While no dead zones have resulted in the small number of controlled ocean fertilization experiments none of these experiments were long-lived thus it is difficult to tell whether the lack of dead zones is due to the short experimental time frame or the lack of conditions to create dead zones, but most non-proponents hypothesize the former over the latter.
- The formation of the bloom may not actually reduce the threat of global warming because while it may absorb CO2 there will be increases in methane and nitrous oxide concentrations, which will provide short-term stressors on global warming relative to the CO2. Basically if this was the only problem one could argue that ocean fertilization is short-term detrimental and long-term beneficial, but that long-term may be too far into the future to ward off most of the detrimental outcomes of global warming.
- There are also questions regarding whether or not the storage, which occurs when the phytoplankton die and sink, is long-term enough to offer sufficient time for environmental recovery.
- There are some genuine questions to whether ocean fertilization even works as one of the most prominent field tests completely failed.13 The joint Germany, India and Chile LOHAFEX project spread 6 tons of iron sulfate over a 300 km^2 area of the Southwest Atlantic Sector of the Southern Ocean and observed results over a 39 day period. While there was a significant increase in phytoplankton growth (doubling of biomass) in the initial stages of the experiment that growth was short-lived as it was soon accompanied by an increase in predation by zooplankton and amphipods. Therefore, even though phytoplankton growth increases that increase does not appear to be sustainable as the limiting factor shifts from iron to predator appetite.
One hypothesis to why predation was so damaging to the LOHAFEX project was that the Southern Ocean does not have large enough quantities of silicic acid to allow the formation of diatoms and their silica protective shells. However, moving fertilization to warmer tropical regions, which have greater supplies of silicic acid, also have problems in nutrition over-consumption elements and how they relate to CO2 absorption. Basically the lack of iron in the Southern Ocean makes it a much more attractive location for iron fertilization versus more tropical regions which have higher iron concentrations; however, as discussed the silicic acid shortage leaves phytoplankton in the Southern Ocean more vulnerable to predation. Overall there really does not appear to be a good area to maximize iron fertilization results.
No discussion was made of shifting ocean currents (up-swell or down-swell) because there is no legitimate positive evidence that either ocean current manipulation strategies will be successful at significantly increasing CO2 absorption rates. Most notably there is no evidence that suggests temperature or organic matter exchange through pipes can transport CO2 to appropriate depth for long-term storage.
An important note with regards to CO2 removal from the atmosphere is that it will be difficult to achieve success if individuals focus too much on an “all of the above” methodology. The sheer amount of CO2 that needs to be removed to aid natural sinks is quite large and the organization and capital required to accomplish this remove demands large scale-up of a small number of strategies versus the narrow application of numerous strategies. Thus, one has to eliminate the mindset of “not picking winners” because there is not enough time to let the “magic of the market” sort out the “best” options. Think of it in a similar vein to economics... which is better having one company with 1 billion dollars in liquid capital or 1000 companies with 1 million dollars each in liquid capital when trying to solve a problem?
Another reason for this required narrow strategy is that most of the costs associated with CO2 removal are based on prevention savings. Despite the hopes of some proponents of DAC and other sequestration demanding strategies the probability that any real economic returns will be seen from these strategies is very slim. The top ideas for creating capital from the capture of atmospheric CO2 contradict with the idea of atmospheric CO2 capture in the first place. For example CO2 removal needs to be negative, not neutral or slightly positive, thus the formation of fuel from the captured CO2 or its use in enhanced oil recovery are eliminated as economic options; incorporation of the captured CO2 as a feedstock for algae for a price is unusual because the algae can simply feed from the atmosphere at little reduced efficiency. Using the captured CO2 for resale in industries that require CO2 for consumer products like soda or greenhouses will produce almost nothing relative to what will be required for the process itself. Therefore, any group funding these absorption strategies will benefit by highlighting the reduced damage costs in the future versus profiting in the present.
Overall the best strategy appears to be to focus on strategies that have the highest potential for scale-up with a secondary focus on speed of CO2 extraction. When applying these criteria DAC is the superior choice, but water concerns may be prohibitory to its expansion. Due to the positive ecological effects and low costs afforestation should be undertaken when possible (not interfering with agricultural lands for food growth). A bio-char program should also be designed and executed on an land appropriate scale due to its absorption ability and its secondary ability to aid soil health, which may become an issue with worsening drought and fertilizer shortages in the future. However, albedo changes must be closely observed to ensure that bio-char integration into the soil is not having a negative influence on crop growth. Expansion of BECS is tricky because of its competition with foodstuffs, bio-char and afforestation, thus probably should be avoided. The remaining strategies do not appear to have the scale or efficiency necessary to warrant their further inclusion in the discussion regarding strategies to implement for CO2 removal.
For another perspective on this remediation issue see:
Lenton, T, and Vaughan, N. “The radiative forcing potential of different climate geoengineering options.” Atmos. Chem. Phys. 2009. 9:5539-5561.
Vaughan, N, and Lenton, T. “A review of climate geoengineering proposals.” Climatic Change. 2011. 109:745-790.
2. “Working Group I: The Physical Science Basis of Climate Change.” Intergovernmental Panel on Climate Change. 2007.
3. Canadell, Josep, et, Al. “Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks.” PNAS. 2007. 104(47): 18866-18870.
4. Le Quéré, C, et Al. “Trends in the sources and sinks of carbon dioxide.” Nature Geoscience. 2009. DOI: 10.1038/NGEO689
5. Hansen, James, et, Al. “Target Atmospheric CO2: Where Should Humanity Aim?” The Open Atmospheric Science Journal. 2008. 2: 217-231.
6. Zeman, Frank. “Energy and Material Balance of CO2 Capture from Ambient Air.” Environ. Sci. Technol. 2007. 41(21): 7558-7563.
7. Turning the Clock Back – Reducing Atmospheric CO2. Bastion of Reason. Sept. 2009. http://www.bastionofreason.blogspot.com/2009/07/turning-clock-back-reducing-atmospheric.html
8. ACS Direct Air Capture Paper;
9. Lehmann, J, Gaunt, J, Rondon, M. “Bio-char sequestration in terrestrial ecosystems – a review.” Mitigation and Adaptation Strategies for Global Change. 2006. 11: 403–427.
10. Glaser, B, et, Al. “The Terra Preta phenomenon – A model for sustainable agriculture in the humid tropics.” Naturwissenschaften. 2001. 88: 37–41.
11. Gerdemann, S.J., et Al. “Ex-Situ and In-Situ Mineral Carbonation as a Means to Sequester Carbon Dioxide.” DOE Analysis.
12. Barker, S, et, Al. “The Future of the Carbon Cycle: Review, Calcification response, Ballast and Feedback on Atmospheric CO2.” Philos. Trans. R. Soc. A. 2003. 361: 1977.
13. “Lohafex project provides new insights on plankton ecology: Only small amounts of atmospheric carbon dioxide fixed.” International Polar Year. March 23, 2009.