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Geoengineering

Geoengineering can refer to any method of deliberately altering the Earth's climate or atmospheric composition. While direct air capture of CO2 from the atmosphere and bioenergy and carbon capture and sequestration (BECCS) can be considered forms of geoengineering, we consider them in our analysis of carbon capture and sequestration.

Approaches

Following is a summary of commonly considered geoengineering methods, their estimated costs, carbon removal or mitigation potential, limitations, and risks and drawbacks. See also our analysis of ocean-based geoengineering.

Geoengineering and Carbon Removal Approaches
MethodDescriptionEmissions Reduction PotentialCost per Ton CO2LimitationsRisks and Drawbacks
Stratospheric Aerosal InjectionRelease aerosols in the stratosphere to reflect sunlight and induce cooling.Unlimited$0.10 to $0.25 (Smith and Wagner)Does not address ocean acidificationTermination could lead to abrupt temperature rise (Parker and Irvine), interstate conflict (Halstead), drought, ozone depletion, loss of sunlight (Rabock et al.).
Marine Cloud BrighteningAdd aerosols to clouds to increase their reflectivity (NRC)Reduce temperatures 0.6-1.1 °C$0.13 (NRC)Does not address ocean acidification.Unknown effects on weather patterns.
AfforestationPlant trees, which absorb CO2116 (Veldman et al.) to 564 (Bastin et al.) tons CO2e (3-14 years of emissions at 2060 levels)$20-$50/ton, depending on amount of afforestation (Busch et al.)Limited land available for forestation, potential land use conflictCarbon sequestration potential of forests is highly debated (Lewis et al.).
Ocean Iron FertilizationbSeed oceans with iron particles, which induce algae growth, absorb CO2, and sequester it in the deep ocean196-790 billion tons CO2 over 100 years (NOAA), the equivalent of about 5-20 years of world emissions at 2020 rates$13-133 (Boyd)---Efficiency of iron in absorbing carbon is disputed, negative feedbacks are uncertain (NOAA). Ocean acidification would be exacerbated (NOAA). Unknown risk to ecosystems (Strong et al.)
Ocean AfforestationSeed oceans with iron particles, which induce algae growth, absorb CO2 and sequester it in the deep ocean36 billion tons CO2 sequestered per year, almost to current emissions (N'Yeurt et al.).$18 (N'Yeurt et al.)---Many stress coastal ecosystems (Royal Society)
Wetland, peatland, and coastal habitat restorationActive restoration0.4-20 billion tons CO2/year$10-100---Short term release of methane and N2O.
Soil carbon sequestrationModify farming practices to increase soil carbon content.1-10 billion tons CO2/year-$10 to +$3Saturation of soil carbon in 10-20 years.Carbon storage can be reversed.
BiocharPyrolsis of biomass into charcoal, to enhance carbon storage and improve soil fertility.3-5 billion tons CO2/year$0-200---Reduced soil albedo may cause warming.
Biomass building materialsIncrease use of plant-based building materials, such as wood, bamboo, straw, or hemp.0.5-1 billion tons CO2/year$0Market size for new buildingsFire risk.
Enhanced terrestrial weatheringMill silicate rocks to increase the rate of weathering, which sequesters CO20.5-4 billion tons CO2/year$50-500---Energy intensive.
Mineral carbonizationGrind and treat minerals and react with CO2 for stable sequestration. Resulting products may be commercially valuableEffectively unlimited$50-300 (above ground), $20 (below ground)Below ground sequestration depends on geological availability.Mining for materials, high energy consumption.
Ocean alkalinityAdd positive charged calcium or magnesium to the ocean to increase alkalinity and accelerate CO2 uptake40 billion tons CO2/year$70-200---Loss of alkalinity could reverse CO2 uptake. Unknown risk to ecosystems.
Low carbon concreteMethods of concrete production that incorporate atmospheric CO2100 million tons CO2/year$50-300Based on market size for concrete.Market acceptance of alternatives to Portland cement.

Sources: Bastin et al. 1, Boyd 2, Busch et al. 3, Halstead 4, Lewis et al. 5, National Research Council 6, N‘Yeurt et al. 7, Robock et al. 8, Royal Society 9, Smith and Irvine 10, Smith and Wagner 11, Strong et al. 12, NOAA 13, Veldman et al. 14.

All proposed methods of geoengineering come with severe risk, limitations, and/or cost, though there is no reason to rule them out a priori.

Problem:
Deforestation
Solution:
Afforestation
Problem:
Risk of Uncontrolled Warming
Solution:
Mineral-Based Carbon Sequestration
Problem:
Risk of Uncontrolled Warming
Solution:
Biochar for Carbon Sequestration - China

Solar Geoengineering

Solar geoengineering refers to a method to reduce the sunlight that reaches Earth's surface, such as stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning, and thus inducing cooling 15. While these are potentially useful and inexpensive tools, they carry possible unknown risks and do not address the problems of atmospheric CO2 concentration or ocean acidification. Modeling and limited field trials are appropriate at this time, but not full scale deployment 15.

Problem:
Risk of Uncontrolled Warming
Solution:
Research Solar Geoengineering

Other Forms

There are several other methods sometimes discussed but not included above. The use of mirrors in orbit to reflect sunlight is at present impractical 6, as is irrigation of the Sahara Desert 16. Efforts to increase Earth's surface albedo, such as by painting roofs white, would make insignificant contribution 16. There are several methods under development that could directly remove greenhouse gases other than CO2, but data on their prospects is limited 9.

References

  1. Bastin, J., Finegold, Y., Garcia, C., Millicone, D., Rezende, M., Routh, D., Zohner, C., Crowther, T. "The global tree restoration potential". Science 365(6448). July 2019. ↩

  2. Boyd, P. "Introduction and synthesis". Marine Ecology Progress Series 364, pp. 213–218. Implications of large-scale iron fertilization of the oceans. July 2008. ↩

  3. Busch, J., Engelmann, J., Cook-Patton, S., Griscom, B., Kroeger, T., Possingham, H., Shyamsundar, P. "Potential for low-cost carbon dioxide removal through tropical reforestation". Nature Climate Change 9, pp. 463–466. May 2019. ↩

  4. Halstead, J. "Stratospheric aerosol injection research and existential risk". Futures 102, pp. 63-77. September 2018. ↩

  5. Lewis, S., Mitchard, E., Prentice, C., Maslin, M., Poulter, B. "Comment on "The global tree restoration potential"". Science 366(6463). October 2019. ↩

  6. National Research Council; Division on Earth and Life Studies; Board on Atmospheric Sciences and Climate; Ocean Studies Board; Committee on Geoengineering Climate: Technical Evaluation and Discussion of Impacts. "Climate Intervention: Reflecting Sunlight to Cool Earth". Washington, DC: The National Academies Press. 2015. ↩ ↩2

  7. N‘Yeurt, A., Chynoweth, D., Capron, M., Stewart, J., Hasan, M. "Negative carbon via Ocean Afforestation". Process Safety and Environmental Protection 90(6), pp. 467-474. November 2012. ↩

  8. Robock, A., Marquardt, A., Kravitz, B., Stenchikov, G. "Benefits, risks, and costs of stratospheric geoengineering". Geophysical Research Letters 36(19). October 2009. ↩

  9. Royal Society, Royal Academy of Engineering. "Greenhouse gas removal". September 2018. ↩ ↩2

  10. Smith, A., Irvine, P. "The Risk of Termination Shock From Solar Geoengineering". Earth's Future 6(3), pp. 456-467. March 2018. ↩

  11. Smith, W., Wagner, G. "Stratospheric aerosol injection tactics and costs in the first 15 years of deployment". Environmental Research Letters 13(12). November 2018. ↩

  12. Strong, A., Chisholm, S., Miller, C., Cullen, J. "Ocean fertilization: time to move on". Nature 461 pp. 347-348. September 2009. ↩

  13. U.S. National Oceanic and Atmospheric Administration. "Ocean Fertilization: The potential of ocean fertilization for climate change mitigation". Report to Congress. 2010. ↩

  14. Veldman, J. et al. "Comment on "The global tree restoration potential"". Science 366(6463). October 2019. ↩

  15. National Academies of Sciences, Engineering, and Medicine 2021. "Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance". Washington, DC: The National Academies Press. 2021. ↩ ↩2

  16. Y Combinator. "Desert Flooding". Accessed April 30, 2020. ↩ ↩2