Module 14: Cleaning Up

Negative CO2 emissions (also known as Carbon Dioxide Removal, CDR) comprises a suite of methods or techniques to sequester carbon from the atmosphere at scale. The easiest methods by far are those that leverage photosynthesis to convert CO2 to biomass and then prevent the resulting organic matter from being decomposed back into CO2 through decomposition. All of these methods require that large amounts of productive land be devoted to carbon storage, so the total potential for carbon storage by these methods is limited. Chemical uptake of CO2 by accelerated mineral weathering or ocean alkalinity enhancement are more speculative, but may provide carbon sequestration without substantial land tradeoffs. Finally, direct air capture (DAC) is an extremely energy-intensive and costly process for concentrating atmospheric CO2 into liquid sorbent and then extracting the resulting material for long-term storage, perhaps underground.

All known methods for negative CO2 emissions are substantially more costly and difficult than preventing positive emissions by substituting low-carbon energy for fossil energy in the world economy. Nevertheless, these methods may become needed if policy fails to achieve deep decarbonization fast enough to prevent catastrophic damage.

Solar Radiation Management (SRM) is a suite of methods to modify Earth’s temperature directly, without addressing CO2 and other greenhouse gases. Stratospheric Aerosol Injection involves adding microscopic particles to the upper atmosphere to reflect sunlight. Marine Cloud Brightening (MCB) uses cloud seeding to condense water droplets in low clouds over the ocean to make them more reflective. Cirrus Cloud Thinning (CCT) uses ice nuclei to grow and then precipitate ice crystals from the highest clouds to reduce their greenhouse warming effect.

Upon completion of this module you should be able to: 

1. List three major proposed methods for CO2 removal, and rank them in terms of their degree of difficulty and expense
2. Compare strengths and weaknesses of various approaches for negative emissions
3. Explain why CO2 removal may be needed even though it’s more expensive than low-carbon energy
4. Describe three methods for cooling Earth’s climate by manipulating clouds and radiation
5. Explain advantages and disadvantages to various proposals for climate intervention

• Module 14: Cleaning Up the Mess [PDF]
• Fuss, S., et al (2014). Betting on negative emissions. Nature Climate Change, Vol. 4. Macmillan Publishers Limited. 
• US Department of Energy, Carbon Dioxide Removal Primer, Chapter 2. Lawrence Berkeley Laboratory. 2021

• Video 14.1: Cleaning Up the Mess
• Video 14.2: Carbon Dioxide Removal
• Video 14.3 Radiation Management for Climate Cooling

Module 14: Cleaning Up the Mess

1. Explain two broad classes of climate intervention intended to mitigate global warming. Why might climate intervention be necessary?
2. Critique the idea that planting trees can offset CO₂ emissions from burning fossil fuels. 
3. Describe three different methods for carbon dioxide removal and explain advantages and disadvantages of each.
4. Explain the following methods of carbon dioxide removal and describe what might limit the potential scope of each one:
   • Ocean alkalinity enhancement
   • Accelerated chemical weathering
   • Soil carbon sequestration
   • Biochar
5. Give some estimates of the range of costs for carbon dioxide removal (dollars per ton of CO₂). If people eventually decide to remove 50 ppm of CO₂ because we overshoot climate targets, what do contemporary cost estimates imply for the total cost of remediation?
6. Explain three proposed radiation management methods for mitigating global warming and compare advantages and disadvantages of each.
7. What is meant by the term “stratospheric aerosol” in climate science and how is it related to intentional climate intervention? What kinds of aerosol are involved? 
8. Explain some potential long-term problems with climate intervention by solar radiation management.