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.