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← Terraforming · Climate intervention

Climate intervention

Cirrus cloud thinning

Seed high ice clouds so they thin and leak more heat to space. Models disagree; IPCC has low confidence in the cooling. It is not SAI with a different name.

MODIS true-color image of high cirrus clouds over the Pacific and central Chile: natural ice clouds, not a cirrus-thinning experiment
Image: MODIS Land Rapid Response Team, NASA GSFC ( Public domain )

Research / proposal: not a deployed planetary fix.

Climate intervention Research Research community (models; no operational national deployment)

Jurisdiction: Research community (models; no operational national deployment)
Years / status: Cirrus cloud thinning (CCT); assessed in IPCC AR6 WG I alongside other radiation-budget proposals; NOAA SRM State of the Science (July 2024) lists CCT separately from SAI and MCB. Status: research.

The plan

CCT would add ice-nucleating particles (often discussed as bismuth triiodide or similar) into cold, high cirrus so ice crystals grow larger, fall faster, and leave a thinner, shorter-lived cloud. The intended effect is more outgoing longwave radiation: cooling by letting Earth’s heat escape: not by reflecting sunlight the way SAI or MCB would. NOAA’s SRM fact sheet draws that distinction explicitly.

Construction / status

Exists as model experiments and microphysics papers. No national CCT fleet, no outdoor seeding programme of record comparable even to cancelled SAI balloon trials. IPCC AR6 WG I assesses CCT among solar-radiation-modification-adjacent options and flags low confidence. Atmospheric chemistry and physics papers (e.g. Gasparini et al., ACP 2017) test how models proxy “seeding” and show that simple sedimentation tricks are a poor stand-in for real ice nucleation.

What went wrong, or only partly worked

Documented today: model ranges and process studies, not a measured planetary cool. Some simulations suggest possible cooling on the order of roughly 1-2 W m⁻² under optimistic assumptions; others find little effect or warming if clouds are “over-seeded.” IPCC’s low-confidence line is the honest summary for a public card.

Lessons

CCT does not remove CO₂ or fix ocean acidification. Rainfall, regional heat, and unintended cloud changes are uncertain. Do not caption this as a ready thermostat or as “the same as SAI.” If a model needs a perfect seeding recipe that nobody has demonstrated outdoors, the card stays Research.

Sources

IPCC AR6 WG I Chapter 4 (SRM / radiation proposals including CCT) and the Chapter 4 PDF; NOAA CSL State of the Science Fact Sheet: Solar Radiation Modification (July 2024); Gasparini et al. 2017 Atmospheric Chemistry and Physics on cirrus seeding proxies in models.