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Stratospheric aerosol injection
Mimic a volcano, cool the planet. It exists as models, reviews and one cancelled balloon. It is not a deployed thermostat.
Jurisdiction: Research community (no operational national deployment)
Years / status: Discussed since the 1990s (and earlier volcanic analogies). IPCC AR6 treats solar radiation modification as researched, not recommended as a substitute for cuts. Harvard SCoPEx cancelled March 2024. Status: research.
The plan
Aircraft or balloons would put reflective particles (often sulfate, sometimes other materials) into the stratosphere to scatter sunlight. Pinatubo 1991 is the usual natural analogue: a temporary global cool, plus monsoon, ozone and precipitation side effects. The policy pitch is a brake while emissions fall. That pitch is a scenario, not a programme of record.
Construction / status
Almost all work is models, labs and paper. SCoPEx, a proposed small stratospheric plume experiment at Harvard, never released particles; a Sweden hardware test was stood down in 2021 after indigenous and civil-society opposition; the principal investigator ended the experiment in March 2024. Other groups continue modelling and governance debates. This is not a fleet.
What went wrong, or only partly worked
IPCC AR6: SRM could cool the globe in models and would not remove CO₂; regional precipitation and ozone effects are uncertain; termination shock is a risk if injections stop while greenhouse gases stay high; governance is thin. No country has a licensed planetary SAI programme. Presenting SAI as a ready fix is false.
Lessons
A volcano is a warning, not a user manual. Research is not deployment. Any outdoor test needs public consent that SCoPEx did not secure. Cut emissions first; do not file SAI under “solutions that already work.”
Sources
IPCC AR6 WG I on solar radiation modification; Harvard SCoPEx termination note; MIT Technology Review 18 March 2024.