Sulfate cooling the Earth is cheap, doable, and coming soon

Sulfate stratospheric aerosol injection is presented as a practical, low-cost way to cool the planet by reflecting a small fraction of incoming sunlight. Sulfate droplets in the stratosphere scatter sunlight and can reduce global radiative forcing by about 2 W/m², roughly undoing the ~1 °C warming since 1980. To work they must be injected above weather systems so particles stay aloft, avoid direct inhalation risks, and accept that the aerosol will disperse globally (tropics-to-poles transport makes purely local control impossible). Returning sulfate becomes dilute acid rain, mostly over oceans, which the calculations argue would be manageable compared with existing acid deposition in some cities.

The back-of-envelope math yields ~2×10²5 droplets or about 1.8×10¹0 kg of sulfuric acid in flight at once, equating to roughly 9×10⁹ kg of SO₂ per year with a one-year residence time. Current industrial sulfur supply can cover that; chemical production CAPEX is modest (~€0.6bn) and commodity costs are on the order of a few billion dollars per year. The hard part is lofting material to ~20 km: tropical stratospheric injection would be cheapest (~$5bn/year plus R&D for a national program), while using unmodified jets from high-latitude sites (Alaska and near South America) raises annual sulfur needs ~4×, requires ~270,000 flights/year, and drives costs toward ~$20bn/year with large startup infrastructure. Existing studies, startups, and government programs are already exploring this pathway.