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To help cure the planet’s ailments, Zhen Dai suggests antacid. In powdered form, calcium carbonate—often used to relieve upset stomachs—can reflect light; by peppering the sky with the shiny white particles, the Harvard researcher thinks it might be possible to block just enough sunlight to achieve some temperature control here on Earth. Dai’s work calls for a custom-designed test balloon that, pending an independent committee’s green light, is set to release up to a kilogram of calcium carbonate 12 miles above the US, in what will be the first solar geoengineering experiment in the lower atmosphere. Small onboard propellers will stir the payload into the air.