In 1965, US president Lyndon Johnson’s science advisers urged research into reflecting sunlight to keep the Earth cool amid projections of an alarming build-up of greenhouse gases in the atmosphere as a result of burning fossil fuels.

Almost six decades later, “solar geoengineering” research has made scant headway.

It attracts less than 1% of climate science budgets, amid fears that tampering with the global thermostat could produce unexpected consequences — and distract from an overriding need for deep cuts in greenhouse gas emissions...