As the climate continues to change and humans are implicated as the cause, the one major back up plan has been geoengineering—somehow devising a way to reduce the impacts of a warming world. Avoiding it al together would be the simplest response, just by turning down the world’s furnace. Collectively we burn some 83 million barrels of oil each day, not to mention coal and natural gas, production of which are sharply increasing. But as the will remains elusive to get the biggest polluters to agree to knock it off, environmental advocates have started to consider Plan B—geoengineering—the new Plan A.

The leading method is known as stratospheric aerosol injection (SAI) and effectively shoots particles into the stratosphere that act as