With the planet on course for global warming of 3 degrees Celsius or more by the end of this century, and with major countries like the United States and Brazilrun by climate deniers dead-set against reducing emissions, a team at Harvard is getting ready to test a last-ditch idea for cooling the planet before it’s too late: solar geoengineering.

Solar geoengineering proposes to reverse the effects of climate change by blocking sunlight, or reflecting it back before it reaches the Earth’s surface. A good model…