In the late 20 th century, the world came together to plug a hole in the ozone layer — the part of Earth’s atmosphere that absorbs most of the Sun’s harmful ultraviolet radiation. If left unchecked, this hole would have exposed life on Earth to dangerous — and in some regions potentially lethal — levels of radiation, but an international treaty brought us back from the brink of disaster. That treaty, the Montreal Protocol, is a lesson in human resilience: We can save the world, because we already did it once before. An epidemic of deadly fridges The story of the Montreal Protocol starts, bizarrely, with an epidemic of deadly fridges in the 1920s. In those pioneer days of electric home refrigeration, everyone’s favorite new kitchen appliance relied on highly toxic, flammable, or corrosive gases to keep food chilled. A faulty compressor or leaky pipe could wipe out an entire family in their sleep, and in the first half of 1929, gas from fridges killed at least 15 people in Chicago alone.…