It’s hard to exaggerate just how much the world changed in just a few weeks this spring. The glittering vehicles that packed suburban office parks disappeared, leaving empty moats of blacktop. Online traffic maps glowed green at what had been reliable deep-red choke points. Families on bikes and delivery trucks moved slowly through newly quiet residential streets. And the fumes from internal combustion engines cleared, with deadly particulate pollution falling by more than half in many cities. The quarantine squeegeed away the smog, leaving sparkling city skylines. The residents of Jalandhar, India, 100 miles south of the Himalayas, had a view of snow-capped mountains for the first time in 30 years.