Of all the trauma of the past year, one moment stands out for Alexandria Villase̴̴nor, a teenage climate activist, as the bellwether of what’s in store for the planet: It was last summer, when toxic wildfire smoke spread through northern California as the coronavirus prompted a lockdown.

“Businesses weren’t open or allowing people inside because it was unsafe,” she says. “But the air quality index was like 300-plus outside, which wasn’t safe either. It was, like, what mask do I wear for wildfire smoke and COVID? How can I stay safe from both threats?”

The arrival of an airborne virus that has killed more than three million people globally has only multiplied her anxieties, and she spent part of the last year