India, May 9 -- "I'm looking out of my closed window, and I have no idea how warm it is out there. It could be 30, 32 or 35 degrees Celsius," says Austin-based journalist and author Jeff Goodell.

This is unlike, say, a typhoon or hurricane, he adds. Trees sway, there is heavy rain and flashes of lightning. "When it comes to heat, there are virtually no visual cues."

Even he, a climate reporter for over two decades (with stories in The New York Times, CNN and The Guardian, among others), never really paused to acknowledge how dangerous heat is. This changed in the summer of 2018.

He was in Phoenix, Arizona, where temperatures had breached 40 degrees Celsius. He was late for a meeting, couldn't find a taxi, and decided to run instead, as...