This week at the United Nations, world leaders are meeting to address, among other things, climate change: the biggest and most urgent crisis humans have ever faced on a global scale.

Activist Bill McKibben was one of the first people to sound the alarm on this subject for a wider audience. His book, The End of Nature, written in 1989, warned people that the earth was at a tipping point. Three decades later, he says, it has tipped.

McKibben spoke recently to host Kai Wright on his show, the United States of Anxiety. It was an intense conversation, but one that addressed a critical question: what do we do RIGHT NOW.

Kai joined WNYC's Morning Edition host Michael Hill to talk through what he learned from that conversation.