SEATTLE — Erika Lundahl writes and performs her own songs. She works in Seattle for a company that publishes books on the environment. She thinks a lot about how best to occupy her place in the world. Yet, despite this full life, Lundahl, at 27, feels a clock ticking.

Her biological clock, yes, but also the one to fix global warming, or face the likelihood that she and her potential children will have to live in a seriously marginalized world.

“There is this sense that if you don’t have kids soon, you could be putting them in a harder position,” Lundahl said. “But if you do have them, that will not be easy either, with the storms, the intense droughts, the…