Just a few years ago, it looked as if humpback whale populations in the Pacific Ocean were booming.

Receding sea ice due to climate change—though bad for polar bears and other wildlife—had lengthened the summer foraging season for hungry whales, and the U.S. government decided that certain populations had recovered enough to be taken off the endangered species list.

But then the warming got out of hand.

Starting in late 2013, a persistent mass of unusually warm ocean water, nicknamed “the blob,” appeared in the Gulf of Alaska and slowly spread south along North America’s Pacific Coast. In its wake followed a few other anomalies, including El Niño.

The mysterious blob persisted for six years, raising sea surface