Chelsea Wegner was shocked when she landed in Anchorage, Alaska, in July, on her way to a research cruise in the Bering Sea. Smoke from wildfires across the state had darkened the skies, and Anchorage was in the midst of a heatwave that saw temperatures soar past 32 °C for the first time in recorded history.

Wegner, a marine biologist at the University of Maryland in Solomons, also knew that the unusual warmth had melted away nearly all of the sea ice in the Bering Sea. “It was a really surreal moment,” she says.

Later, while sailing aboard a Canadian icebreaker off the coast of Alaska, Wegner watched…