The ice shelf was cracking up. Surveys showed warm ocean water eroding its underbelly. Satellite imagery revealed long, parallel fissures in the frozen expanse, like scratches from some clawed monster. One fracture grew so big, so fast, scientists took to calling it “the dagger.”

“It was hugely surprising to see things changing that fast,” said Erin Pettit. The Oregon State University glaciologist had chosen this spot for her Antarctic field research precisely because of its stability. While other parts of the infamous Thwaites Glacier crumbled, this wedge of floating ice acted as a brace, slowing the melt. It was supposed to be boring, durable, safe.

Now climate change has turned the ice shelf into a threat — to Pettit’s field work, and to the world.

Planet-warming pollution from burning fossil fuels and other human activities has already raised global temperatures more than 1.1 degrees Celsius (2 degrees Fahrenheit). But the effects are particularly profound at the poles, where rising temperatures have seriously undermined regions […]