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 once locked in ice.

In research presented this week at the world’s biggest earth science conference, Pettit showed that the Thwaites ice shelf could collapse within the next three to five years, unleashing a river of ice that could dramatically raise sea levels. Aerial surveys document how warmer conditions have allowed beavers to invade the Arctic tundra, flooding the landscape with their dams. Large commercial ships are increasingly infiltrating formerly frozen areas, disturbing wildlife and generating disastrous amounts of trash. In many Alaska Native communities, climate impacts compounded the hardships of the coronavirus pandemic, leading to food shortages among people who have lived off this land for thousands of years.

“The very character of these places is changing,” said Twila Moon, a glaciologist at the National Snow and Ice Data Center and co-editor of the Arctic Report Card, an annual assessment of the state of the top of the world. “We are seeing conditions unlike those ever seen before.”

The rapid transformation of the Arctic and Antarctic creates ripple effects all over the planet. Sea levels will rise, weather patterns will shift and ecosystems will be altered. Unless humanity acts swiftly to curb emissions, scientists say, the same forces that have destabilized the poles will wreak havoc on the rest of the globe.

“The Arctic is a way to look into the future,” said Matthew Druckenmiller, a scientist at the National Snow and Ice Data Center and another co-editor of the Arctic Report Card. “Small changes in temperature can have huge effects in a region that is dominated by ice.”