The devastating effects of climate change can feel abstract when we talk about carbon-dioxide levels and shifting weather patterns. But they are distressingly real when we see what they’re doing to the world’s polar bears.

Nature photographer Paul Nicklen recently posted a video to social media that he shot on Somerset Island, in the Canadian territory of the Arctic Circle. The clip shows a dying, emaciated polar bear searching desperately for food, its skin hanging from its bones, its legs so weak it can hardly stand. “The muscles atrophy,” he wrote in the caption. “No energy. It’s a slow, painful death.”

Polar bears have become the poster animals for climate change because they offer a clear, concrete image to convey the harm global warming is already beginning to wreak on the planet. “We are a visual species,” Nicklen told the Washington Post (paywall). He had traveled to the remote region where he shot the footage with Sea Legacy, a group that uses visual storytelling to prompt action on climate change.

No one can prove that particular bear was starving because of climate change. But there is plenty of evidence indicating that warming is causing sea ice—the bears’ main hunting ground—to dwindle, which threatens the bears’ existence.

One 2015 study found that the polar bear population in the southern Beaufort Sea had declined 40% because of the loss of sea ice. A 2017 study by the US Geological Survey and the University of Wyoming concluded that bears are also spending more energy walking across a “treadmill” of drifting sea ice caused by warming, and needed to eat more to compensate. According to the Washington Post (paywall), researchers believe 80% of polar bear populations could die off if sea ice continues to disappear.

Nicklen told National Geographic, where he is a contributor, that the video he shot of the starving bear was one of the most painful scenes he’s witnessed. “We stood there crying—filming with tears rolling down our cheeks,” he said.

But he thought it was necessary to show to people who would otherwise never see such a scene. “When scientists say bears are going extinct, I want people to realize what it looks like,” he said. “Bears are going to starve to death. This is what a starving bear looks like.”