A third of Antarctica’s vast offshore ice shelves could collapse into the ocean if the world warms by 4 degrees Celsius (7.2 degrees Fahrenheit) above pre-industrial levels. These floating platforms of solid water wouldn’t directly raise sea levels if they melted; they already sit in the ocean. But they’re important barriers preventing the immense bulk of the frozen continent’s glaciers from rolling out to sea. If those inland glaciers reached open water, sea level could rise catastrophically.

Of the 34% of ice shelves at risk of collapse by the end of the 21st century, many are concentrated in the Antarctic Peninsula — a region of West Antarctica that juts northward toward South America. The at-risk ice makes up two-thirds of the peninsula’s ice shelf extent. In total, 190,000 square miles (500,000 square kilometers) of Antarctic ice would be at risk. That’s a region much bigger than California.