Vast and white, easily visible from space, Earth’s Arctic ice cap seems such a permanent fixture—a frozen country at the top of the world—that the idea that it could ever vanish almost defies comprehension. But by the middle of the century most of it will in fact vanish, thanks to our burning of fossil fuels. The North Pole and most of the ocean around it will be free of sea ice in summer for the first time in thousands of years.

Will we ever bring the ice back?

That’s the question a team of researchers from Columbia University pondered in a presentation last week at the annual meeting of the American Geophysical Union in San Francisco. It’s not as academic as it