Not bad for data jockeys. Turns out, Arctic ice is getting pretty thin. For most of the 1990s, Rothrock and his partners have been studying measurements of North Pole ice taken by U.S. Navy nuclear submarines. The Arctic Ocean is fairly deep in many places, and military submarines regularly glide beneath the ice, trying to remain strategically invisible to competing navies, while taking sonar soundings of the ice pack above. Since the end of the Cold War in 1990, the military has been increasingly willing to let scientists use their vessels, vehicles and aircraft to study the planet.
Arctic ice is very different than the stuff found on land, like glaciers.
For starters, ice formed in seawater never gets all that thick. Most of the actual freezing takes place on the bottom of the ice sheet, where ice contacts the ocean. Ice traps air, which acts as an insulator, which eventually slows down the ice-forming process. Even in the sub-freezing temperatures of midwinter, Rothrock commonly finds Arctic ice measurements only a few inches thick, although in some places it can pile up to 10 feet or more. The Navy has been recording ice data for decades, but like most Cold War calculations, the information has been top secret.