For the past 50 years, the U.S. Navy has used submarines to patrol underneath the ice at the North Pole. These submarines can only surface where the ice shelf is less than three feet thick, so the Navy must routinely measure the thickness of the Arctic. Gore says their records show that "starting in the early '70s, [the ice shelf] started declining rapidly, both in the extent and the thickness. The North Pole is melting."
One consequence of this, as shown in An Inconvenient Truth, is a decline in polar bear populations as they drown. "They live on the ice and now the ice is sometimes 60 miles from the land. And the distance between the ice floes is very large, and so for the first time now in significant numbers they're finding them dying by drowning. ... Something similar is happening to the penguins near the South Pole."
Additionally, the melting North Pole could lead to even more global warming. "It's also really dangerous for us to have this big expanse of open water at the top of our world," Gore says. "Normally that's like a giant mirror. It's one of the ways the planet cools itself. But as it melts, the open ocean absorbs all that heat."