POTSDAM, Germany

--In 1845, Capt. Sir John Franklin of the British Royal Navy led 128 men on two iron-plated steam ships, Erebus and Terror, into the Arctic, where they eventually disappeared. The voyage was the culmination of four centuries of failed attempts to navigate the Northwest Passage--a direct route from Europe to Asia across the Arctic Ocean--and remains one of the greatest tragedies in the history of polar exploration.

Today, a far greater Arctic tragedy is unfolding: The Arctic sea-ice cap is melting. Just recently, an unprecedented new low was reached after decades of decline. Indeed, the ice cap's area has decreased by half since the 1980s, when summer sea-ice still extended over roughly 7 million square kilometers, as opposed to less than 4 million today. It is now likely smaller than it has been for at least a millennium and a half.

In 2007, the Northwest Passage was ice-free for the first time in living memory. Boats of all sizes--including cruise ships--have sailed through easily in summers since then.

Walt Meier of the United States' National Snow and Ice Data Center describes today's ice cap as "crushed ice." And it is getting thinner. In the last three decades, its volume has shrunk by roughly three-quarters. As the University of Laval's Louis Fortier puts it, "we are three-quarters of the way to ice-free summers in the Arctic Ocean."

In addition to the sea-ice loss, satellite data show that Greenland's three-kilometer-thick continental ice sheet is also melting at a record rate. In July, 97 percent of the sheet's surface was affected. The meltwater runoff in western Greenland was so strong that it swept away an important road bridge across the Watson River.

This ice loss, caused largely by human-induced global warming, has far-reaching environmental, geopolitical, and economic consequences.

For starters, Greenland's meltwater is flowing into the ocean, raising global sea levels. As temperatures have increased, the sea level's rise has accelerated from one centimeter per decade in the early 20th century to more than three centimeters in each of the last two decades--an overall increase of nearly 20 centimeters since 1900. While the numbers may seem small, the rise significantly increases the likelihood of severe flooding along vulnerable coasts worldwide.