In 1845, Captain 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. Last month, 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 seven million square kilometers, as opposed to less than four 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.

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.

Greenland's meltwater accounts for one-fifth of the global sea-level rise over the last decade. If its ice sheet melted completely, sea levels would rise by seven meters - meaning that we cannot afford to lose even a small fraction of the ice sheet. Meanwhile, satellite data show that Antarctica's ice sheet, which is 10 times larger than Greenland's, is losing ice as well.

The vanishing Arctic sea ice also affects the atmosphere. Less ice reflects less sunlight, and more open, darker ocean absorbs more heat, which is then released into the atmosphere, affecting wind and pressure patterns throughout the northern hemisphere.

Further exacerbating the problem, the disappearance of Arctic sea ice has triggered a rush to secure newly accessible resources, particularly fossil fuels, which are a primary cause of global warming.

The longer powerful interests deny humanity's contribution to global warming, the more difficult it will be to arrest and reverse its effects. One hopes that the satellite footage of the Arctic meltdown will help to inspire serious action.

Stefan Rahmstorf is professor of physics of the oceans at Potsdam University and department head at the Potsdam Institute for Climate Impact Research. His most recent book is "The Climate Crisis." Copyright: Project Syndicate