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Even as debate continues about the role global warming has played in the recent burst of violent hurricanes, more bad news on the climate front emerged today from a decidedly untropical part of the world: the Arctic. According to a study sponsored by the National Snow and Ice Data Center (NSIDC) and other groups, the Arctic ice sheet—a frozen expanse measuring millions of square miles—is shrinking faster than was previously thought. At the current rate it could melt away to nothing before the end of the century.

The numbers are straightforward. In the past 50 years, air temperatures across the Arctic Ocean have climbed by as much as 5.4 degrees F—huge by climate standards. This has had an unsurprising effect on the ice. Ordinarily, the ice sheet expands and shrinks with the changing seasons, dwindling to its smallest size in late September. Even then, however, it used to measure about 2.7 million sq. mi., roughly the size of the contiguous U.S. Not anymore. On the last day of summer this year, the ice measured just 2.05 million sq. mi.—a loss of area twice the size of Texas. That continues a four-year trend of dwindling ice, reducing the sheet to perhaps the smallest size ever recorded in the 100 years measurements have been taken.

Even a little shrinking can be a self-reinforcing process. Ice reflects sunlight back into space, helping to keep Earthly temperatures under control. But dark seawater absorbs the sun's energy, raising the temperature of both the Arctic and the planet as a whole. What's more, a smaller ice sheet in summer means less to build on in winter, when temperatures plummet and the ice should rebound. In the winter of 2004 and 2005, the rate of regrowth was the smallest ever measured.

If you're a whale or a codfish—which use the Arctic waters for breeding—this is good news. It's also a bonus for cargo ships taking advantage of a shortcut through the open sea. For other animals—to say nothing of humans—it could spell disaster. Polar bears spend the summer stranded on land, surviving on fat reserves and waiting until the ice creeps back, when they can hop aboard and resume their wintertime fishing. More and more hungry bears are now remaining marooned later and later in the season.

For humans, melting Arctic ice can mean rising seas, but less than we might think. Since the ice cap already floats on water, the mere act of melting has little or no effect on sea level. But the general warming of the oceans does, since warm water expands, increasing overall volume and eroding already inundated coastlines. What's more, if melting worsens global warming, and global warming is widely believed to have helped fuel the killer hurricanes, all the other dangers associated with higher temperatures—from droughts to crop failures to the migration of tropical diseases—start to look more real.

The question, as always, is whether all of this is a part of natural cycling, or whether the greenhouse gasses we produce in such abundance have simply busted a fragile system. Ocean temperatures historically run in cycles, with shifting currents carrying warmth to different parts of the world at different times. But the planet's own metabolism and the damage we do to it with our industrial exhausts don't exist separately. Greenhouse emissions undeniably raise global temperatures. Whether we're entirely responsible for the loss of Arctic ice or only exacerbating a natural phenomenon, now is the time to dial down the gasses. The planet—as Katrina and Rita showed—knows how to hit back.

• Jeffrey Kluger
• Global Warming Update: A new study shows the Arctic ice cap is shrinking faster than expected