Arctic sea ice extent fell to an unprecedented annual low last weekend, shattering the record set in 2007 by nearly 20 percent.

Ice extent fell to 1.32 million square miles Sunday, leaving the Arctic's icy cap half the average size recorded from 1979 to 2000, the National Snow and Ice Data Center said yesterday.

The record comes more than three weeks after Arctic sea ice smashed the 2007 mark, 1.61 million square miles, catapulting the Far North into conditions that normally cautious sea ice experts have described as "undiscovered territory," "surprising" and "astonishing."

"If you want evidence for climate change, just look at this," said Walt Meier, a scientist at the National Snow and Ice Data Center. "Summer sea ice used to cover an area roughly the size of the lower 48 United States. We've now lost [the equivalent of] half of the United States in ice cover. That's a big area that is completely different than it used to be."

The Arctic's sea ice begins to thaw each spring, as sunlight reaches the Far North and temperatures rise. It normally hits its annual low in mid-September and reforms in autumn as sunlight begins to wane, bringing cooler temperatures.

That summer melt has intensified over the last decade, in what scientists say is a clear indicator of climate change. The five lowest summer sea ice minimums in the satellite era -- which began in 1979 -- have occurred in the last five years, outpacing the predictions of climate models.

Until the dramatic thaw in 2007, many scientists believed the Arctic wouldn't see an ice-free summer until the end of this century. Newer climate models project ice-free summers could begin in the 2030s.

But a few scientists, frustrated by the disconnect between model output and real life, are making more dramatic predictions.

Thaw outpaces the models

Peter Wadhams, a sea ice expert at Cambridge University, told reporters last week that the Arctic could become ice-free in summer by 2014 or 2015.

"My extreme view is that for summer, August to September, it could be ice-free in as soon as two or three years' time," he said.

James Overland, an oceanographer with the National Oceanic and Atmospheric Administration, wasn't as bold.

"We're talking about differences between five years to 20 years," he said. "People were talking even end of the century as recently as five years ago. We keep shortening the estimate. Most of us are moving toward what Peter said, but we're not quite ready to go that far."

Experts noted that climate models have underestimated the pace of Arctic sea ice loss in part because they haven't accurately captured the rate at which ice has thinned.

A few decades ago, a quarter of Arctic waters was capped by old ice, 10 to 12 feet thick, but that is hard to find these days. The area covered by sea ice that has survived three or more summer thaws has decreased by 17.2 percent per decade since 1980, according to research by NASA scientist Josefino Comiso. One- to 2-year-old "perennial" sea ice coverage has dropped 13.5 percent per decade over the same period.

The Arctic is now dominated by thin, "first-year" ice that is more vulnerable to rising temperatures, sunny days and ocean circulation patterns that can shove blocks of ice toward the warmer waters of the Atlantic Ocean.

When that ice melts, it opens wide patches of dark Arctic Ocean waters that absorb more heat than the reflective ice they replace, a process that accelerates Arctic warming.

This year, that thinner ice cover may have been pushed toward a record by a storm that hit the Arctic in early August, breaking up fragile sea ice thinned by long-term warming.

Opening to trade, fishing, oil and gas explorers, and tourists

In the meantime, the ongoing decline of sea ice is changing the face of Arctic. Rising temperatures are opening the once-forbidding region to new shipping, tourism, fishing, and oil and gas exploration.

The eight nations that make up the Arctic Council inked a search-and-rescue agreement last year, anticipating a rise in traffic as industry expands into the Far North. In Alaska, Royal Dutch Shell PLC spent the summer preparing to drill for oil in the Beaufort and Chukchi seas, while the Coast Guard expanded its summer patrols of U.S. Arctic waters.

And in northern Canada, an international crew of explorers took advantage of melting ice to pilot a 31-foot fiberglass boat, the Belzebub II, through the M'Clure Strait, which lies at the northwestern end of the Northwest Passage.

Those developments, coupled with this summer's historic melt, have Greenpeace Executive Director Kumi Naidoo "shit scared," he said yesterday at a briefing on the eve of climate talks among members of the U.N. General Assembly.

"What happens in the Arctic doesn't stay in the Arctic," he said, warning of the potential for future resource wars.

Arctic experts and representatives of Arctic nations have generally rejected such concerns, arguing that relationships established under the multinational Arctic Council have paved the way for cooperation in the developing north.

But Naidoo's warning -- "what happens in the Arctic doesn't stay in the Arctic" -- is a message echoed in recent scientific studies predicting that Arctic warming, and specifically the loss of the region's sea ice, could reshape winter weather patterns hundreds or thousands of miles away by altering the behavior of the jet stream.

Researchers suspect that Arctic warming intensified by sea ice's shrinking footprint had a hand in unusual weather over the past few years, including the powerful snowstorms that buried the U.S. East Coast in 2009-2010 and 2010-2011 (ClimateWire, Sept. 13).

Reporter Lisa Friedman contributed.