Arctic sea ice has shrunk to its smallest surface area since record-keeping began, taking the world into "uncharted territory" as climate change intensifies, US scientists warned Wednesday.

Satellite images show the ice cap has melted to 1.32 million square miles (3.4 million square kilometers) as of September 16, the predicted lowest point for the year.

That's the smallest Arctic ice cover since record-keeping began in 1979, according to the National Snow and Ice Data Center.

"We are now in uncharted territory," NSIDC director Mark Serreze said in a statement.

"While we've long known that as the planet warms up, changes would be seen first and be most pronounced in the Arctic, few of us were prepared for how rapidly the changes would actually occur."

Arctic sea ice expands and contracts seasonally, with the lowest extent usually occurring in September.

This year's minimum followed a season already full of records for shrinking ice, with the lowest ever extents recorded on August 26 and again on September 4.

And in the last two weeks, the ice cover melted by more than 200,000 square miles, a large margin for the end of the summer.

"The strong late season decline is indicative of how thin the ice cover is," said NSIDC scientist Walt Meier.

"Ice has to be quite thin to continue melting away as the sun goes down and fall approaches."

Scientists use Arctic sea ice extent as a gauge of the overall climate. Despite year-to-year fluctuations from natural weather variations, the ice cap has shown a clear trend toward shrinking over the last 30 years, according to the NSIDC.

"This year's minimum will be nearly 50 percent lower than the 1979 to 2000 average," it said.

The Colorado-based center said the Arctic is shifting in composition. Whereas most of the ice previously stayed frozen through several summers, much of it now melts and refreezes each season.

"Twenty years from now in August, you might be able to take a ship right across the Arctic Ocean," once blocked year-round by ice, said NSIDC scientist Julienne Stroeve.

Climate models predict "ice free conditions" before 2050, she added, but said the decline appears to be happening faster than predicted.

The NSIDC warned that increased heat and moisture from the melting Arctic ice cover could have global climate implications.

"This will gradually affect climate in the areas where we live," he said. "We have a less polar pole -- and so there will be more variations and extremes."

Environmental activist group Greenpeace lamented the announcement, expressing hope it would trigger a sense of urgency for action to slow the trend.

"In just over 30 years, we have altered the way our planet looks from space, and soon the North Pole may be completely ice free in summer," Greenpeace chief Kumi Naidoo said in a statement.

"I hope that future generations will mark this day as a turning point, when a new spirit of global cooperation emerged to tackle the huge challenges we face."

Scientists say climate change is caused when carbon dioxide and other human-produced gases rise into the atmosphere and make it more difficult for the planet to reflect the sun's heat back into space, creating a greenhouse effect.

As the ice cap melts over Greenland vast amounts of methane -- a greenhouse gas -- trapped in the permafrost are likely to be released.

The methane comes from the remains of the region's plant and animal life trapped in sediment and later covered by ice sheets in the last Ice Age.

Methane is 25 times more efficient at trapping solar heat than carbon dioxide.

As the methane is released into the atmosphere and the planet warms further due to the greenhouse effect, more ice in turn melts, freeing up locked-up carbon.