Arctic sea ice has melted to a level not recorded since satellite observations started in 1972 – and almost certainly not experienced for at least 8,000 years, say polar scientists.
Daily satellite sea-ice maps released by Bremen university physicists show that with a week's more melt expected this year, the floating ice in the Arctic covered an area of 4.24 million square kilometres on 8 September. The previous one-day minimum was 4.27m sq km on 17 September 2007.
The US National Snow and Ice Data Centre (NSIDC) in Boulder, Colorado, which also tracks the extent of sea ice, has not posted data for a week but is expected to announce similar results in the next few days.
The German researchers said the record melt was undoubtedly because of human-induced global warming. "The sea-ice retreat can no more be explained with the natural variability from one year to the next, caused by weather influence," said Georg Heygster, head of the Institute of Environmental Physics at Bremen.
"It seems to be clear that this is a further consequence of the man-made global warming with global consequences. Climate models show that the reduction is related to the man-made global warming, which, due to the albedo effect, is particularly pronounced in the Arctic," he said. The albedo effect is related to a surface's reflecting power – whiter sea ice reflects more of the sun's heat back into space than darker seawater, which absorbs the sun's heat and gets warmer.
Floating Arctic sea ice naturally melts and re-freezes annually, but the speed of change in a generation has shocked scientists – it is now twice as great as it was in 1972, according to the NSIDC, with a decline of about 10% per decade.
Arctic temperatures have risen more than twice as fast as the global average over the past half century.
Separate, less reliable, research suggests that Arctic ice is in a downward spiral, declining in area but also thinning. Using records of air, wind and sea temperature, scientists from the Polar Science Centre of the University of Washington, Seattle, announced last week that the Arctic sea-ice volume reached its lowest ever level in 2010 and was on course to set more records this year.
The new data suggests that the volume of sea ice last month appeared to be about 2,135 cubic miles – just half the average volume and 62% lower than the maximum volume of ice that covered the Arctic in 1979. The research will be published in a forthcoming issue of the Journal of Geophysical Research.
If current trends continue, a largely ice-free Arctic in the summer months is likely within 30 years –that is up to 40 years earlier than was anticipated in the last Intergovernmental Panel on Climate Change (IPCC) assessment report.
The last time the Arctic was uncontestably free of summertime ice was 125,000 years ago, at the height of the last major interglacial period, known as the Eemian.
"This stunning loss of Arctic sea ice is yet another wake-up call that climate change is here now and is having devastating effects around the world," Shaye Wolf, climate science director at the Centre for Biological Diversity in San Francisco told journalists.
Arctic ice plays a critical role in regulating Earth's climate by reflecting sunlight and keeping the polar region cool. Retreating summer sea ice is widely described by scientists as both a measure and a driver of global warming, with negative impacts on a local and planetary scale.
This year, both the North-west and North-east passages were mostly ice free, as they have been twice since 2008.
Last month, the 74,000-tonne STI Heritage tanker passed through the North-east Passage with the assistance of ice breakers in just eight days on its way from Houston, Texas, to Thailand.
The north-east sea route, which links the Atlantic to the Pacific, is likely to become a commercial ship operator's favourite, saving thousands of miles and avoiding tolls on the Suez Canal tolls.
Further evidence of dramatic change in the Arctic came last week from Alan Hubbard, a Welsh glaciologist at Aberystwyth University, who has been studying the Petermann glacier in northern Greenland for several years.
The glacier, which covers about 6% of the icecap, is 186 miles (300km) long and up to 3,280ft (1km) high. In August last year, a 100 square-mile (260 sq km) block of ice calved from the glacier. Photographs show that by July this year it had melted and disappeared.
"I was gobsmacked. It [was] like looking into the Grand Canyon full of ice and coming back two years later to find it full of water," said Hubbard.
Last year (2010) tied with 2005 as the warmest year on record.
• This article was amended on 16 September 2011 to remove an inaccurate quote.