By Jonathan Amos
Science reporter, BBC News, San Francisco
ICE BUOY - MEASURING SEA-ICE THICKNESS IN THE ARCTIC
(1) Data collector, batteries, barometer, satellite transmitter; (2) Satellite antenna; (3) Sensor to measure temperature through ice to the water; (4) Air temperature sensor; (5) Pinger monitors surface melting/growth; (6) Pinger monitors underside erosion/growth
Scientists have detailed what was an extraordinary melting season in the Arctic during the summer of 2007.
The record withdrawal of sea ice has been well documented, but the region also hit a number of other firsts.
Some ocean temperature measurements were unprecedented, and 2007 also set a new record for melting snow over the Greenland ice sheet.
The researchers told a meeting of the American Geophysical Union that some of the observations had been astonishing.
"The further you go down this path, the harder it is to get back," observed Don Perovich from the US Army Cold Regions Research and Engineering Laboratory.
"Things could come back, but basically it's the fourth quarter and we're down two touchdowns," he said, using an analogy from American football.
The big thin
The extent of the sea-ice cover fell to a record minimum in September of 4.13 million sq km (1.59 million sq miles), beating the previous low mark, set in 2005, by 23%.
This was well publicised at the time, but some of the other "Arctic numbers" have not been so widely reported.
Scientists say they demonstrate the step changes in environmental conditions in the northern polar region.
Dr Perovich himself has a number of ice mass balance buoys dotted around the Arctic measuring ice thickness.
One of them, in the Beaufort Sea, saw a 3.3m-thick (130 inches) slab of perennial sea ice reduced to just half a metre. The slab lost some 70cm (27 inches) off the top and 2.2m (86 inches) off the bottom.
The ablation on the bottom is five times what one would normally expect.
Dr Peter Wadhams, an expert who studies thickness using sonar data from submarines, called this observation "unprecedented".
"It's incredible, amazing," he told BBC News. "Ice cover can't stand that for many years: it'll all go."
Dr Perovich told the meeting: "One of the major concerns is that there's less ice there than we think. Ice thickness is a really important component in all this."
Warmth from below
A big driver behind the melt is the current warmth of the waters in the Arctic. In the summer of 2007, Arctic Ocean surface temperatures hit new maximums.
In waters just north of the Chukchi Sea (above Alaska and Eastern Siberia), sea-surface temperatures (SSTs) were 3.5C warmer than the historical average and 1.5C warmer than the historical maximum. SSTs of 4C were recorded.
This warming was probably the result of having increasing amounts of open water that readily absorb the sun's rays, a phenomenon known as the ice-albedo feedback: less ice means less reflection and more absorption, leading to more warming and more melting.
Warm waters from the Pacific Ocean are also flowing into the Arctic basin through the Bering Strait.
"In 2007, we had off-the-charts warming," said Dr Michael Steele, from the Polar Science Center, Applied Physics Laboratory at the University of Washington.
But it is not just at the surface. The warmth of Atlantic sub-surface waters (100-500m down) that move into the Arctic are also in "record territory", the AGU meeting heard.
And this picture is reflected on land, where a new record was set on the Greenland ice sheet, with snowmelt for areas above 2,000m observed to be 150% above the long-term average for the period 1988-2006.
These high elevation areas melted for 25-30 days longer this year than the observed average in the previous 19 years.
At lower altitudes, the melt was still higher than average by 30%, but it did not break any record. This year can be put in fifth place over the observed average, behind 2005, 2002, 1998 and 2004.
Dr Marco Tedesco from the University of Maryland and the Nasa Goddard Space Flight Center said the increased melting was probably the consequence of extremely warm temperatures over the Greenland ice sheet.
"Another factor which we think might be one of the concurrent causes for the record we observe is the low accumulation [of snow] for these areas," he added.
"Low accumulation means that the older snow, which absorbs more solar radiation than young snow, was exposed earlier than previous years," he added.
There has been much talk at the meeting of climate "tipping points", and, in the case of the Arctic, when precisely the ocean will be totally free of ice in summer months.
One group of modellers came to the AGU conference with a projection of 2013.
While a number of scientists thought that date a little early, they said ice-free summers would come sooner than many people thought possible.
"If I could take a different angle on that question, and rather than look at when the last piece of ice goes away, ask 'when does the ice cover change in summer in a way that affects people?'," Dr Perovich said.
"I think we're already headed in that direction. This past year, the Northwest Passage was open; there's more potential for exploration for resources; we're already seeing for coastal villages that erosion is a serious problem.
"So, we're already beginning to see impacts on people that, as the melt continues, will just become more obvious."