NASA has discovered the Arctic has lost more sea ice this year than at any time since satellite records began in 1979.
Scientists involved in the calculations say it is part of a fundamental change.
What is more, sea ice normally reaches its low point in September so it is thought likely that this year’s melt will continue to grow.
NASA says the extent of sea ice was 1.58 m sq miles (4.1 m sq km) compared with a previous low of 1.61 m sq miles (4.17 m sq km) on 18 September 2007.
The sea ice cap grows during the cold Arctic winters and shrinks when temperatures climb again, but over the last three decades, satellites have observed a 13% decline per decade in the summertime minimum.
The thickness of the sea ice is also declining, so overall the ice volume has fallen far – although estimates vary about the actual figure.
Joey Comiso, senior research scientist at NASA’s Goddard Space Flight Center, said this year’s ice retreat was caused by previous warm years reducing the amount of perennial ice – which is more resistant to melting. It’s created a self-reinforcing trend.
“Unlike 2007, temperatures were not unusually warm in the Arctic this summer. [But] we are losing the thick component of the ice cover,” he said.
“And if you lose [that], the ice in the summer becomes very vulnerable.”
Walt Meier, from the National Snow and Ice Data Center that collaborates in the measurements, said: “In the context of what’s happened in the last several years and throughout the satellite record, it’s an indication that the Arctic sea ice cover is fundamentally changing.”
Professor Peter Wadhams, from Cambridge University, said: “A number of scientists who have actually been working with sea ice measurement had predicted some years ago that the retreat would accelerate and that the summer Arctic would become ice-free by 2015 or 2016.
“I was one of those scientists – and of course bore my share of ridicule for daring to make such an alarmist prediction.”
But Prof. Peter Wadhams said the prediction was now coming true, and the ice had become so thin that it would inevitably disappear.
“Measurements from submarines have shown that it has lost at least 40% of its thickness since the 1980s, and if you consider the shrinkage as well it means that the summer ice volume is now only 30% of what it was in the 1980s,” he added.
“This means an inevitable death for the ice cover, because the summer retreat is now accelerated by the fact that the huge areas of open water already generated allow storms to generate big waves which break up the remaining ice and accelerate its melt.
“Implications are serious: the increased open water lowers the average albedo [reflectivity] of the planet, accelerating global warming; and we are also finding the open water causing seabed permafrost to melt, releasing large amounts of methane, a powerful greenhouse gas, to the atmosphere.”
Opinions vary on the date of the demise of summer sea ice, but the latest announcement will give support to those who err on the pessimistic side.
A recent paper from Reading University used statistical techniques and computers to estimate that between 5-30% of the recent ice loss was due to Atlantic Multi-decadal Oscillation – a natural climate cycle repeating every 65-80 years. It’s been in warm phase since the mid 1970s.
But the rest of the warming, the paper estimates, is caused by human activity – pollution and clearing of forests.
If the ice continues to disappear in summer there will be opportunities as well as threats.
Some ships are already saving time by sailing a previously impassable route north of Russia.
Oil, gas and mining firms are jostling to exploit the Arctic – although they’re being strongly opposed by environmentalists. Greenpeace has been protesting at drilling by the Russian giant Gazprom.
Among the many threats, the warming is bad for Arctic wildlife. Thanks to the influence of sea ice on the jet stream the changes could affect weather in the UK.
The changes – if they happen – could unlock frozen deposits of methane which would further overheat the planet.
Warmer seas could lead to more melting of Greenland’s ice cap which would contribute to raising sea levels and changing the salinity of the sea, which in turn could alter ocean currents that help govern our climate.
Cryosat, the European radar spacecraft launched in 2010 to monitor changes in the thickness and shape of polar ice, is now watching the ebb and flow of Arctic sea ice with high precision.
Scientists have spent the past two years getting to grips with its data.
And on Tuesday, they reported that Cryosat was now delivering an unprecedented view of the seasonal growth and retreat of sea ice spanning the entire Arctic basin.
The researchers also released a map showing the difference in height across the Greenland ice sheet.
“The message is that Cryosat is working extremely well. Its data are very reliable and the measurements we have match reality,” said Prof. Volker Liebig, the director of Earth Observation at the European Space Agency (ESA).
“We now have a very powerful tool to monitor the changes taking place at the poles,” he said.
The ESA director delivered an update on the mission at London’s Royal Society. The information was also being released here at the European Geosciences Union (EGU) meeting in Vienna, Austria.
Several satellites have already detailed the recent and rapid erosion of summer sea ice extent as the Arctic has warmed.
But Crysosat’s innovation has been to provide a means to get at a figure for ice volume – a far more significant number in terms of understanding the long-term viability of the ice.
To do this, the satellite carries one of the highest resolution synthetic aperture radars ever put in orbit.
The instrument sends down pulses of microwave energy which bounce off both the top of the ice and the water in the cracks, or leads, which separate the floes.
Cryosat was launched in 2010 to monitor changes in the thickness and shape of polar ice
By measuring the difference in height between these two surfaces, scientists can, using a relatively simple calculation, work out the overall volume of the marine cover.
The Cryosat team, led from University College London, has spent the period since launch working through the satellite’s measurements, validating and calibrating them against a number of independent observations.
These include data from plane-borne instruments, from direct on-the-ice assessments, and even from scientific sea-floor moorings that profile the ice floes as they pass overhead.
“We can now say with good confidence that Cryosat’s maps of ice thickness are correct to within 10-20 cm,” said Dr. Seymour Laxon, from UCL’s Centre for Polar Observation and Modelling (CPOM).
Tuesday’s release shows a complete seasonal cycle, from October 2010, when the Arctic Ocean was beginning to freeze up following the summer melt, right through to March 2011, when the sea ice was approaching peak thickness. Cryosat found the volume (area multiplied by thickness) of sea ice in the central Arctic in March 2011 to have been 14,500 cubic kilometres.
This figure is very similar to that suggested by PIOMAS (Panarctic Ice Ocean Modeling and Assimilation System), an influential computer model that has been used to estimate Arctic sea ice volume, and which has been the basis for several predictions about when summer sea ice in the north might disappear completely.
In addition to the announcement on sea ice, the Cryosat team also published a digital elevation model (DEM) of Greenland.
The big island, too, has experienced some rapid changes of late and is losing tens of billions of tons of its ice cover to the ocean annually.
The DEM is a map of varying height, and the visualization on this page incorporates a year’s worth of data.
For Cryosat, it is another illustration of its capability. Radar satellites have traditionally struggled to discern the detail in the steep slopes and ridges that mark the edges of ice sheets, but the ESA spacecraft can recover far more information thanks to a special interferometric observing mode that uses two antennas.
“This is really the first demonstration of the interferometer in action,” said Prof. Andrew Shepherd from Leeds University.
“The DEM contains about 7.5 million data points, and we’re pretty confident this will be the best elevation model for Greenland, by some margin. Our next step is to compare it to previous data to see how Greenland has changed.”
Cryosat’s principal investigator, Prof. Duncan Wingham – formerly of UCL but now chief executive of the UK’s National Environment Research Council – summed up: “We have years of data to come, but I think it’s quite clear that we will provide synoptic, accurate, Arctic-wide thickness; and that we will be able to determine the accuracy of the predictions of when the Arctic will be ice-free in Summer.
“And I think it’s also clear we can now sustain coverage of [ice sheets on Antarctica and Greenland] right down to the coast.”
The Cryosat update was timed to coincide with this week’s 50th anniversary of UK activity in orbit.
April 1962 was the month Britain became a space-faring nation with the launch of its first satellite, Ariel-1.
