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Summer ice is melting at a faster rate in the Antarctic peninsula than at any time in the last 1,000 years, new research has shown.
The evidence comes from a 364-metre ice core containing a record of freezing and melting over the previous millennium.
Layers of ice in the core, drilled from James Ross Island near the northern tip of the peninsula, indicate periods when summer snow on the ice cap thawed and then refroze.
By measuring the thickness of these layers, scientists were able to match the history of melting with changes in temperature.
Lead researcher Dr Nerilie Abram, from the Australian National University and British Antarctic Survey (BAS), said: "We found that the coolest conditions on the Antarctic peninsula and the lowest amount of summer melt occurred around 600 years ago.
"At that time temperatures were around 1.6C lower than those recorded in the late 20th century and the amount of annual snowfall that melted and refroze was about 0.5%.
"Today, we see almost 10 times as much (5%) of the annual snowfall melting each year.
"Summer melting at the ice core site today is now at a level that is higher than at any other time over the last 1,000 years. And while temperatures at this site increased gradually in phases over many hundreds of years, most of the intensification of melting has happened since the mid-20th century."
Levels of ice melt on the Antarctic peninsula were especially sensitive to rising temperature during the last century, he said.
"What that means is that the Antarctic peninsula has warmed to a level where even small increases in temperature can now lead to a big increase in summer melt," Abram added.
Dr Robert Mulvaney, from the British Antarctic Survey, who led the ice core drilling expedition in 2008 and co-authored a paper on the findings published on Sunday in the journal Nature Geoscience.
He said: "Having a record of previous melt intensity for the Peninsula is particularly important because of the glacier retreat and ice shelf loss we are now seeing in the area.
"Summer ice melt is a key process that is thought to have weakened ice shelves along the Antarctic peninsula leading to a succession of dramatic collapses, as well as speeding up glacier ice loss across the region over the last 50 years."
The ice core record suggested a link between accelerated melting and man-made global warming. But a different and more complex picture has emerged from another region of Antarctica.
A separate US study, published in the same journal, shows that thinning ice from the West Antarctic Ice Sheet Divide cannot confidently be blamed on greenhouse gas emissions.
An ice core record from this site indicates a strong influence from unusual conditions in the tropical Pacific during the 1990s.
In that decade, an El Niño event – a cyclical system of winds and ocean currents that can affect the world's weather – caused rapid thinning of glaciers in the west Antarctic.
Summer ice is melting at a faster rate in the Antarctic peninsula than at any time in the last 1,000 years, a new study has shown. Photo courtesy: Nasa/AFP/Getty Images
The spike in temperature was little different from others that occurred in the 1830s and 1940s, which also saw prominent El Niño events.
"If we could look back at this region of Antarctica in the 1940s and 1830s we would find that the regional climate would look a lot like it does today, and I think we also would find the glaciers retreating much as they are today," said lead author Prof Eric Steig, from the University of Washington.
He said the same was not true for the Antarctic peninsula, the part of the continent closer to South America. Here, more dramatic changes were "almost certainly" a result of human-induced global warming.
Here's a video of a city-sized glacier collapsing.
The Greenland ice sheet on July 8, left, and four days later on the right. An estimated 97% of the ice sheet surface had thawed by July 12. Photo courtesy: Nasa
The Greenland ice sheet melted at a faster rate this month than at any other time in recorded history, with virtually the entire ice sheet showing signs of thaw.
The rapid melting over just four days was captured by three satellites. It has stunned and alarmed scientists, and deepened fears about the pace and future consequences of climate change.
In a statement posted on Nasa's website on Tuesday, scientists admitted the satellite data was so striking they thought at first there had to be a mistake.
"This was so extraordinary that at first I questioned the result: was this real or was it due to a data error?" Son Nghiem of Nasa's jet propulsion laboratory in Pasadena said in the release.
He consulted with several colleagues, who confirmed his findings. Dorothy Hall, who studies the surface temperature of Greenland at Nasa's space flight centre in Greenbelt, Maryland, confirmed that the area experienced unusually high temperatures in mid-July, and that there was widespread melting over the surface of the ice sheet.
Climatologists Thomas Mote, at the University of Georgia, and Marco Tedesco, of the City University of New York, also confirmed the melt recorded by the satellites.
However, scientists were still coming to grips with the shocking images on Tuesday. "I think it's fair to say that this is unprecedented," Jay Zwally, a glaciologist at Nasa's Goddard Space Flight Center, told the Guardian.
The set of images released by Nasa on Tuesday show a rapid thaw between July 8-12, 2012. Within that four-day period, measurements from three satellites showed a swift expansion of the area of melting ice, from about 40% of the ice sheet surface to 97%.
Zwally, who has made almost yearly trips to the Greenland ice sheet for more than three decades, said he had never seen such a rapid melt.
About half of Greenland's surface ice sheet melts during a typical summer, but Zwally said he and other scientists had been recording an acceleration of that melting process over the last few decades. This year his team had to rebuild their camp, at Swiss Station, when the snow and ice supports melted.
He said he was most surprised to see indications in the images of melting even around the area of Summit Station, which is about two miles above sea level.
It was the second unusual event in Greenland in a matter of days, after an iceberg the size of Manhattan broke off from the Petermann Glacier. But the rapid melt was viewed as more serious.
"If you look at the 8 July image that might be the maximum extent of warming you would see in the summer," Zwally noted. "There have been periods when melting might have occurred at higher elevations briefly – maybe for a day or so – but to have it cover the whole of Greenland like this is unknown, certainly in the time of satellite records."
Lora Koenig, another Goddard glaciologist, told Nasa similar rapid melting occurs about every 150 years. But she warned there were wide-ranging potential implications from this year's thaw.
"If we continue to observe melting events like this in upcoming years, it will be worrisome." she told Nasa.
The most immediate consequences are sea level rise and a further warming of the Arctic. In the centre of Greenland, the ice remains up to 3,000 metres deep. On the edges, however, the ice is much, much thinner and has been melting into the sea.
The melting ice sheet is a significant factor in sea level rise. Scientists attribute about one-fifth of the annual sea level rise, which is about 3mm every year, to the melting of the Greenland ice sheet.
In this instance of this month's extreme melting, Mote said there was evidence of a heat dome over Greenland: or an unusually strong ridge of warm air.
The dome is believed to have moved over Greenland on 8 July, lingering until 16 July.
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