By Jason Palmer Science and technology reporter, BBC News
Scientists drilling deep into the edge of modern Antarctica have pulled up proof that palm trees once grew there.
The study in Nature suggests Antarctic winter temperatures exceeded 10C, while summers may have reached 25C.
Better knowledge of past "greenhouse" conditions will enhance guesses about the effects of increasing CO2 today.
The early Eocene - often referred to as the Eocene greenhouse - has been a subject of increasing interest in recent years as a "warm analogue" of the current Earth.
Now, the Integrated Ocean Drilling Program (IODP) has literally got to the bottom of what the Eocene Antarctic was like, dropping a drilling rig through 4km of water off Wilkes Land on Antarctica's eastern coast.
The rig then drilled through 1km of sediment to return samples from the Eocene. With the sediment came pollen grains from palm trees and relatives of the modern baobab and macadamia.
Crucially, they contained also the remnants of tiny single-celled organisms called Archaea.
The creatures' cell walls show subtle molecular changes that depend on the temperature of the soil surrounding them when they were alive. The structures are faithfully preserved after they die.
They are, in essence, tiny buried thermometers from long ago.
Together, the data suggest that even in the darkest period of Antarctic winter, the temperature did not drop below 10C; and summer daytime temperatures were in the 20Cs.
The lowland coastal region sported palm trees, while slightly inland, hills were populated with beech trees and conifers.
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By Richard Black Environment correspondent, BBC News
A rift in the Antarctic rock as deep as the Grand Canyon is increasing ice melt from the continent, researchers say.
A UK team found the Ferrigno rift using ice-penetrating radar, and showed it to be about 1.5km (1 mile) deep.
Antarctica is home to a geological rift system where new crust is being formed, meaning the eastern and western halves of the continent are slowly separating.
The team writes in Nature journal that the canyon is bringing more warm sea water to the ice sheet, hastening melt.
The Ferrigno rift lies close to the Pine Island Glacier where Nasa scientists found a giant crack last year; but the newly discovered feature is not thought to be influencing the "Pig", as it is known.
The rift lies beneath the Ferrigno Ice Stream on a stretch of coast so remote that it has only been visited once previously.
The team towed ice-penetrating radar kit behind a snowmobile, traversing a total of about 2,500km (1,500 miles).
"What we found is that lying beneath the ice there is a large valley, parts of which are approximately a mile deeper than the surrounding landscape," said Dr Bingham.
"If you stripped away all of the ice here today, you'd see a feature every bit as dramatic as the huge rift valleys you see in Africa and in size as significant as the [US] Grand Canyon.
Penetrating between the Antarctic bedrock and the ice that lies on it and lubricating the join, the water allows ice to flow faster into the sea.
"We know that the ice loss from the West Antarctic Ice Sheet is governed by delivery of warm water, and that the warm water is coming along channels that were previously scoured by glaciers," said Prof David Vaughan of BAS.
"So the geology and the present rate of ice loss are intricately linked, and they feed back - if you have fast-flowing ice, that delivers ice to the edge where it can be impacted by warm water, and warm water makes the ice flow faster," he told BBC News.
Prof Vaughan doubted there would be more such features around the West Antarctic coast, though in the remoter still regions of the east, it was a possibility.
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By Jonathan Amos Science correspondent, BBC News, Vienna
Most of the ice being lost from Antarctica is going as a result of warm water eating the fringes of the continent, scientists say.
The researchers used a satellite laser to measure the thinning occurring on ice shelves - the floating tongues of ice that jut out from the land.
The team's analysis found the shelves' shrinkage could not be attributed simply to warmer air.
Rather, it is warm water getting under the floating ice to melt it from below.
This is leading to a weakening of the shelves, permitting more and more ice to drain from the continent's interior through tributary glaciers.
"What we realise now is that we're looking at a very sensitive system," Dr Hamish Pritchard, from the British Antarctic Survey (BAS), told BBC News.
"Previously, you would have thought that we needed a lot of warming in the atmosphere to get a substantial loss of ice from Antarctica - because it's such a cold place. But what we show is that that's not necessary; you don't need radical change.
"All you need are quite subtle changes - such as a change in the winds - and that can produce effects at the edges of Antarctica that then lead to a loss of a lot of ice."
Some of the greatest melting has been seen where deep troughs cut across the continental shelf, allowing the water easier access to the shelves' undersides.
The picture is not uniform all around Antarctica. Indeed, on the peninsula - the long stretch of land pointing towards South America - the shelves show a different set of thinning symptoms, which very probably can be tied to a warming atmosphere.
But again, the originator is probably the same changing wind patterns.
"Strong westerlies go up over the chain of the peninsula mountains and these winds descend, they warm up, melting the surface of the shelves on the eastern side. So, although we have two different melting mechanisms, the ultimate cause is the same - it's the wind," said Dr Pritchard.
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Antarctic ... Summers were in the low 20c and the winters during the 3 months of darkness the temperature did not go below 10c. (Also noted by one researchers(Dr James Bendle) at the end of an interview said: 'If all the ice melted it could raise the ocean sea levels 60 meters.').
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By Alex Kirby - BBC News Online environment correspondent.
The Arctic Coring Expedition (Acex) has recovered sediment cores from nearly 400m (1,300ft) below the sea floor.
It says fossilised algae in the cores show the sea temperature was once about 20C, instead of the average now, -1.5C.
The expedition, which has relied on three icebreakers during its work, is now heading back to Tromso in Norway.
Unlocking the Arctic's history
The scientists, from eight nations, recovered the cores from below the sea floor in waters 1,300m (4,260ft) deep.
Acex has been taking cores from the Lomonosov Ridge between Siberia and Greenland. The ridge, 1,500km (930 miles) long, rises to 800m (2,625ft) below sea level and is topped by 450m (1,475ft) of layered sediments.
The scientists said before they set sail from Tromso last month their findings would help science to work out how long the Arctic sea ice, now in retreat, had persisted.
The cores they have extracted show the Arctic Ocean was once a subtropical, shallow sea. The evidence, Acex says, is in the form of tiny algal fossils found in the cores, which were once marine plants and animals.
It was characterized by an extremely warm climate that created a natural greenhouse effect, which caused massive amounts of carbon to be deposited in both sea and air.
The algae found in the Lomonosov cores, which lived only in subtropical conditions, prove how warm the Arctic once was, Acex says. It says the ocean's temperature was once similar to the waters off New York in August.
There is also evidence that part of the Arctic Ocean was once a freshwater lake.
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More info found at BBC news.
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