Antarctic Peninsula in 'dramatic' ice loss

Satellites have seen a sudden dramatic change in the behaviour of glaciers on the Antarctica Peninsula, according to a Bristol University-led study.

The ice streams were broadly stable up until 2009, since when they have been losing on the order of 56 billion tonnes of ice a year to the ocean.

Warm waters from the deep sea may be driving the changes, the UK-based team says.

The details of the satellite research are published in Science Magazine.

They include more than 10 years of space observations of a broad swathe of coastline roughly 750km in length, on the south-western sector of the peninsula.

Here there is a multitude of glaciers slipping down mountainous terrain and terminating in the Bellingshausen Sea.

"Around 2009/2010, the surface in this part of the southern Antarctic Peninsula started to lower at a really quite dramatic rate, of 4m per year in some places. That's a pretty big signal," said Bristol's Prof Jonathan Bamber.

"The total loss of ice per year is about 60 cubic km. Just to put that into some kind of context: 4 cubic km is roughly equivalent to the domestic water supply of the UK every year."

Antarctica's contribution to sea level rise from melting ice, although growing, is still less than 0.5mm per year. The reported behaviour, however, would mean the south-western peninsula sector now has the second biggest input to that contribution behind the large glaciers that drain into the Amundsen Sea even further to the south and west.

One of the key elements of the new study was the use of the European Space Agency's Cryosat platform, which circles the Earth at a height of over 700km.

This satellite has a remarkable radar altimeter that measures the shape of the ice surface below, and this instrument can be tuned to see rugged regions like the peninsula with a previously unobtainable resolution.

For a check on its work, the Bristol team also used a completely different type of measurement from the US space agency's Grace satellites. This pair of platforms senses the Earth's gravity field and can, in a coarse way, calculate how much ice mass has been lost from a particular region of the continent. These observations are said to be in good agreement with the altimetry data.

The scientists say the Antarctic climate models indicate no significant changes in snowfall or air temperature over the study period, which leads them to think the rapid ice loss is the result of warmer ocean waters.

"The westerly winds flowing around Antarctica have increased in strength in recent decades, probably as a result of global warming and changes in the ozone hole," explained lead author Dr Bert Wouters.

"Now, because these winds have become much stronger, they are pushing more water from the deep ocean on to the continental shelf of Antarctica. This water is relatively warm. It's not warm like in Majorca, for example, but it has a temperature of 1-2 degrees centigrade, which is above the freezing temperature of ice, so it carries enough heat to melt the glaciers and their ice shelves from below."

Some other glaciologists who have seen the Science paper are concerned the numbers reported by Wouters and colleagues may be too high. Certainly, they are out of step with recent studies that could find losses that were only a third to a half as big.

Last year, Veit Helm and colleagues reported annual losses of about 35 cubic km from the entire peninsula. Malcolm McMillan and colleagues, also in 2014, calculated the deficit to be about 25 cubic km per annum, again from the whole peninsula.

Cryosat's principal scientific adviser, Prof Andy Shepherd, said: "Cryosat first spotted this pattern of thinning last year, and although the basic measurements in this new study do seem to be consistent with the older estimates, I think the extremely high rates of ice loss that have been computed are incredible.

"For this much ice to have been lost so quickly, the glaciers would need to have speeded up dramatically, but all the evidence suggests that just hasn't happened. So I will treat these huge ice losses with caution for the time being," the Leeds University researcher told BBC News.

At issue will be the models that describe snow conditions on the ice. The top snow is much less dense than the underlying ice, and if the elevation changes observed by the satellites are attributed to the wrong fraction then the calculation of any mass loss will go awry.

But the Bristol team is adamant that it has captured the situation properly.

"We've done a very thorough and careful analysis of all the various processes, to separate out the impact of each of those processes - and the dynamic signal is so strong it's pretty unequivocal," said Prof Bamber.

"The other thing to say about the other papers is that they were looking at the whole continent whereas we really drilled down into this area."

The way the south-western sector of the peninsula behaves is being closely monitored.

Much of the coastal ice actually sits below sea level, with the bedrock under the glaciers sloping back towards the land interior.

It is a geometry that the theorists is say is potentially unstable, and makes the region's ice streams particularly sensitive to any changes in the temperature of ocean water.