Major Antarctic glacier melting speeds up
27 June 2011, by Tamera Jones
One of Antarctica's largest glaciers is melting more than 50 per cent faster than it was just over 15 years ago, a major study has revealed.
A glacier meets the ocean in the Antarctic.
The research found that in 1994, the ice shelf that floats on the ocean in front of Pine Island Glacier in West Antarctica was already melting into the sea at a rate of just over 50 cubic kilometres per year.
But by 2009, that rate had risen to a worrying 80 cubic kilometres of ice each year.
Some scientists predict that if Pine Island Glacier melted entirely, it could raise sea levels by around 25cm.
The researchers who led the latest study say that while some of the melting they detected can be blamed on rising sea temperatures, the real problem is that more warm water is reaching a cavity beneath the ice shelf. This is making the ice shelf much more vulnerable to melting.
'The rate at which the ice shelf is melting has increased significantly, because more warm water is circulating in the cavity beneath it,' explains Dr Adrian Jenkins from the British Antarctic Survey (BAS), one of the study's co-authors.
The study revealed that the temperature of the ocean in Pine Island Bay has gone up by just 0.2° Celsius over the same period.
'This isn't enough to account for the increased melting,' says Jenkins.
But it wasn't until they sent an autonomous submarine, dubbed Autosub, underneath the ice shelf that they realised why it is melting so rapidly.
'The rate at which the ice shelf is melting has increased significantly, because more warm water is circulating in the cavity beneath it.'
Dr Adrian Jenkins, the British Antarctic Survey
The submarine, built and operated by the National Oceanography Centre, revealed an underwater ridge on the sea floor beneath the ice shelf. Jenkins and colleagues from BAS and Columbia University concluded that the ridge must have once slowed the glacier like a giant retaining wall.
But as soon as the receding glacier broke free from the ridge, at some point before the 1970s, this allowed warmer water access to the 'underbelly' of the ice shelf. The warm water would have made the cavity grow, allowing more warm water to flow in and meltwater to flow out, and allowing the glacier to accelerate towards the sea.
'The inner cavity didn't exist at all before, so this is the most likely explanation for why a subtle change in temperature can have a huge effect,' Jenkins says.
Even back in 1994, researchers found that warm water around the ice shelf was melting it 'rapidly', and already causing it to thin. This means the main glacier, which scientists call 'grounded' because it's on the land, drains more ice and contributes to sea-level rise.
Right now, Pine Island Glacier and the neighbouring Thwaites Glacier contribute about 0.25mm per year to global sea level rise.
Just two years ago, UK researchers found that the rate at which the glacier is thinning has accelerated, and that this thinning has spread inland. These scientists calculated that the central 'trunk' of the glacier lost four times more ice in 2006 than it did in 1995: around 10.2 cubic kilometres compared with 2.6 cubic kilometres.
Since then other measurements have suggested the situation is worsening. Scientists had always assumed that the warming ocean was the culprit and it's only with this new study that scientists have shown that this picture is too simplistic.
'Earlier research had found that the glacier is speeding up and thinning, which infers that the glacier has melted faster. Now we can actually say that's what's happening. No one could say this until now,' says Jenkins.
Pine Island Glacier is one of a handful of glaciers that transport huge amounts of ice from the West Antarctic Ice Sheet to the sea. It moves more ice than any other Antarctic glacier and is the fastest-moving in the entire continent.
The study is published in Nature Geoscience.
Stanley S. Jacobs, Adrian Jenkins, Claudia F. Giulivi & Pierre Dutrieux, Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf, Nature Geoscience, Published online 26 June 2011, doi:10.1038/ngeo1188
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