Using phase-sensitive radar - an exceptionally accurate version of the systems used by ships and aircraft to detect objects in their path - Dr Adrian Jenkins and colleagues from BAS studied the internal structure of the enormous Filchner-Ronne Ice Shelf, as well as the rate at which the bottom of the ice shelf is melting.

Lead author Dr Jenkins of BAS says, "The radar provides an unprecedented insight into the flow of the ice shelf. Internal structures are formed as layers of snow are laid down each year. These layers produce radar reflections that give us a totally new view of the internal workings of an ice sheet. This will help us understand how the ice flows and improve our ability to predict how the ice sheet as a whole will evolve in the future, which is important because growth or shrinkage of the ice sheet has a direct impact on global sea level."

As well as shedding new light on the makeup of the ice shelf, Dr Jenkins and his colleagues used the phase-sensitive radar to measure the rate at which the underside of the ice shelf is melting. These are the first-ever direct measurements of ice shelf melting and are extraordinarily accurate. According to Dr Jenkins,

"The new technique allows us to measure centimetre-scale changes in the 2-km thickness of the ice. We found that an average of 1 m of ice is melted from the bottom of the ice shelf every year. At this rate, all the ice lost by melting can be replenished by flow of ice from upstream, so that this part of the ice shelf is showing no signs of change. Elsewhere in Antarctica ice shelves and ice streams are thinning and now we have a tool to measure the thinning rates to unparalleled accuracy."

Ice sheet - is the huge mass of ice, up to 4 km thick, that covers Antarctica's bedrock. It flows from the centre of the continent towards the coast where it feeds ice shelves.

Ice shelf - is the floating extension of the grounded ice sheet. It is composed of freshwater ice that originally fell as snow, either in situ or inland and brought to the ice shelf by glaciers. As they are already floating any disintegration (like Larsen B) will have no impact on sea level. Sea level will rise only if the ice held back by the ice shelf flows more quickly onto the sea.

Located to the east of the Antarctic Peninsula, the Filchner-Ronne Ice Shelf is an Antarctic ice mass twice the size of Great Britain and contains some of the thickest floating ice anywhere on Earth. Ice continually flows into the ice shelf, while seawater flowing underneath it melts ice from its undersurface. If there is an imbalance between the amount of ice flowing into the ice shelf and that lost by melting, the ice shelf will grow or shrink.

Loss of ice shelves near the Antarctic Peninsula is one of the most obvious signs of climate change. Elsewhere in Antarctica ice shelves are shrinking, which most scientists believe is because of a recent increase in the rate at which the ocean melts the ice. So far, the Filchner-Ronne Ice Shelf - which is further south - seems to be immune from such changes.

The new technique is based on phase-sensitive radar. The radar signal is sent down from the surface of the ice shelf and bounces off the bottom. The time taken for the signal to reach the bottom and return to the surface is then converted to a distance. The exceptional accuracy in the timing, made possible through measurement of the phase of the returning signal, means that changes in distance of only a few millimetres can be detected. Until now, scientists have had to calculate ice shelf melting by indirect means that are far less accurate.

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