Scientists have never laboured under the illusion that Patagonia’s ice fields are small. The three major sheets – North, South and Cordillera Darwin – make up the largest expanse of ice in the Southern Hemisphere outside of Antarctica. The biggest of these, the South, stretches 350km across the Andes between Chile and Argentina. But to really get a grip on the size of these structures you have to look down as well as across.
Following a seven-year study of the region, glaciologists from the University of California, Irvine (UCI) and their partners in Argentina and Chile, have now concluded that Patagonian ice sheets are considerably larger than expected, containing roughly 40 times the ice volume of the European Alps and with some of the largest glaciers being as much as a mile (1,600 metres) thick.
These findings only became possible when the team abandoned the radar-based ground systems usually employed to measure glaciers and instead took to the skies, flying over the vast terrain in helicopters and fixed-wing aircraft equipped with gravimeters. These devices can determine the ice volume by reading changes in Earth’s gravitational field.
The reason Patagonia’s glaciers have proven so tricky to measure before is specific to the make-up of the ice in the region. Unlike the ice in Greenland or Antarctica, Patagonia’s ice is temperate (near to the melting point) with the result that it is spotted with big pockets of melt water. This feature means that the high-frequency radar soundings used elsewhere won’t work because they react too vigorously with the water at the surface.
‘We have to use lower radar frequencies in order to go by the wetter layers,’ explains Eric Rignot, chair of Earth system science at UCI. ‘But then the signals start interacting with the pockets which act like scattering centres, making the ice volume shine like a Christmas tree. Even with the lowest frequencies, at some point, there is just so much scattering, it doesn’t matter how much more power you radiate it just doesn’t work. Most radar-based ground systems in Patagonia have a hard time probing ice thicker than about 600-800 metres. That’s where gravity comes in.’
The new gravity-based data can now be used by scientists who model the dynamics of Patagonia’s glaciers, which are among the fastest moving in the world. Most of the ice slabs in the region have been thinning rapidly over the past four decades. Accurate models will allow scientists to better predict the reaction of the ice to climate change, its potential contribution to sea-level rise and flooding, and the ways in which it may need to be managed to ensure continued freshwater access for local people. ‘This is why having accurate maps of the ice thickness is a priority,’ says lead author Romain Millan. ‘It is fundamental to get the right contours and depth of the glacial valleys otherwise simulations of glacier retreat will always be wrong.’
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