Scientists in the US have used on-the-ice measurements and military-grade satellite imagery to take a much closer look at just how Greenland’s icesheet melts.
They already knew that huge icebergs fall from the glaciers into the ocean, and that surface lakes drain suddenly in the summer warmth. But now they know considerably more about what happens to the network of streams, rivers and ponds that collect in the summer sunshine, and then flow across the top of the icesheet into moulins, or sinkholes.
They report in the Proceedings of the National Academy of Sciences that they used the data collected to chart 523 streams in a catchment area of about 6,800 square kilometres during the freak thaw of 2012, when almost the whole of the Greenland icecap was covered in slush. They measured the run-off at between 1,550 and 1,700 cubic metres per second − twice the average flow of the Colorado River.
Thereafter, all this water drained into moulins, and began to filter towards the base of the icesheet. What happened to it then is not yet certain, but the guess is that a percentage was soaked up within the iceshelf, while a proportion reached the sea.
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If we can get better estimates, then we can have better projections for the extent and impact of global warming
Marco Tedesco, founder and director of the City College of New York’s Cryospheric Processes Laboratory
Only the start
The study placed 11 researchers on the ice for six days in July 2012, during a massive and unusual melt. Only on one other occasion in the last 700 years, in 1889, did Greenland’s ice melt on such a scale.
The scientists were moved around by helicopter and equipped with a specially-designed automaton boat, buoys fitted with GPS technology, and sophisticated satellite imagery.
“It was a real preview of just how quickly that ice sheet can melt and the meltwater can escape”, said lead author Laurence Smith, Professor of Earth, Planetary, and Space Sciences at University of California Los Angeles. “The question was whether the ice sheet acts like a sponge or like Swiss cheese.”
The provisional answer is: both. Some meltwater stays, and some certainly escapes altogether. But it will take more than just one visit to arrive at more precise calculations.
The scientists also took measurements of Greenland’s Isortoq river − just one of about 100 large terrestrial rivers delivering Greenland meltwater to the oceans.
They found that the Isortoq carries water from the ice sheet to the ocean at between 650 and 1,300 cubic metres per second, which is less than models have projected.
Such direct measurements are important because they make predictions of melt rate and sea level rise more accurate − and more credible.
“If we can get better estimates, then we can have better projections for the extent and impact of global warming”, said another of the report’s authors, Marco Tedesco, founder and director of the City College of New York’s Cryospheric Processes Laboratory. “Greenland is really the big player for sea level rise in the future, so improving climate models is extremely crucial.”
