Thursday, November 29, 2012

Antarctic Melting and Sea Level

Antarctic Melting and Sea Level

Due to its location at the South Pole, Antarctica receives relatively little solar radiation. This means that it is a very cold continent where water is mostly in the form of ice or snow. This accumulates and forms a giant ice sheet which covers the land. New data which more accurately measures the rate of ice-melt could help us better understand how Antarctica is changing in the light of global warming. The rate of global sea level change is reasonably well-established but understanding the different sources of this rise is more challenging. Using re-calibrated scales that are able to weigh ice sheets from space to a greater degree of accuracy than ever before, the international team led by Newcastle University has discovered that Antarctica overall is contributing much less to the substantial sea-level rise than originally thought.

Instead, the large amount of water flowing away from West Antarctica through ice-melt has been partly cancelled out by the volume of water falling onto the continent in the form of snow, suggesting some past studies have overestimated Antarctica’s contribution to fast-rising sea levels.

Using Gravity Recovery and Climate Experiment (GRACE) satellite data, the team calculated ice sheet mass loss by more accurately mapping and removing the mass changes caused by the flow of rock beneath Earth’s surface.

Publishing their findings in the academic journal Nature, project lead Professor Matt King said the data meant we were at last close to understanding how Antarctica is changing.

"We have tried to weigh the ice in the past but GRACE only measures the combined effect of the ice changes and the land mass changes occurring beneath the Earth’s surface," explains Professor King, Professor of Polar Geodesy at Newcastle University. "The step forward we have made is to provide a better calculation of the land mass changes so we can correct the satellite measurements to more accurately calculate the changes in ice mass alone."

"Our ice change calculations rely heavily on how well we can account for these important changes taking place beneath the Earth’s surface.  While the land beneath the ice is moving by no more than a few millimetres-per-year — the thickness of a fingernail —that seemingly small effect significantly alters the rate at which we estimate the ice is changing."

"By producing a new estimate of the land motion we’re effectively re-calibrating the scales — in this case the GRACE satellite —so we can more accurately weigh the ice.  And what we’ve found is that present sea level rise is happening with apparently very little contribution from Antarctica as a whole."

"We’re now confident it is shrinking," says Professor King.  "Our new estimate of land motion helps us narrow the range and shifts the best estimate to the lower end of the ice melt spectrum."

"Worryingly, though, the rate of shrinking has sped up in some important locations. The parts of Antarctica that are losing mass most rapidly are seeing accelerated mass loss and this acceleration could continue well into the future."

"The sea level change we’re seeing today is happening faster than it has for centuries with just a small contribution from the massive Antarctic ice sheet. What is sobering is that sea levels will rise even faster if Antarctica continues to lose increasingly more ice into the oceans."

Sea levels around the world are rising.  Between 1870 and 2004, global average sea levels rose 17 cm as reported in 2006 in  Geophysical Research Letters.  From 1950 to 2009, satellite data showed a rise of 3.3 ± 0.4 mm from 1993 to 2009.  In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected that during the 21st century, sea level will rise another 18 to 59 cm (7.1 to 23 in).

Ice sitting on the Antarctic continent at the peak of the last ice age 20,000 years ago forced the rock beneath to deform and slowly flow away. After that time ice levels generally reduced and the rock within the Earth's mantle more than 100km below the surface has been slowly flowing back in. That change affects the GRACE satellites in exactly the same way as ice moving into and out of the continent and it has to be factored in to get an accurate measurement of the total ice.

For further information see Sea Levels.

Antarctica image via  Wikipedia.

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