Posts Tagged ‘Wilkins’

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The “Collapse” of the Wilkins ice shelf

April 1, 2008

A few quick calculations put the size and effect of latest broken piece of Wilkins ice into perspective

The recent “collapse” of the Wilkins ice shelf is causing quite a stir in the blogosphere.  The issue of disintegrating ice shelves is often entangled with the issue of sea level rise.  The Los Angeles Times carried an AP story on March 25th that reported:

…the western peninsula, which includes the Wilkins Ice Shelf, juts out into the ocean and is warming.  Scientists are most concerned about melting ice in this part of the continent triggering a rise in sea level.

The next day, CNN reported on the Wilkins ice shelf, saying:

…the poles will be the leading edge of what’s happening in the rest of the world as global warming continues.  Even though they seem far away, changes in the polar regions could have an impact on both hemispheres, with sea level rise and changes in climate patterns.

Although most reports do admit that this floating ice will not raise the sea level at all, they paint an ominous picture of land bound glaciers rapidly sliding into the sea.  In fact, the Wilkins ice shelf, like other ice shelves, is the product of a land glacier or ice sheet flowing over the coast and onto the water.

The piece of the ice shelf that broke off over the last month is reported to be 160 square miles (about 400 square kilometers).   It is “up to” 650 feet (200 meters) thick according to the Times Online.  A BBC video report corroborates the thickness by saying “Those cliffs are about 60 feet high,” when referring to the floating ice, which indicates that the total thickness is about 10 times that (because most of it is underwater), or about 600 feet (180 meters).  So, lets say the ice is about 0.2 kilometers thick (200 meters).  Then the total volume of the piece that broke off is about

400 km²  x  0.2 km  = 80 km³

One km³ of water will raise the sea level by a miniscule 2.78 microns (less than 3 millionths of a meter).  So, over the course of time that it took this 80 km³ volume of ice to move from the land to the sea it contributed to the sea level by:

80 km³  x  2.78 microns/km³  =  220 microns  =  0.22 millimeters  =  0.009 inches

That’s not very much, considering that it took many years. 

In general, it takes 360 km³ of water to raise the sea level by 1 mm.  In order for the Antarctic peninsula to contribute 12 inches (about 300 mm) to the sea level in 100 years, it would have to drop 1,080 km³ of ice into the ocean  (more really, because the density of the ice is less than the density of water) EVERY SINGLE YEAR FOR 100 YEARS!!  If the ice at the grounding line (where the ice leaves the land) were 0.33 km thick on average, then more than 3000 km² of ice would have to move into the ocean every single year.  Of course, this estimate is based on the unrealistic assumption that there would be no new ice accumulation on land from precipitation to offset the sea level rise.  The difference in the amount of ice sliding into the sea and the amount of ice building up on land due to snowfall is call the mass balance.

Typical estimates for the ice mass balance in the Antarctic Peninsula are nowhere near the 1,080 km³ (roughly 1,080 Gt).  The mass balance for the entire Antarctic continent doesn’t even come close.  Estimates for the entire continent vary greatly and have huge uncertainties.  Vilaconga and Wahr (2006) estimate a net ice loss of “152 ± 80 cubic kilometers of ice per year, which is equivalent to 0.4 ± 0.2 millimeters of global sea-level rise per year.”  Davis (2005) estimates a net increase in Antarctic ice, which would cause a net drop in sea levels.  Either way, the Antarctic is a very, very long way from any kind of catastrophic meltdown.

Then there is Greenland.  Luthcke (2006) estimates the mass balance for Greenland at a loss of 101 Gigatonnes per year.  This translates into a puny sea level rise of only 0.28 mm per year.

While we are at it, let’s consider James Hansen’s estimate of a 15 foot sea level rise this century. 

On the average, a 15 foot sea level rise in a hundred years translates into 46 millimeters per year, requiring 16,500 km³ of additional water per year!  This is about 65 times the current rate of ice melt, if we accept the mass balances of Vilaconga and Wahr for the Antarctic and Luthcke for Greenland.  If the ice sliding into the ocean is a third of a kilometer thick, then Hansen’s doomsday scenario would require 50,000 square kilometers of ice to move from land to ocean every single year!!!!

The bottom line

Pictures of huge chunks of ice and making scary comparisons like “Seven times the size of Manhattan” may get people excited, but they are not very enlightening.

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Davis, C., et. al., Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise, Science Vol. 308. no. 5730, pp. 1898 – 1901, 2005  Get copy here

Luthcke, et. al., Recent Greenland Ice Mass Loss by Drainage System from Satellite Gravity Observations, Science, Vol. 314. no. 5803, pp. 1286 – 1289, 2006   Get copy here

Velicogna, I. and Wahr, J., Measurements of Time-Variable Gravity Show Mass Loss in Antarctica, Science, Vol. 311. no. 5768, pp. 1754 – 1756, 2007  Get copy here

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