NASA: New View of Antarctic Melt has “Profound Implications”.

June 14, 2013

New study from JPL has big implications for ice dynamics at the South Pole.  I interviewed study author Eric Rignot in December at the AGU conference in San Francisco, above.  You’ll hear him discuss IMBIE, the Ice Mass Balance Inter-Comparison Exercise  an earlier, 2012 effort.
Interesting to compare perspective from the interview, and the press release below.

NASA:

PASADENA, Calif. — Ocean waters melting the undersides of Antarctic ice shelves are responsible for most of the continent’s ice shelf mass loss, a new study by NASA and university researchers has found.

Scientists have studied the rates of basal melt, or the melting of the ice shelves from underneath, of individual ice shelves, the floating extensions of glaciers that empty into the sea. But this is the first comprehensive survey of all Antarctic ice shelves. The study found basal melt accounted for 55 percent of all Antarctic ice shelf mass loss from 2003 to 2008, an amount much higher than previously thought.

Antarctica holds about 60 percent of the planet’s fresh water locked into its massive ice sheet. Ice shelves buttress the glaciers behind them, modulating the speed at which these rivers of ice flow into the ocean. Determining how ice shelves melt will help scientists improve projections of how the Antarctic ice sheet will respond to a warming ocean and contribute to sea level rise. It also will improve global models of ocean circulation by providing a better estimate of the amount of fresh water ice shelf melting adds to Antarctic coastal waters.

The study uses reconstructions of ice accumulation, satellite and aircraft readings of ice thickness, and changes in elevation and ice velocity to determine how fast ice shelves melt and compare the mass lost with the amount released by the calving, or splitting, of icebergs.

basalmelt

Rates of basal melt of Antarctic ice shelves (melting of the shelves from underneath) overlaid on a 2009 mosaic of Antarctica created from data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA’s Terra and Aqua spacecraft. Red shades denote melt rates of less than 5 meters (16.4 feet) per year (freezing conditions), while blue shades represent melt rates of greater than 5 meters (16.4 feet) per year (melting conditions). The perimeters of the ice shelves in 2007-2008, excluding ice rises and ice islands, are shown by thin black lines. Each circular graph is proportional in area to the total ice mass loss measured from each ice shelf, in gigatons per year, with the proportion of ice lost due to the calving of icebergs denoted by hatched lines and the proportion due to basal melting denoted in black. Image credit: NASA/JPL-Caltech/UC Irvine/Columbia University

“The traditional view on Antarctic mass loss is it is almost entirely controlled by iceberg calving,” said Eric Rignot of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and the University of California, Irvine. Rignot is lead author of the study to be published in the June 14 issue of the journal Science. “Our study shows melting from below by the ocean waters is larger, and this should change our perspective on the evolution of the ice sheet in a warming climate.”

Ice shelves grow through a combination of land ice flowing to the sea and snow accumulating on their surface. To determine how much ice and snowfall enters a specific ice shelf and how much makes it to an iceberg, where it may split off, the research team used a regional climate model for snow accumulation and combined the results with ice velocity data from satellites, ice shelf thickness measurements from NASA’s Operation IceBridge — a continuing aerial survey of Earth’s poles — and a new map of Antarctica’s bedrock. Using this information, Rignot and colleagues were able to deduce whether the ice shelf was losing mass through basal melting or gaining it through the basal freezing of seawater.

In some places, basal melt exceeds iceberg calving. In other places, the opposite is true. But in total, Antarctic ice shelves lost 2,921 trillion pounds (1,325 trillion kilograms) of ice per year in 2003 to 2008 through basal melt, while iceberg formation accounted for 2,400 trillion pounds (1,089 trillion kilograms) of mass loss each year.

Basal melt can have a greater impact on ocean circulation than glacier calving. Icebergs slowly release melt water as they drift away from the continent. But strong melting near deep grounding lines, where glaciers lose their grip on the seafloor and start floating as ice shelves, discharges large quantities of fresher, lighter water near the Antarctic coastline. This lower-density water does not mix and sink as readily as colder, saltier water, and may be changing the rate of bottom water renewal.

“Changes in basal melting are helping to change the properties of Antarctic bottom water, which is one component of the ocean’s overturning circulation,” said author Stan Jacobs, an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. “In some areas it also impacts ecosystems by driving coastal upwelling, which brings up micronutrients like iron that fuel persistent plankton blooms in the summer.”

The study found basal melting is distributed unevenly around the continent. The three giant ice shelves of Ross, Filchner and Ronne, which make up two-thirds of the total Antarctic ice shelf area, accounted for only 15 percent of basal melting. Meanwhile, fewer than a dozen small ice shelves floating on “warm” waters (seawater only a few degrees above the freezing point) produced half of the total melt water during the same period. The scientists detected a similar high rate of basal melting under six small ice shelves along East Antarctica, a region not as well known because of a scarcity of measurements.

The researchers also compared the rates at which the ice shelves are shedding ice to the speed at which the continent itself is losing mass and found that, on average, ice shelves lost mass twice as fast as the Antarctic ice sheet did during the study period.

“Ice shelf melt doesn’t necessarily mean an ice shelf is decaying; it can be compensated by the ice flow from the continent,” Rignot said. “But in a number of places around Antarctica, ice shelves are melting too fast, and a consequence of that is glaciers and the entire continent are changing as well.”

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11 Responses to “NASA: New View of Antarctic Melt has “Profound Implications”.”

  1. Sir Charles Says:

    “Earth’s Climate Changes Gradually… We Can Easily Adapt.”
    Maybe, maybe not… because sometimes climate goes over the edge, and quickly. Richard Alley describes the 10 degrees C jump in temperature seen in the ice core record as like “bungee jumping off the climate roller coaster” which we see him do, in person in New Zealand, and in computer graphics. Perhaps we’d better take out some insurance.

  2. rayduray Says:

    Well, this gives a whole new twist to that old expresion, “bottom’s up!” ….

  3. takver Says:

    An interesting report and confirms other recent studies about the importance of ocean currents on basal melting of ice shelves contributing to the ice mass loss in Antarctica

    Antarctic Paradox: ocean warming melting ice shelves causing sea ice expansion (April 1, 2013)

    http://takvera.blogspot.com/2013/04/antarctic-paradox-ocean-warming-melting.html

    Increasing Sea Surface temperatures in tropical pacific impacting West Antarctica (December 24, 2012)

    http://takvera.blogspot.com/2012/12/global-warming-on-west-antarctic-ice.html

  4. mspelto Says:

    Great figure indicates why the region from Pine Island to Getz Glacier is the area of greatest immediate concern. When you look at the calving from Thwaites, this puts the basal melting in perspective.

    • greenman3610 Says:

      Mauri,
      have not read the paper yet – is this a surprise insofar as total mass loss, or just where the mass loss is coming from?
      and –
      if basal melt is the culprit, does that raise the “speed limit” – in that, the melt is coming right off the bottom of the ice, and is not limited by the physical constraints on glacier calving ie friction, grounding lines etc?

      • mspelto Says:

        Basal melting is a large component of the story, but as the diagram shows calving is quite crucial. In fact thinning of the ice is what typically leads to acceleration of calving as we have seen from Wilkins Ice Shelf to Jakobshavn, for reasons explained in an article at Realclimate I wrote 5 years ago. “It appears then that glacier or ice shelf thinning is the key preconditioning factor for collapse, retreat and acceleration, whether you are in Antarctica of Greenland. The mechanisms for ice shelf thinning include basal melting (from warming ocean waters), surface melting, reduction in glacier inflow and rift development. These are interrelated mechanisms that precondition the ice shelves to collapse. “


  5. James Hansen’s back of the envelope 5 meter rise by 2107 looks better and better.


  6. […] New study from JPL has big implications for ice dynamics at the South Pole. I interviewed study author Eric Rignot in December at the AGU conference in San Francisco, above. You'll hear him discu…  […]


  7. […] NASA: New View of Antarctic Melt has “Profound Implications”. (climatecrocks.com) […]

  8. mspelto Says:

    This is not a surprise, it would have been a surprise 10 years ago. All results in the last five years have been indicating the crucial importance of basal melt. Calving is still quite important, and in fact ice shelf thinning due to basal melting enhances calving. We have seen this from Wilkins to Jakobshavn as I noted in an article in Realclimate five years ago. http://www.realclimate.org/index.php/archives/2008/10/what-links-the-retreat-of-jakobshavn-isbrae-wilkins-ice-shelf-and-the-petermann-glacier/


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