Tech

June 14, 2013

Warm ocean, not icebergs, causing most of Antarctic ice shelves’ mass loss

This photo shows the ice front of Venable Ice Shelf, West Antarctica, in October 2008. It is an example of a small-size ice shelf that is a large melt water producer. The image was taken onboard the Chilean Navy P3 aircraft during the NASA/Centro de Estudios Cientificos, Chile campaign of fall 2008 in Antarctica.

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.

“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.”

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 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.

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 – an 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 2003-2008 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 coast line. 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.

Calving front of the calving front of an ice shelf in West Antarctica. The traditional view on ice shelves, the floating extensions of seaward glaciers, has been that they mostly lose ice by shedding icebergs. A new study by NASA and university researchers has found that warm ocean waters melting the ice sheets from underneath account for 55 percent of all ice shelf mass loss in Antarctica. This image was taken during the 2012 Antarctic campaign of NASA’s Operation IceBridge, a mission that provided data for the new ice shelf study.

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.”

 




All of this week's top headlines to your email every Friday.


 
 

 

Headlines October 24, 2014

News: U.S., South Korea delay transfer of wartime control - The U.S. and South Korea have delayed transferring wartime operational control of allied forces by taking on a “conditions-based approach” and scrapping the previously set deadline of 2015.   Business: Exclusive: Lockheed, Pentagon reach $4 billion deal for more F-35 jets - Lockheed Martin and U.S. defense...
 
 

News Briefs October 24, 2014

French moving troops toward Libyan border A top French military official says the country is moving troops toward the Libyan border within weeks and, along with U.S. intelligence, is monitoring al Qaeda arms shipments to Africa’s Sahel region. A French base will go up within weeks in a desert outpost just a hundred kilometers (60...
 
 
Navy photograph

Navy to commission submarine North Dakota

Navy photograph The PCU North Dakota (SSN 784) during bravo sea trials. The crew performed exceptionally well on both alpha and bravo sea trials. The submarine North Dakota is the 11th ship of the Virginia class, the first U.S....
 

 

Boeing announces SF Airlines order for Boeing converted freighters

Boeing announced Oct. 23 that SF Airlines has placed an order for an undisclosed number of 767-300ER passenger-to-freighter conversions (Boeing Converted Freighters). SF Airlines, a subsidiary of Shenzhen, China-based delivery services company SF Express, will accept its first redelivered 767 in the second half of 2015. “SF Express aims to become China’s most respected and...
 
 
LM-C130

Another Super Herc Little Rock Rollin’

  Lockheed Martin delivered another C-130J Super Hercules to the 61st Airlift Squadron at Little Rock Air Force Base, Ark., Oct. 23. Little Rock AFB’s new C-130J was ferried from the Lockheed Martin Aeronautics facility ...
 
 

United Technologies beats third quarter profit expectations

United Technologies Corp. Oct. 23 reported third-quarter profit of $1.85 billion as sales increased across all its businesses and the aerospace giant reported favorable tax settlements. The Hartford, Conn.,-based company said it had profit of $2.04 per share and earnings, adjusted for non-recurring gains, came to $1.82 per share. The results topped Wall Street expectations,...
 




0 Comments


Be the first to comment!


Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>