Measuring the Irreversible Retreat of the West Antarctic Ice Sheet

The West Antarctic Ice Sheet is in a state of irretrievable decline, warns a study released by NASA and the University of California, Irvine.




Antarctica. Image Credit: Ben Holt Sr., NASA.

Antarctica. Image Credit: Ben Holt Sr., NASA.



Glacial retreat on the West Antarctic Ice Sheet has been a matter of scientific interest for decades and holds serious implications for sea level rise, but difficulties involved in conducting research in this inaccessible location limited early investigation. As remote sensing improved, so too did the capacity to observe glacial patterns on the Antarctic continent.

Now, a new study from NASA and the University of California, Irvine combines 40 years’ worth of ground, aerial and satellite radar observations.

The paper, “Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith and Kohler glaciers, West Antarctica from 1992 to 2011,” was published by Geophysical Research Letters.  It concludes that observational evidence indicates that a large area of the West Antarctic Ice Sheet has entered a state of irreversible retreat. Lead author Eric Rignot says, “It has passed a point of no return.”

Thwaites Glacier. Image credit: NASA.

Thwaites Glacier. Image credit: NASA.



Since the 1970s, glaciologists have identified the West Antarctic Ice Sheet as a weak spot on the continent, likely to be susceptible to changes in climate.  Many of the changes that have taken place are visible on the surface of the ice, allowing this 40-year data record to provide a valuable snapshot of change. As radar technologies improved, the satellite data were augmented with ground and aerial measurements.

In an interview with Earthzine, radar systems expert and microwave engineer Dr. Prasad Gogineni described the types of measurements currently available.

Glacial melt and decline (which are equivalent to ice sheet decline) are closely tied to how rapidly a glacier moves. The rate at which a glacier flows is related to bed topography and bed conditions. Changes in elevation, mass or the shape of the glacier’s margins can be important indicators in how rapidly a glacier is currently declining and the likely rate and extent of future decline.

There are three approaches to measuring alterations in ice sheets: volume change, flux approach, and mass change.

Dr. Prasad Gogineni. Image Credit: University of Kansas.

Dr. Prasad Gogineni. Image Credit: University of Kansas.



Volume change is determined by measuring the ice surface elevation using airborne and satellite laser radar altimeters to conduct surveys, such as those being done through the NASA IceBridge  and ICESat programs. Satellite sensors also can be used to measure changes in the margin of the ice sheet, surface temperature, and velocity of movement.

Velocity measurements are also a key component of the flux approach to studying ices sheets. This approach compares data on ice thickness, snow accumulation, and velocity to create a summative overview of change on the ice sheet.

Snow accumulation and ice thickness are both measurements that require a closer look than satellites provide. Measuring changes in snow accumulation is done by taking ice cores, while ice thickness requires aircraft-based equipment. Gogineni has visited glaciers in Greenland and the Antarctic and taken measurements of ice thickness and snow accumulation using aircraft-based sensors.

“You can’t reach large areas with the surface-based sensors, so the only effective way to measure ice thickness is with radars on the aircraft,” Gogineni explains.

There are two main types of measurements taken by these areal radars:  ice thickness measurements and surface elevation.  Changes in surface elevation indicate that the glacier may be spreading out over a broader area, and unless the glacier is also gaining mass, then spreading out means thinning out as well. Mass of the glaciers can be measured using gravimeters, which operate on satellites and aircraft.

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NASA’s Multiple Altimeter Beam Experimental Lidar flies over Southwest Greenland’s glaciers and sea ice to test a new method of measuring the height of Earth from space. Image Credit: Tim Williams, NASA.



“You are measuring the gravitational force between the objects, which depends upon the mass. You measure the change in gravity very, very accurately, and then convert that to mass change using models. Changes in mass can be compared to changes in the margin of the glacier to determine if it is thinning or thickening.”

These recent analyses add complexity to more basic historic records and provide a more comprehensive understanding of change in the West Antarctic than was previously possible.

An overview of change is especially valuable to researchers because ice sheets usually alter at a geologic rate: their movements and responses to changing conditions can be as slow as they are monolithic. Therefore, as Gogineni explained, it can be difficult to tell if an ice sheet is responding to current conditions or something that occurred a long time ago.

“Are (previous events) the major reason for the change or are the recent changes in warming the ones that are causing these changes? That’s where the models and all these things come in to play—to understand what is happening now and what is going to happen in the future.”

The observations published in Geophysical Research Letters by Rignot and co- authors indicate that human-induced climate change is indeed playing a role in the decline of the West Antarctic Ice Sheet.

The last two decades have involved a change in the wind regime of Antarctica, which has increased in intensity and shifted pole-ward. The changing wind regimes result in a greater circulation of warm water toward the glaciers of the West Antarctic.



The consequence of the alterations to the ocean patterns has led to such an increased rate of flow for the glaciers that melting will likely continue even if climate patterns were to hold steady from this point on and rate of atmospheric warming were to level-off.  This irreversible cycle is what is known as positive feedback. In science, positive feedback refers to a self-perpetuating cycle.

This cycle is occurring for multiple glaciers in the West Antarctic, on a massive scale that involves thousands of kilometers of ice. Although the collapse of the entire ice sheet may occur slowly in human terms of time, the result will be dramatic and directly significant for coastal communities. Glacial melting on the West Antarctic Ice Sheet alone may contribute as much as 1.2 meters to sea level rise by the year 2100, according to the Rignot study.

Evidence from the latest study on West Antarctic Ice Sheet indicates that decline of the ice sheet is unavoidable, but the rate at which this decline occurs is still undetermined. Because glacial retreat is climate-linked, society’s current actions or lack thereof will determine whether the ice sheet’s decline unfolds at its current pace or with more disastrous rapidity.

 

 


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