Satellites track variations in retreat of West Antarctic glaciers

An almost 25-year long record of elevation change of some of Antarctica’s fastest flowing and receding glaciers reveals important differences in the timing of retreat between single glaciers and the pace by which the ice loss spreads to Antarctica’s interior, according to a study now published in Geophysical Research Letters.

Combining data from five separate satellite missions, research carried out by CPOM shows how these glaciers have been thinning in patterns spreading upwards from the ice edge.

Focusing on Pine Island Glacier – which contributes more to sea level rise than any other ice stream on the planet – alongside the neighbouring Thwaites and smaller Pope, Smith and Kohler (PSK) Glaciers, the study shows how glacier heights have been falling, most likely as the glaciers are thinning due to the warmer sea temperatures recorded around Antarctica in recent decades. What is particularly interesting, however, is that they have all been responding differently over the last two decades.

Lead author Dr Hannes Konrad explained: “Scientists generally agree that it is warm ocean water that melts the floating part of the glacier, which then allows the glacier to flow more easily because it’s no longer held back by the floating ice shelf. As the glacier flows faster, it starts to become thinner”.

Grounding line retreat.  Credit: ESA/Planetary Visions

“If there’s not enough snow and ice accumulating higher up to compensate, the glaciers lose more and more of their mass as they flow towards the sea, and that’s exactly what we are seeing here, but the detail varies considerably between the three systems, and even within each glacier.”

Bringing together data from 1992 (when satellite altimetry records began) to the present day, the team reconstructed surface heights along a series of flowlines to see how thinning at the grounding lines had affected the glacier further inland.

By 1992, all three were already experiencing height loss at or near the grounding line, with Pine Island Glacier losing height by around 1m every year, although the interior’s surface was stable. Thinning at Pine Island Glacier then spread steadily, firstly up the glacier’s main trunk, and then further inland.  While the pace at which it spread across the glacier surface varied, rates of thinning reached up to 13km/yr in places.

Pace and onset of drawdown at Pine Island, Thwaites and PSK Glaciers 1992-2015. Credit: CPOM, NERC, iSTAR, ESA, BEDMAP2, MEaSUREs, NASA

Changes at Thwaites Glacier were more erratic: the surface at the grounding line was already falling by up to 3 m/yr at the start of the satellite record, but thinning ceased around 2000. Since restarting in 2004, thinning has spread at similar rates to those seen at Pine Island Glacier, but the offset of about ten years means that it has not spread so far inland.

Finally, although PSK experienced the largest falls in surface height (up to 7m/yr), most likely beginning around 1980, the thinning spread much more slowly than at Pine Island Glacier or Thwaites Glacier.

Possible reasons include differences in glacier catchment size, bedrock, topography and hydrology, but what remains clear is that, over the past 25 years, all three systems have seen thinning which has spread from the grounding line across the glacier surface.

With climate models indicating that ocean warming is set to continue and the glaciers expected to retreat further, even without the influence of warm water due to their resting on downwards-sloping bedrock, further ice melt at the glacier margins and subsequent surface lowering seems likely.

Dr Konrad summarised: “Combining these altimetry records has given us a unique insight into how some of Antarctica’s most important glaciers are changing, including the fact that they do not all respond in the same way.  We are even seeing interesting differences between the tributaries of individual glaciers.”

Stressing the importance of continued monitoring, CPOM Director Professor Andy Shepherd added: “As well as being able to routinely monitor the polar ice sheets as a whole, these results show the ability of satellites to pinpoint how individual glaciers are responding to environmental change”.

“The next steps are to refine our calculations of ice loss and sea level rise from the Antarctic ice sheet as a whole, and, in turn, improve our models of what might happen in the future”.

Konrad, H. et al. (2016) Uneven onset and pace of ice-dynamical imbalance in the Amundsen Sea Embayment, Geophysical Research Letters, doi:10.1002/2016GL070733