Satellites track variations in retreat of West Antarctic glaciers

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.”

“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.

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

Lowest early-winter sea ice growth on record

There is set to be only 10,500 cubic kilometres of sea ice in the Arctic this month, the joint lowest of any November on record, according to satellite observations processed by CPOM. Although sea ice volume did not hit a record low at the end of summer, early winter growth has been 9 % slower than usual, leading to today’s conditions.

Trends in early-winter Arctic sea ice volume recorded by CryoSat. Sea ice growth this month has been 9 % lower than usual, and November 2016 is tied as a record low.

The measurements were made using the European Space Agency’s CryoSat satellite, which is dedicated to monitoring the polar regions and is uniquely able to detect the thickness of sea ice floes. Not only are these observations vital for tracking climate change, they are also an essential resource for maritime operators who increasingly navigate ice infested waters.

This year, the US National Snow and Ice Data Centre reported that the extent of Arctic sea ice fell to 4.1 million km2 on 7th September tied with 2007 as the second lowest on record. However, CryoSat shows that the ice was thicker at the end of summer than in most other years – 116 cm, on average – and so there was substantially more ice than in two other years (2011 and 2012). Thicker ice can occur if melting is lower, or if snowfall or floe-compaction is higher.

CPOM Research Fellow Rachel Tilling explained, “Sea ice is highly mobile and susceptible to deformation, so we need to measure its thickness as well as its extent to be sure how much is there.”

However, although the Arctic sea ice pack usually gains 161 cubic kilometres per day in November, this year’s growth has been 10 % lower at 139 cubic kilometres per day, and it is estimated that the final amount will rise to only 10,500 cubic kilometres by the month end.

This would essentially tie with conditions in the Novembers of 2011 and 2012, when levels of Arctic sea ice were at their lowest on record for this time of year.

Although sea ice in the central Arctic is currently thicker than it was in 2011 and 2012, there is far less ice in more southerly regions such as the Beaufort, East Siberian and Kara Seas.

Maps of November Arctic sea ice thickness recorded by CryoSat-2. Although ice is thicker than usual north of Canada, there is less ice overall in southerly regions such as the Beaufort, East Siberian and Kara Seas.

Rachel Tilling added: “Because CryoSat can measure Arctic sea ice thickness in autumn, it gives us a much clearer picture of how it has fared during summer. Although sea ice usually grows rapidly after the minimum extent each September, this years’ growth has been far slower than we’d expect probably because this winter has been warmer than usual in the Arctic.”

As demand for information on Arctic conditions increases, CryoSat has become an essential source of information for polar stakeholders ranging from ice forecasting services to scientists studying the impacts of climate change.

CPOM Director and Principal Scientific Advisor to the CryoSat mission, Professor Andrew Shepherd, summarised: “In its short, six years of life, we have learnt more about Arctic sea ice from CryoSat than from any other satellite mission. But to really understand the role that sea ice plays in the climate system, and the restrictions it places on maritime operations, we must ensure that its measurements are continued into the future.”

A complete assessment of 2016 sea ice conditions will be available from CPOM in the coming weeks.