Reporting this week (Wednesday Nov. 23) in the journal Nature an international team led by British Antarctic Survey (BAS) explains that present-day thinning and retreat of Pine Island Glacier, one of the largest and fastest shrinking glaciers of the West Antarctic Ice Sheet, is part of a climate trend that was already underway as early as the 1940s.
It is already known that Pine Island Glacier -- roughly two-thirds the size of the UK -- has been thinning and retreating at an alarming rate since 1992 when satellite observations first started. The ice lost from this glacier and its neighbours, has added significantly to sea-level rise, and currently this area is one of biggest single unknowns in future projections. Until now, it was not known when the retreat of Pine Island Glacier started, or its underlying cause.
In this study, seabed sediment cores obtained from beneath the floating part of Pine Island Glacier have revealed that a cavity started to form beneath the shelf prior to the mid-1940s. This allowed warm sea water to flow under the shelf, and cause it to lift-off from a prominent sea-floor ridge which held it in place. This strongly suggests that current retreat was initiated by strong warming of the region associated with El Niño activity.
Lead author, marine geologist Dr James Smith from British Antarctic Survey, says:
'We are very excited about this new finding as it provides the first direct evidence of the timing of glacier retreat even before we had satellites to measure them. The sediment cores were obtained through a 450-m deep hole in of ice, and up to 500 m of ocean. The sediment reveals climate events that initiated the current thinning of Pine Island Glacier. They show us how changes half-way across the planet in the tropical Pacific, reached through the ocean to influence the Antarctic ice sheet.
"Pine Island Glacier is one of the most inhospitable and remote areas of Antarctica, so to get all the equipment needed to hot-water drill through the ice shelf required a major effort from our collaborators at the US Antarctic Programme. On the ground it was real team effort to lower the drill by hand to the seabed on nearly 1000 m of rope. After all that work, the cores show us something so unexpected."
Co-author and principal scientist Professor Bob Bindschadler of NASA says:
'A significant implication of our findings is that once an ice sheet retreat is set in motion it can continue for decades, even if what started gets no worse. It is possible that the changes we see today on Pine Island Glacier were essentially set in motion in the 1940s'.
Professor David Vaughan, co-author and Director of Science at British Antarctic Survey, says:
"Ice loss from this part of West Antarctica is already making a very significant contribution to global sea level rise, and is actually one of the largest uncertainties in global sea-level predictions. Understanding what initiated the current changes is one major piece of the jigsaw, and now we are already looking for the next -- how long will these changes continue and how much ice will Pine Island Glacier and its neighbours lose in the coming century? Data from the UK science programme iSTAR will tell us even more about Pine Island Glacier, but these are big questions that need the international science community to work together."
A new joint programme recently announced by the UK Natural Environment Research Council (NERC) and the US National Science Foundation will allow a more focussed study of Pine Island Glacier and provide a new opportunity to understand West Antarctica and quantify how much sea level rise it might cause in the coming century .
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Notes to Editors:
Sub-ice shelf sediments record 20th Century retreat history of Pine Island Glacier by Smith J.A, Anderson T.J, Shortt M, Gaffney A.M, Truffer M, Stanton T.P, Bindschadler R, Dutrieux P, Jenkins A, Hillenbrand C.D, Ehrmann W, Corr H.F.J, Farley N, Crowhust S, Vaughan D.G is published in the journal Nature on Wednesday 23 November.
Pine Island Glacier drains an area equivalent to two thirds the size of the United Kingdom; it is the most rapidly shrinking glacier on the planet and is contributing more to sea level rise than any other ice stream; it is approximately 2km thick, but is thinning by more than 1 metre per year. It is responsible for a greater contribution of ice into the sea than any other glacier on Earth which, given the global concern about rising sea level, makes it very important. Significantly, this contribution appears to be increasing. PIG is also the fastest shrinking on the planet and it is contributing to sea level rise faster than any other glacier.
Over the past 15 years, PIG has thinned at a rate of more than 1 metre per year. Glaciers thin when more mass (ice) is lost during the summer than is replaced by snowfall in the winter. When this happens it is said to have a negative mass-balance. Temperature, precipitation and the speed at which a glacier is moving are the main factors controlling this relationship.
It is not known for certain why PIG retreat is accelerating but one theory attributes it to the warmer sea temperatures felt around Antarctica in recent years. This warm water acts to melt the underside of an ice shelf, making it weaker and more likely to crack and fall into the sea. In recent years the location at which PIG starts to float on the sea, its grounding line, has retreated by more than 1 km per year, and in July 2013 a 720 km2 section (roughly eight times the size of Manhattan Island) of PIG's ice shelf broke away. Having a smaller ice shelf means that less 'back pressure' is exerted on the rest of the glacier and results in an increased rate of flow.
The point at which a glacier meets the sea bed is called the grounding line. As warmer water pushes underneath the glacier, the lower ice is melted away, effectively pushing the grounding line backwards. The fast the grounding line retreats, the faster a glacier is melting.
This research was supported by NSF's Office of Polar Programs under NSF grants including ANT-0732926 and ANT 0732730 and funding from NASA's Cryospheric Sciences Program; by New York University Abhu Dabi grant 1204; and by the Natural Environment Research Council (NERC)-British Antarctic Survey 'Polar Science for Planet Earth Program'. Work at the Lawrence Livermore National Laboratory (LLNL) was performed under contract DE-AC52-07NA27344 and grant LLNL-JRNL-697878.
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