̽��ֱ�� of Cambridge - ice cover

‘Scars’ left by icebergs record West Antarctic ice retreat

sc604 — Wed, 25 Oct 2017 17:00:00 +0000

Researchers from the ̽��ֱ�� of Cambridge, the British Antarctic Survey and Stockholm ̽��ֱ�� imaged the seafloor of Pine Island Bay, in West Antarctica. They found that, as seas warmed at the end of the last ice age, Pine Island Glacier retreated to a point where its grounding line – the point where it enters the ocean and starts to float – was perched precariously at the end of a slope.

Breakup of the floating ‘ice shelf’ in front of the glacier left tall ice ‘cliffs’ at its edge. ̽��ֱ��height of these cliffs made them unstable, triggering the release of thousands of icebergs into Pine Island Bay, and causing the glacier to retreat rapidly until its grounding line reached a restabilising point in shallower water.

Today, as warming waters caused by climate change flow underneath the floating ice shelves in Pine Island Bay, the Antarctic Ice Sheet is once again at risk of losing mass from rapidly retreating glaciers. Significantly, if ice retreat is triggered, there are no relatively shallow points in the ice sheet bed along the course of Pine Island and Thwaites glaciers to prevent possible runaway ice retreat into the interior of West Antarctica. ̽��ֱ��results are published in the journal Nature.

“Today, the Pine Island and Thwaites glaciers are grounded in a very precarious position, and major retreat may already be happening, caused primarily by warm waters melting from below the ice shelves that jut out from each glacier into the sea,” said Matthew Wise of Cambridge’s Scott Polar Research Institute, and the study’s first author. “If we remove these buttressing ice shelves, unstable ice thicknesses would cause the grounded West Antarctic Ice Sheet to retreat rapidly again in the future. Since there are no potential restabilising points further upstream to stop any retreat from extending deep into the West Antarctic hinterland, this could cause sea-levels to rise faster than previously projected.”

Pine Island Glacier and the neighbouring Thwaites Glacier are responsible for nearly a third of total ice loss from the West Antarctic Ice Sheet, and this contribution has increased greatly over the past 25 years. In addition to basal melt, the two glaciers also lose ice by breaking off, or calving, icebergs into Pine Island Bay.

Today, the icebergs that break off from Pine Island and Thwaites glaciers are mostly large table-like blocks, which cause characteristic ‘comb-like’ ploughmarks as these large multi-keeled icebergs grind along the sea floor. By contrast, during the last ice age, hundreds of comparatively smaller icebergs broke free of the Antarctic Ice Sheet and drifted into Pine Island Bay. These smaller icebergs had a v-shaped structure like the keel of a ship and left long and deep single scars in the sea floor.

High-resolution imaging techniques, used to investigate the shape and distribution of ploughmarks on the sea floor in Pine Island Bay, allowed the researchers to determine the relative size and drift direction of icebergs in the past. Their analysis showed that these smaller icebergs were released due to a process called marine ice-cliff instability (MICI). More than 12,000 years ago, Pine Island and Thwaites glaciers were grounded on top of a large wedge of sediment and were buttressed by a floating ice shelf, making them relatively stable even though they rested below sea level.

Eventually, the floating ice shelf in front of the glaciers ‘broke up’, which caused them to retreat onto land sloping downward from the grounding lines to the interior of the ice sheet. This exposed tall ice ‘cliffs’ at their margin with an unstable height, and resulted in the rapid retreat of the glaciers from marine ice cliff instability between 12,000 and 11,000 years ago. This occurred under climate conditions that were relatively similar to those of today.

“Ice-cliff collapse has been debated as a theoretical process that might cause West Antarctic Ice Sheet retreat to accelerate in the future,” said co-author Dr Robert Larter, from the British Antarctic Survey. “Our observations confirm that this process is real and that it occurred about 12,000 years ago, resulting in rapid retreat of the ice sheet into Pine Island Bay.”

Today, the two glaciers are getting ever closer to the point where they may become unstable, resulting once again in rapid ice retreat.

̽��ֱ��research has been funded in part by the UK Natural Environment Research Council (NERC)

Reference:
Matthew G. Wise et al. ‘Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks.’ Nature (2017). DOI: 10.1038/nature24458

Thousands of marks on the Antarctic seafloor, caused by icebergs which broke free from glaciers more than ten thousand years ago, show how part of the Antarctic Ice Sheet retreated rapidly at the end of the last ice age as it balanced precariously on sloping ground and became unstable. Today, as the global climate continues to warm, rapid and sustained retreat may be close to happening again and could trigger runaway ice retreat into the interior of the continent, which in turn would cause sea levels to rise even faster than currently projected.

Today, the Pine Island and Thwaites glaciers are grounded in a very precarious position, and major retreat may already be happening.

Matthew Wise

Martin Jakobsson

Ice cliffs in Pine Island Bay

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Antarctic Ice Sheet study reveals 8,000-year record of climate change

sjr81 — Mon, 12 Dec 2016 16:02:17 +0000

Results of the study, co-authored by Michael Weber, a paleoclimatologist and visiting scientist at the ̽��ֱ�� of Cambridge, along with colleagues from the USA, New Zealand and Germany, are published this week in the journal Nature.

Global climate models that look at the last several thousand years have failed to account for the amount of climate variability captured in the paleoclimate record, according to lead author Pepijn Bakker, a climate modeller from the MARUM Center for Marine Environmental Studies at the ̽��ֱ�� of Bremen in Germany.

̽��ֱ��researchers first turned their attention to the Scotia Sea. “Most icebergs calving off the Antarctic Ice Sheet travel through this region because of the atmospheric and oceanic circulation,” explained Weber. “ ̽��ֱ��icebergs contain gravel that drop into the sediment on the ocean floor – and analysis and dating of such deposits shows that for the last 8,000 years, there were centuries with more gravel and those with less.”

̽��ֱ��research team’s hypothesis is that climate modellers have historically overlooked one crucial element in the overall climate system. They discovered that the centuries-long phases of enhanced and reduced Antarctic ice mass loss documented over the past 8,000 years have had a cascading effect on the entire climate system.

Using sophisticated computer modelling, the researchers traced the variability in iceberg calving (ice that breaks away from glaciers) to small changes in ocean temperatures.

“There is a natural variability in the deeper part of the ocean adjacent to the Antarctic Ice Sheet that causes small but significant changes in temperatures,” said co-author Andreas Schmittner, a climate modeller from Oregon State ̽��ֱ��. “When the ocean temperatures warm, it causes more direct melting of the ice sheet below the surface, and it increases the number of icebergs that calve off the ice sheet.”

Those two factors combine to provide an influx of fresh water into the Southern Ocean during these warm regimes, according to Peter Clark, a paleoclimatologist from Oregon State ̽��ֱ��, and co-author on the study.

“ ̽��ֱ��introduction of that cold, fresh water lessens the salinity and cools the surface temperatures, at the same time, stratifying the layers of water,” he said. “ ̽��ֱ��cold, fresh water freezes more easily, creating additional sea ice despite warmer temperatures that are down hundreds of meters below the surface.”

̽��ֱ��discovery may help explain why sea ice is currently expanding in the Southern Ocean despite global warming, the researchers say.

“This response is well-known, but what is less-known is that the input of fresh water also leads to changes far away in the northern hemisphere, because it disrupts part of the global ocean circulation,” explained Nick Golledge from the ̽��ֱ�� of Wellington, New Zealand, an ice-sheet modeller and study co-author. “Meltwater from the Antarctic won’t just raise global sea level, but might also amplify climate changes around the world. Some parts of the North Atlantic may end up with warmer temperatures as a consequence of part of Antarctica melting.”

Golledge used a computer model to simulate how the Antarctic Ice Sheet changed as it came out of the last ice age and into the present, warm period.

" ̽��ֱ��integration of data and models provides further evidence that the Antarctic Ice Sheet has experienced much greater natural variability in the past than previously anticipated,” added Weber. “We should therefore be concerned that it will possibly act very dynamically in the future, too, specifically when it comes to projecting future sea-level rise.”

Two years ago Weber led another study, also published in Nature, which found that the Antarctic Ice Sheet collapsed repeatedly and abruptly at the end of the Last Ice Age to 19,000 to 9,000 years ago.

An international team of researchers has found that the Antarctic Ice Sheet plays a major role in regional and global climate variability – a discovery that may also help explain why sea ice in the Southern Hemisphere has been increasing despite the warming of the rest of the Earth.

̽��ֱ��Antarctic Ice Sheet has experienced much greater natural variability in the past than previously anticipated.

Michael Weber

Iceberg in the Weddell Sea

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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