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Antarctic past points to sea level threat

January 9, 2017, by Tim Radford

sea level

Global warming is causing land-based ice to melt in parts of Antarctica such as the Weddell Sea region. Image: NASA/Jeremy Harbeck via Flickr

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Evidence of Antarctic ice sheet melting and sea level rise almost 15,000 years ago raises alarm over current climate change dangers.

LONDON, 9 January, 2017 – Scientists have identified a fearful lesson from the past. Some 14,700 years ago, the Antarctic continent experienced a warm phase, when ice sheets melted and the global sea level rose by three metres.

And they warn that it could happen again, as conditions in the southern ocean that triggered the bygone event are being repeated.

Changes in ocean-atmosphere circulation have left the southern ocean stratified − a cold layer at the surface, and a warmer ocean lapping the base of the ice below.

And this is making the ice sheets melt more strongly, the scientists say in Scientific Reports journal.

“The changes that are currently taking place in a disturbing manner resemble those 14,700 years ago,” says one of the authors, Michael Weber, an expert in paleoclimatology, geology and oceanography at the University of Bonn, Germany.

Land-based ice

And Chris Fogwill, senior research associate at the Climate Change Research Centre at the University of New South Wales, Australia, who led the study, says: “The reason for the layering is that global warming in parts of Antarctica is causing land-based ice to melt, adding massive amounts of cool freshwater to the ocean surface.

“At the same time as the surface is cooling, the deeper ocean is warming, which has already accelerated the decline of glaciers in the Amundsen Sea embayment. It appears global warming is replicating conditions that, in the past, triggered significant shifts in the stability of the Antarctic ice sheet.”

The team had been examining cores drilled from the Antarctic ice to “read” the pattern of temperatures of the past.

Earlier research, based on deep sea sediments deposited between the last Ice Age and the present warm period, has found evidence of eight melting events in the region, the largest occurring 14,700 years ago.

Melting ice delivers fresh water to the oceans. This makes the formation of sea ice more likely, and this same interplay between melting ice and the formation of more sea ice has been confirmed by other studies.

“The big question is whether the ice sheet will
react to these changing ocean conditions
as rapidly as it did 14,000 years ago”

The latest finding counts as an alarm signal rather than a prediction. It is, however, only the latest in a series of reports that the Antarctic ice cap is responding to global warming as a consequence of rising carbon dioxide levels in the atmosphere.

In the last two or three years, scientists have repeatedly warned that a warming ocean could accelerate the melt of sea ice and then of the land-based glaciers of the continent.

Sea level rise

They have pinpointed mechanisms that could be causing the ice shelf to break up, and they have even warned that loss of ice could cause a sea level rise of three metres.

So the latest study is a confirmation of familiar anxieties, but this time the researchers have pinpointed a feedback mechanism that might trigger melting.

They based their findings on analysis of the chemical isotopes locked in ancient ice from the Weddell Sea embayment, and the evidence suggests that in the past, when polar waters became more stratified, the ice sheets melted much more quickly.

What happened in the past could happen again. The process near the end of the last Ice Age took perhaps 300 years. In geological terms, this is rapid. More disconcertingly, once such a process starts, there is no obvious reason why it should stop.

“The big question,” says Nick Golledge, senior research fellow at the Antarctic Research Centre at Victoria University in New Zealand, “is whether the ice sheet will react to these changing ocean conditions as rapidly as it did 14,000 years ago.” – Climate News Network

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