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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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  • The question I have is–What would cause such a similar phenomenon to the world’s current rise in global temperatures and their subsequent destabilization of Anarctic sea ice caused by the increase of CO2 emissions due to anthropogenic interference 14,500 years ago?

  • There just isn’t the ability to hold anyone accountable for this; in the future, history may cite the reasons why things occurred in the past, but are there names like Stalin, Hitler, Mussolini, Tojo that could be held accountable for the failure of Earths decision-makers to avert environmental disaster? Make a list now, and put names on it for those making decisions or obstructing decisions that will affect the future climate.

  • This alarm echoes a Mexico geological study showing a 30 foot rise in sea level within a 50 year period.

  • You are referring to meltwater pulses 1a and 1b, during the last glacial termination. This gets to asking the wrong question. Comparing what happened as we melted our way out of the last ice age to present is exactly the wrong question. The question you should be asking, as as a great many geologists and paleoclimatologists long ago realized, was what were the ends of the last interglacials like?

    Half of the warming that brought us out of the Wisconsin ice age occurred in less than a decade, the other half took 10,000 years. This interglacial is termed the Holocene Epoch, and is presently, exactly, 11,700 years old. We are at the 23kyr point in the 19kyr-23kyr precession cycle. Only 1 of the past 8 interglacials has lasted longer than about half a precession cycle, which we are now past by 2 centuries…..

    This necessarily drags the focus from the beginning of an interglacial to its end. And this is where it gets ugly indeed. Formally, it is termed glacial inception, informally we geologists refer to it as “the climatic madhouse”. No one knows why, but the ends of the most recent interglacials have been quite the wild climate ride. The end of the last interglacial serves as the most recent and best preserved. There were 2 rapid and extreme positive thermal excursions at the very end of the last interglacial, the 2nd one being the strongest. It netted a sea level rise somewhere between +6 meters above present to possibly as high as +52 meters above present. Between the two thermal excursions, climate darted to near glacial conditions, and then went all-in after the 2nd one.

    These events occurred on the centennial scale, with 10’s of centimeters of sea level falls and rises on the decadal scale.

    “We will illustrate our case with reference to a debate currently taking place in the circle of Quaternary climate scientists. The climate history of the past few million years is characterised by repeated transitions between `cold’ (glacial) and `warm’ (interglacial) climates. The first modern men were hunting mammoth during the last glacial era. This era culminated around 20,000 years ago [3] and then warmed rapidly. By 9,000 years ago climate was close to the modern one. The current interglacial, called the Holocene, should now be coming to an end, when compared to previous interglacials, yet clearly it is not. The debate is about when to expect the next glacial inception, setting aside human activities, which may well have perturbed natural cycles.”

    Crucifix, M. and J. Rougier, 2009, “On the use of simple dynamical systems for climate predictions: A Bayesian prediction of the next glacial inception”, Published in Eur. Phys. J. Spec. Topics, 174, 11-31 (2009)
    http://arxiv.org/pdf/0906.3625.pdf

    If you really want to know what the really BIG question is in climate science, see if you can formulate it from this:

    “The possible explanation as to why we are still in an interglacial relates to the early anthropogenic hypothesis of Ruddiman (2003, 2005). According to that hypothesis, the anomalous increase of CO2 and CH4 concentrations in the atmosphere as observed in mid- to late Holocene ice-cores results from anthropogenic deforestation and rice irrigation, which started in the early Neolithic at 8000 and 5000 yr BP, respectively. Ruddiman proposes that these early human greenhouse gas emissions prevented the inception of an overdue glacial that otherwise would have already started.”

    conclude Muller and Pross (2007)
    https://www.researchgate.net/profile/Ulrich_Mueller7/publication/222561971_Lesson_from_the_past_present_insolation_minimum_holds_potential_for_glacial_inception/links/0c96051e593f3a593d000000.pdf

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