Another Antarctic ice shelf at risk of melt

By Bryan Walker 03/06/2012

In the last episode of the Climate Show Gareth drew attention (at 13:40)  to two recent papers suggesting that the Weddell Sea area of the West Antarctic Ice Sheet may be more vulnerable to warming than previously realised. One paper, published in Nature Geoscience, recorded that radar mapping has uncovered a deep sub-glacial basin close to the edge of the ice sheet at the head of the Filchner-Ronne ice shelf. The basin measures 100 by 200 km and is well below sea level, nearly 2km deep in places. The ice sheet, currently grounded above the deep basin, may be more unstable than previously thought and could quickly undergo ice loss.

In a related paper, published in Nature, models reveal that the Weddell Sea region may experience warmer ocean conditions at the end of the 21st century, which could provide the trigger for ice sheet change. Professor Martin Siegert of the University of Edinburgh, who led the project, said:

“This is a significant discovery in a region of Antarctica that at present we know little about. The area is on the brink of change, but it is impossible to predict what the impact of this change might be without further work enabling better understanding of how the West Antarctic Ice Sheet behaves.”

I’ve been reading an interesting collection of expert opinion on the second paper, made available by the New Zealand Science Media Centre (SMC) ; it was gathered by the Canadian SMC.  The seven experts who commented all thought the paper worthy of respect, and together provided a sense of the breadth and liveliness of scientific interest in the behaviour of the ice shelves.  I thought Hot Topic readers might be interested to get the flavour of the comments.

Dr Jeffrey Kavanaugh of the University of Alberta provides a useful summary of the paper and an assessment:

“Model investigations by the authors show that continued warming in the region will likely result in a reorganization of circulation patterns in the Weddell Sea basin.  The primary cause of these changes will be a decrease in local sea ice production, which will allow greater coupling between surface winds and water currents (even in the absence of increased wind speeds).  Model results indicate that as early as 2070, the relatively warm coastal ocean current will be consistently directed southwards, towards the Ronne-Filchner Ice Shelf via the Filchner Trough.  This will warm the waters contacting the underside of the ice shelf by ~2°C, resulting in a 20-fold increase in the rate of melting (to an average of ~4 meters per year).  This result appears robust, with the onset of these flow changes varying by a couple of decades depending on the climatic scenarios tested.  Two different coupled ice-ocean models of different architecture are also employed. Results for both are roughly similar, with the finite-element FESOM model (able to resolve finer features and additional dynamics) showing earlier increases in basal ice melting rates.”

Dr Robert Bindschadler, former Chief scientist, Cryospheric Sciences Laboratory, NASA Goddard Space Flight Centre, places the paper in the context of other studies:

“Ice shelves are thinning in multiple locations around Antarctica and the grounded glaciers are responding to this by accelerating and thinning.  These connections have been emerging in various scientific papers over the past 5 years.  The limitation has been being able to see into the future.  Hellmer’s paper shows how this troubling process could rapidly spread to one of the really large ice shelves.   This is a BIG DEAL because the ocean’s impact on the ice shelves/ice sheets is THE way ice sheets can lose the largest amount of ice in the shortest amount of time. … This work is a large step in the right direction to be able to get credible numbers in the hands of those who will have to decide how to adapt to these changes in the next few decades.”

Dr Shawn Marshall of the University of Calgary also sets the paper in the context of developing scientific understanding:

“This is solid new work which increases the emphasis on what glaciologists are just starting to understand: that in Antarctica, ice sheet collapse and sea level rise are all about the ocean.  The ice shelf in the Weddell Sea sector is one of the two ‘linchpin’ ice shelves in West Antarctica, and it would be a frightening prospect for global sea level rise if this unhinged. It has long been known that the West Antarctic Ice Sheet is intrinsically unstable, and it is currently losing mass, but I am not sure how many glaciologists expect the ice sheet to unravel this century. Their prediction that circumpolar deep water could get into this sector of the ice sheet this century is the first time anyone has put a timetable on large-scale ice sheet changes.  Even if there is only a 10% chance that this proves true, this has huge implications for forecasts of sea level rise.  It could easily double the current IPCC [International Panel on Climate Change] projections for 2100.

“The mechanisms of ocean-ice shelf interactions are only just being understood in the scientific community, but everything we have learned in the last decade (e.g., in the Amundsen Sea and in Greenland) tells us that this is a real threat to the ice sheet.  Certainly waters that are 2°C in contact with the ice shelf would lead to its quick demise. It is just a question of whether the Antarctic sea ice changes really occur as forecast in the model, as this leads to the increased mixing and shifts in ocean circulation that deliver the warm water to the ice sheet. So far, Antarctic sea ice has proven to be very resilient to climate change, so we have not yet seen the initial stages of what is projected in this climate model.”

Dr Christian Schoof of the University of British Columbia recognises the implications:

“This paper shows that we may see a drastic increase in melting of floating ice tongues off the coast of Antarctica during this century. If this does occur, it is likely that ice flow from the interior of Antarctica towards the coast will accelerate, pushing more ice into the ocean. Just like adding ice cubes to a glass of water raises the water level in the glass, pushing more ice into the oceans will increase sea levels.

“The potential of ice stored inland in Antarctica to raise sea levels is large: West Antarctica alone contains enough ice to raise sea levels on average by 3.5 metres. Sea level rise on that scale will not happen instantly, and is likely to take several hundred years. But sea level even on a smaller scale of tens of centimetres is enough to put coastal infrastructure and settlements into jeopardy.”

And assesses the modelling:

“The methodology used is sound by today’s standards. It uses a computer model of ocean and atmosphere movements to calculate how much warm water will reach the bottom of the floating ice tongues. Confidence in the model comes from its ability to reproduce ocean behaviour during the 20th century [i.e. in the past]. As with all projections into the future, we will only be able to see exactly how well the model works for a warmer climate after the fact – so in 50 to a100 years’ time.”

Dr J Graham Cogley of Trent University also acknowledges that the paper is a projection, but sees it as a likely one:

“The Hellmer study is a projection into future decades, not a description of what is happening now. But it is valuable because it dramatizes what is very likely to happen to a part of the margin of Antarctica that is relatively sheltered today. There is no reason to expect that continued global warming will not have consequences like those described by Hellmer and his co-authors, and on the scale that they suggest. The best time to start trying to avert these consequences was a couple of decades ago.”

Dr Jeremy Fyfe of the Los Alamos National Laboratory recognises the disturbing nature of the findings:

“Ultimately, this study implies that regional Southern Ocean circulation changes due to human-caused planetary warming will cause dramatic increases in melt rates under the massive Ronne-Filchener Ice Shelf (and probably other Antarctic ice shelves as well). This melting has the potential to drive Antarctic ice loss and sea level to heights that have not been seen for millions of years.  Ultimately, it is a profoundly disturbing statement from the climate science community about the consequences of unabated greenhouse gas emissions.”

Dr Martin Sharp of the University of Alberta is cautious, pointing out that it is a model-based study demonstrating something that potentially could happen under specific circumstances:

“…– it should not be viewed as making a definitive projection, or as showing something that already is happening. Different climate models forced in different ways might lead the ice-ocean model to produce quite different results – either more or less dramatic.

“Having said that, the model results do demonstrate something that could be watched for in observations – and the changes suggested are sufficiently large that it might be a good idea to try to do this.”

The paper he finds clearly written and the study carefully conducted.

“The authors are clear (and realistic) about both the implications and limitations of the results. We should take them for what they are and not over-react.”

A few impressions formed for me as a lay person as I read the comments from which the above quotes are extracted. They illustrate the fact that climate studies emerge from a wide community of scientific study and are closely scrutinised by that community, giving solidity to the consequent overall picture of global warming. I was aware of the sober tone of the writers, careful in their recognition of the limits of the study, rational in their estimates of its relative significance. The restraint in their statements about sea level rise is apparent, but there is no mistaking the potential magnitude of what the gradual disintegration of the ice sheets will mean.

The comments reinforced for me the essential sanity in what the science is telling us. It contrasts with the madness of imagining that we can carry on mining and burning coal, or enter into a “golden age of gas” as the current vision holds, without causing the great ice sheets at the poles to slowly surrender their mass to the sea, with appalling consequences for coming generations.