Deshielo ' irreversible' de la Antártida
La contracción de los glaciares en la Antártida occidental ha cruzado un umbral y es ahora "irreversible" e "imparable", según afirmaron el lunes 12 de mayo de 2014 científicos que han estudiado los datos recogidos durante cuatro décadas por la NASA.
"Este estudio muestra que hemos cruzado un umbral crítico", afirmó Tom Wagner, científico del programa de criósfera en la agencia espacial estadounidense, y añadió que "no se sustenta en simulacros de computadora o modelos numéricos".
"Esto se apoya en la interpretación empírica de más de cuarenta años de observaciones desde satélites de la NASA", agregó.
Eric Rignot, profesor de Ciencias del Sistema de la Tierra de la Universidad de California y glaciólogo de la NASA, sostuvo que "la observación deja en evidencia un gran sector de retirada irreversible de los glaciares".
"Hemos pasado ya el punto sin retorno y esto tendrá consecuencias mayores para los niveles de los mares en todo el mundo", dijo Rignot, quien agregó que este proceso "podría triplicar su contribución al nivel de los océanos".
Ya desde la década de 1970, los científicos que estudiaban los glaciares señalaron como amenazada la región de la Antártida sobre el mar de Admunsen, donde se encuentran seis glaciares gigantes: Pine Island, Thwaites, Haynes, Pople, Smith y Kohler.
Todos ellos bajan de las montañas hasta el mar y los científicos explicaron que la línea de asentamiento, el área donde la base del glaciar toca el suelo submarino, ha estado retrocediendo de manera rápida en las últimas décadas.
A medida que el agua más templada se desliza debajo del manto flotante de hielo, erosiona la base del glaciar y la línea de asentamiento "ha estado retrocediendo a una velocidad que no se ve en ninguna otra parte de la Antártida".
Derretirse hasta desaparecer
"El hielo que se descarga en el océano ha estado incrementándose continuamente durante más de cuarenta años", añadió Rignot, quien calculó la contracción en el caso del glaciar Smith en unos 35 kilómetros, a razón de 2 kilómetros por año.
"Estos glaciares continuarán derritiéndose hasta que desaparezcan y el hecho de que los glaciares reaccionan casi simultáneamente indica una causa común", que es el "aumento de las temperaturas en el océano", aseguró.
Sridhar Anandakrishnan, profesor de geociencias en la Universidad estatal de Pensilvania, dijo en la misma teleconferencia que "el cambio que está ocurriendo es enorme".
"Es bien claro que el derretimiento del manto de hielo tendrá un papel creciente en el aumento de los niveles del mar", dijo. "La situación en la Antártida occidental -agregó- es particularmente mala, con una retirada acelerada de la línea de asentamiento"
West Antarctic glaciers melting - NASA
Updated 13 May 2014
NASA has warned that part of the Antarctic ice-sheet appears to be in what it calls irreversible decline with serious consequences for the world's sea levels.
Researchers analysed 40 years of observations of six big ice streams draining into the Amundsen Bay and concluded that nothing can stop them melting away.
Although these are abrupt changes - in terms of geological time - the timescales involved are likely measured in centuries.
The BBC reported Amundsen Bay had some of the biggest and fastest moving glaciers on Earth.
Scientists said sea levels could rise 3m to 5m over hundreds of years as a result.
The lead author Eric Rignot said countries - and especially coastal communities - needed to act.
The new study has been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union, but Nasa held a teleconference on Monday to brief reporters on the findings.
Prof Rignot said warm ocean water was relentlessly eating away at the glaciers' fronts and that the geometry of the sea bed in the area meant that this erosion had now entered a runaway process.
"We present observational evidence that a large section of the West Antarctic Ice Sheet has gone into a state of irreversible retreat; it has passed the point of no return," the agency glaciologist explained.
"This retreat will have major consequences for sea level rise worldwide. It will raise sea levels by 1.2m .... but its retreat will also influence adjacent sectors of the West Antarctic Ice Sheet which could triple this contribution to sea level."
The Amundsen Bay sector includes some of the biggest and fastest moving glaciers on Earth.
Pine Island Glacier (PIG), over which there had been intense research interest of late, covers about 160,000 sq km, or about two-thirds the area of the UK.
Like the Thwaites, Smith, Haynes, Pope, Smith and Kohler Glaciers in this region - the PIG has been thinning rapidly.
And its grounding line - the zone where the glacier enters the sea and lifts up and floats - has also reversed tens of km over recent decades.
What makes the group vulnerable is that their bulk actually sits below current sea level with the rock bed sloping inland towards the continent.
This is a geometry, say scientists, that invites further melting and further retreat.
The new study includes radar observations that map the underlying rock in the region, and this finds no ridge or significant elevation in topography that could act as a barrier to the glaciers' reverse.
"In our new study, we present additional data that the junction of the glaciers with the ocean - the grounding line - has been retreating at record speeds unmatched anywhere in the Antarctic," said Prof Rignot.
"We also present new evidence that there is no large hill at the back of these glaciers that could create a barrier and hold the retreat back. This is why we conclude that the disappearance of ice in this sector is unstoppable."
The researcher, who is also affiliated to the University of California, Irvine, attributed the underlying driver of these changes to global warming.
This, together with atmospheric behaviours influenced by a loss of ozone in the stratosphere, had created stronger winds in the Southern Ocean that were now drawing more warm water towards and under the glaciers.
Dr Tom Wagner, the cryosphere program scientist with Nasa, said it was clear that, in the case of these six glaciers, a threshold had been crossed.
"The results are not based on computer simulations or numerical models; they are based on the interpretation of observations," he told reporters.
"And I think this is an important point because this sometimes can get lost on the general public when they're trying to understand climate change and the implications."
Professor Rignot and colleagues put no real timescales on events, but a paper released by the journal Science to coincide with the Nasa media conference tries to do just this.
It does include computer modelling and was led by Dr Ian Joughin, a glaciologist at the University of Washington's Applied Physics Laboratory. The study considers the particular case of Thwaites Glacier.
In the model, Dr Joughin's team is able to reproduce very accurately the behaviour of the glacier over the past 20 years.
The group then runs the model forwards to try to forecast future trends.
This, likewise, indicates that a collapse of the glacier is inevitable, and suggests it will most likely occur in the next 200 to 500 years.
Professor Andy Shepherd, from Leeds University, UK, is connected with neither Rignot's nor Joughin's work.
He told the BBC: "[Joughin's] new simulations are a game changing result, as they shine a spotlight on Thwaites Glacier, which has until now played second fiddle to its neighbour Pine Island Glacier in terms of ice losses.
"There is now little doubt that this sector of West Antarctica is in a state of rapid retreat, and the burning question is whether and how soon this retreat might escalate into irreversible collapse. Thankfully, we now have an array of satellites capable of detecting the tell-tale signs, and their observations will allow us to monitor the progress and establish which particular scenario Thwaites Glacier will follow."
Prof Shepherd said the EU's newly launched Sentinel-1a radar satellite would have a unique capability to assess the glaciers' grounding lines.
"As soon as the satellite reaches its nominal orbit, we will turn its eye on Thwaites Glacier to see whether it has indeed changed as predicted."
The Amundsen Sea glacier beds are below sea level and slope deeper as they continue inland. This image shows the beds of Thwaites and Haynes glaciers, with colors indicating depth. The blue area under Thwaites Glacier is almost three-quarters of a mile below sea level. Image credit: Eric Rignot
The glaciers studied by Rignot's research team. Red indicates areas where flow speeds have increased over the past 40 years. The darker the color, the greater the increase. The increases in flow speeds extend hundreds of miles inland. Image credit: Eric Rignot