Antarctic ice sheet may melt more rapidly with thinning of ice shelf

May 10th, 2012 by Alfred Wegener Institute for Polar and Marine Research

The edge of the Filchner-Ronne ice shelf in the Weddell Sea, Antarctica (click image to expand - image courtesy of Alfred Wegener Institute ©Ralph Timmermann)

The Filchner-Ronne Ice Shelf fringing the Weddell Sea, Antarctica, may start to melt rapidly in this century and no longer act as a barrier for ice streams draining the Antarctic Ice Sheet.

These predictions are made by climate researchers of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association in the British science magazine “Nature“.

They refute the widespread assumption that ice shelves in the Weddell Sea would not be affected by the direct influences of global warming due to the peripheral location of the Sea.

The results of the climate modelers from the Alfred Wegener Institute will come as a surprise as the majority of experts assumed that the consequences of global warming for Antarctica would be noticeable primarily in the Amundsen Sea and therefore in the western part of Antarctica.

“We all thought that unlike the Amundsen Sea, the warm waters would not be able to reach the ice shelves of the Weddell Sea. But we found a mechanism which drives warm water towards the coast with an enormous impact on the Filchner-Ronne Ice Shelf in the coming decades”, said Dr. Hartmut Hellmer, oceanographer at the Alfred Wegener Institute and lead author of the paper, “Twenty-first century warming of a large Antarctic ice shelf cavity by a redirected coastal current.”

Using different model calculations, Dr. Hellmer and his colleagues, Dr. Frank Kauker, Dr. Ralph Timmermann and Dr. Jürgen Determann as well as Dr. Jamie Rae from Met Office Hadley Centre, U.K., demonstrated that as a result of a chain reaction large ice masses could presumably slide into the ocean within the next six decades.

This chain reaction is triggered by rising air temperatures above the southeastern Weddell Sea.

Today a consolidated sea-ice cover forms high-salinity shelf water (blue arrow) that prevents the warm coastal current (red arrow) from passing the coastal shelf and moving into the shelf ice cavity (click diagram to expand - diagram courtesy of the Alfred Wegener Institute)

“Our models show that the warmer air will lead to the currently solid sea ice in the southern Weddell Sea becoming thinner and therefore more fragile and mobile in a few decades“, says Frank Kauker.

If this happens, fundamental transport processes will change.

“This will mean that a hydrographic front in the southern Weddell Sea will disappear which has so far prevented warm water from getting under the ice shelf. According to our calculations, this protective barrier will disintegrate by the end of this century”, explained Hartmut Hellmer.

An inflow of warmer water beneath the Filchner-Ronne Ice Shelf will melt the ice from below.

“We expect the greatest melting rates near the so-called grounding line, the zone in which the ice shelf settles on the sea floor at the transition to the glacier.”

By 2070 the consolidated sea-ice cover is much reduced and no longer prevents the warm coastal current from moving into the shelf ice cavity (click diagram to expand - diagram courtesy of the Alfred Wegener Insitute)

“At this point the Filchner-Ronne Ice Shelf is melting today at a rate of around five metres per year. By the turn of the next century the melt rates will rise to up to 50 metres per year,” says Hellmer’s colleague Jürgen Determann.

Jürgen Determann is investigating how the ice streams behind will react in the event of a melt of such enormous proportions.

“Ice shelves are like corks in the bottles for the ice streams behind them. They reduce the ice flow because they lodge in bays everywhere and rest on islands. If, however, the ice shelves melt from below, they become so thin that the dragging surfaces become smaller and the ice behind them starts to move,” explained Hartmut Hellmer.

Simulated evolution of near bottom temperatures in the Weddell Sea. a-d. Values are from 60 m above bottom for the period 2030-2099 of the HadCM3-B/A1B scenario. Warm pulses into the Filchner Trough (2037) are followed by a return of the shelf water masses to the cold state typical for present conditions. The final (unrevoked) destruction of the slope front starts in 2066; by 2075, the tongue of slightly modified warm deep water reaches the Filchner Ice Shelf front. It fills the deeper part of the Filchner Ice Shelf cavity and enters the Ronne Ice Shelf cavity near the grounding line south of Berkner Island in 2081. By 2095, warm water fills most of the bottom layer of the Filchner-Ronne Ice Shelf cavity, reaching a quasi-steady state. (click diagram to expand - diagram courtesy of the Alfred Wegener Institute)

“If the high melting rates are completely compensated by inland ice flow, this loss in mass would correspond to an additional rise in global sea level of 4.4 millimetres per year,” Jürgen Determann said.

According to the latest estimates based on remote sensing data, global sea level rose for the period 2003-2010 at a rate of 1.5 millimetres per year due to melting of glaciers and ice shelves. This occurs in addition to the 1.7 millimetres per year due to thermal expansion of the oceans.

The forecasts of the current study are based on independent calculations of the ocean models BRIOS (Bremerhaven Regional Ice Ocean Simulations) and FESOM (Finite Element Sea Ice Ocean Model).

The scientists used the atmospheric projections of the British Met Office Hadley Centre in Exeter as forcing data.

These included, for example, information on the future development of the wind and of the temperature in Antarctica.

Floating ice shelves typically produce giant icebergs. An abrupt increase of basal ice shelf melting may cause a substantial thinning of the large Filchner-Ronne Ice Shelf in the Weddell Sea, Antarctica (click image to expand - image courtesy of the Alfred Wegener Institute ©Ralph Timmermann)

Hartmut Hellmer and his colleagues have thoroughly checked the model results for being realistic.

“We started the BRIOS model in 1860 to see whether its results also represent the current situation. We found that this condition was satisfied. For example, the water temperatures for the Weddell Sea predicted by BRIOS are close to those we have actually measured in the recent past,” Ralph Timmermann said.

“The BRIOS model has been verified on many occasions in the past. It correctly predicts sea ice thickness, concentration, and drift as well as circulation patterns. And FESOM is well on the way to attaining BRIOS status. However, it has a far higher resolution, which is why we have to wait a long time until the computer has calculated several decades and more. BRIOS only needs less than a week for a century.”

The study was conducted as part of the EU-funded research programme “Ice2sea” which was set-up to estimate the future contribution of continental ice to sea-level rise.

This project brings together scientists from 24 leading research institutions of the European Union and from Chile, Norway and Iceland.

Together, the scientists aim for decoding the interactions between ice and climate and in this way facilitate more precise predictions about the effects of melting ice on sea level.

 

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