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Saturday, May 26, 2012

Waking the giant: Global Warming in the Weddell Sea, West Antarctic Ice Sheet and sea level rise

Warm ocean currents are projected to melt the Filchner-Ronne Ice Shelf in the Weddell Sea area of Antarctica opening instabilities in the West Antarctic Ice sheet (WAIS) which will impact global sea level rise. Climate change is waking up the sleeping giant of Antarctica.

Significant scientific research has been published in recent weeks on the impact of global warming on changing wind patterns and southern ocean currents and the flow-on impact on Antarctic ice shelves and glaciers. The most recent studies reveal the potential instability of the Filchner-Ronne Ice Shelf in the Weddell Sea area. But the real questions to be asked concern the long term stability of the West Antarctic Ice Sheet (WAIS) and how rapidly it could collapse raising global sea levels by up to 6 metres.

I highlighted in November 2011 that the Thwaites and Pine Island Glaciers accelerating, West Antarctic Ice Sheet losing mass in the Amundsen Sea area. Now climate researchers of the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association studying the opposite side of the West Antarctic Ice Sheet (WAIS) in the Weddell Sea area have discovered a mechanism which will drive warm ocean water towards the coast in the later decades of this century, allowing a higher rate of basal melting - the melting from underneath of ice shelves and glaciers by warmer ocean currents - leading to greater discharge of ice from the icesheet.

Another research team has found a large deep basin, about 20,000 square kilometres in size and up to 2 kilometres deep, underneath the ice sheet in the Weddell Sea area, which makes the ice sheet inherently unstable if pinion points for the ice shelf retreat and warmer water enters the basin.

Warm ocean currents to melt ice shelf in the Weddell Sea

The results surprised many researchers who assumed the Weddell Sea area would remain insulated from the impacts of climate change for a long while due to theever present pack ice. There has been substantially more attention focussed on the Amundsen Sea side of WAIS where ice mass loss is already accelerating with warmer ocean currents causing basal melt.

"The Weddell Sea was not really on the screen because we all thought that unlike the Amundsen Sea its warm waters would not be able to reach the ice shelves. But we found a mechanism which drives warm water towards the coast with an enormous impact on the Fichner-Ronne Ice Shelf in the coming decades", says Dr. Hartmut Hellmer, oceanographer at the Alfred Wegener Institute and lead author of the study.

According to the model to analyze ocean current changes, a chain reaction triggered by rising air temperatures above the south-eastern Weddell Sea could lead to large ice masses sliding into the ocean within the next six decades.

"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", explains Hartmut Hellmer.

With the protective barrier gone 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. At this point the Filchner-Ronne Ice Shelf is melting today at a rate of around 5 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.

Huge ice streams are held in place by the ice shelves. "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", explains Hartmut Hellmer. "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", adds Jürgen Determann. 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. Another 1.7 millimetres per year is due to thermal expansion of the oceans.

The results of the ocean models used for the projection - BRIOS (Bremerhaven Regional Ice Ocean Simulations) and FESOM (Finite Element Sea Ice Ocean Model) - were systematically checked against Ocean data dating back over the last 150 years.

"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", says Ralph Timmermann and adds: "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 authors highlight the implications for global sea level rise in the abstract of the study:

The projected ice loss at the base of the Filchner-Ronne Ice Shelf represents 80 per cent of the present Antarctic surface mass balance. Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise.

Caption: 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. Source: Alfred Wegener Institute

But the story doesn't end there. Another paper published in Nature Geoscience at the same time highlighted a new potential instability in West Antarctic Ice Sheet in the Weddell Sea area.

Subglacial basin discovered in Weddell Sea area highlights ice sheet instability

A research team from the U.S. and U.K. have uncovered a previously unknown sub-glacial basin nearly the size of New Jersey or Wales beneath the West Antarctic Ice Sheet (WAIS) near the Weddell Sea. According to the research, the location, shape and texture of the mile-deep basin suggest that this region of the ice sheet is at a greater risk of collapse than previously thought.

Image Caption: Bottom Image: This radar image of bedrock elevation reveals the new sub-glacial basin (purple and blue regions). The basin is divided into two components (A and B) and lies just inland of the West Antarctic Ice Sheet's grounding line (black line), where streams of ice flowing toward the Weddell Sea begin to float. Top Image: White box indicates location of bottom image. Pine Island Glacier (PIG) and Thwaites Glacier—two parts of the West Antarctic Ice Sheet previously studied by the U.S. and U.K. researchers—drain into the Amundsen Sea.

"If we were to invent a set of conditions conducive to retreat of the West Antarctic Ice Sheet, this would be it," said Don Blankenship, senior research scientist at The University of Texas at Austin's Institute for Geophysics and co-author on the new paper. "With its smooth bed that slopes steeply toward the interior, we could find no other region in West Antarctica more poised for change than this newly discovered basin at the head of the Filchner-Ronne Ice Shelf. The only saving grace is that losing the ice over this new basin would only raise sea level by a small percentage of the several meters that would result if the entire West Antarctic Ice Sheet destabilized."

The study was carried out by climate researchers at the University of Edinburgh with the British Antarctic Survey and the Universities of Aberdeen, Exeter and York, as well as The University of Texas at Austin. The researchers identified that the basin covers 20,000 square kilometers (7,700 square miles), nearly the size of New Jersey or Wales, and is well below sea level, nearly 2 kilometers (about 1.2 miles) deep in places.

"This is a significant discovery in a region of Antarctica that at present we know little about," said Professor Martin Siegert of the University of Edinburgh, who led the project. "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."

The scientists have identified that the seaward edge of the basin lies just inland of the ice sheet's grounding line, where streams of ice flowing toward the sea begin to float. The basin is shaped like a cereal bowl with its edges sloping down steeply. If the grounding line retreats upstream, seawater will replace it and more ice will float. This will allow warmer seawater to increase basal melting - a positive feedback mechanism which would sustain retreat of the ice sheet until eventually all of the ice filling the basin goes afloat. The bed of the basin is also very smooth with very few big bumps to act as "pinning points' to hold back sliding ice.

Warm ocean currents identified as primary process melting Ice shelves

A NASA and British Antarctic Survey study, published in Nature in April 2012, highlighted that 20 of the 54 ice shelves studied in Antarctica are being melted by warm ocean currents. Most of the present impact is in West Antarctica where ocean driven thining is responsible for rapid ice losses by the Thwaites and Pine Island Glaciers.

"We can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt," said the study's lead author Hamish Pritchard of the British Antarctic Survey in Cambridge, United Kingdom. "The oceans can do all the work from below."

This highlights that much of the global warming taking place is happening in the world's oceans. Atmospheric temperature rises are just the metaphorical tip of the iceberg of the global warming effect on increasing ocean warming.

In Antarctica and above the southern ocean wind patterns are changing, which are changing ocean currents. The incease in basal melt and glacier acceleration has been linked to changes in wind patterns. "Studies have shown Antarctic winds have changed because of changes in climate," Pritchard said. "This has affected the strength and direction of ocean currents. As a result warm water is funnelled beneath the floating ice. These studies and our new results suggest Antarctica's glaciers are responding rapidly to a changing climate."

Dr Hamish Pritchard explains further from the British Antarctic Survey media release:

"In most places in Antarctica, we can't explain the ice-shelf thinning through melting of snow at the surface, so it has to be driven by warm ocean currents melting them from below."

"We've looked all around the Antarctic coast and we see a clear pattern: in all the cases where ice shelves are being melted by the ocean, the inland glaciers are speeding up. It's this glacier acceleration that's responsible for most of the increase in ice loss from the continent and this is contributing to sea-level rise."

"What's really interesting is just how sensitive these glaciers seem to be. Some ice shelves are thinning by a few metres a year and, in response, the glaciers drain billions of tons of ice into the sea. This supports the idea that ice shelves are important in slowing down the glaciers that feed them, controlling the loss of ice from the Antarctic ice sheet. It means that we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt -- the oceans can do all the work from below."

"But this does raise the question of why this is happening now. We think that it's linked to changes in wind patterns. Studies have shown that Antarctic winds have changed because of changes in climate, and that this has affected the strength and direction of ocean currents. As a result warm water is funnelled beneath the floating ice. These studies and our new results therefore suggest that Antarctica's glaciers are responding rapidly to a changing climate."

Further north on the eastern Antarctic peninsula warm summer winds also directly melt the snow on the ice-shelf surfaces, as well as basal melting from the warmer ocean currents.

ICESat laser altimetry measurements combined with ocean and tidal models were used in the study. The data of 4.5 million surface height measurements was collected from October 2003 to October 2008. They measured how the ice shelf height changed over time and ran computer models to discard changes in ice thickness because of natural snow accumulation and compaction. The researchers also used a tide model that eliminated height changes caused by tides raising and lowering the ice shelves. ICESat operated from 2003 to 2009. It's absence leaves a hole in collecting valuable data on accelerating changes happening in polar regions due to climate change. A replacement satellite - ICESat-2 - is not due for launch until 2016.

The study authors conclude in their paper:

that the most profound contemporary changes to the ice sheets and their contribution to sea level rise can be attributed to ocean thermal forcing that is sustained over decades and may already have triggered a period of unstable glacier retreat.

I am going to repeat myself here, but this series of scientific papers adds increasing weight to the research and modelling discussed in my November 2011 post - Global Warming in Antarctica: Thwaites and Pine Island Glaciers accelerating, West Antarctic Ice Sheet losing mass, in particular the last section West Antarctic Ice Sheet losing mass - stability threatened. I stated then:

The melting of the Greenland ice sheet is already well documented with Greenland seting a new melt record in 2010, and Greenland melting in 2011 well above average with near-record mass loss. We may be witnessing the start of the destabilization of the West Antarctic Ice Sheet (WAIS). The collapse of these ice sheets, once started will be impossible to stop and will contribute to substantial sea level rise that will affect coastal areas of Australia and around the world. Sea levels will rise slowly, then accelerate and continue for several centuries. In the distant past sea levels have risen at a speed of up to one metre per 20 years, although we are unlikely to see that rate this century.

Scientists studying the geological record have determined that at slightly above current temperatures we are about 20 metres below what the sea level equilibrium should be I reported in March 2012.

Are we seeing the first signs of the rapid collapse of the West Antarctic Ice Sheet? Will it take 300 years or 3000 years? Either way, the rise in global sea level will prove catastrophic for coastal areas and populations around the world.

Antarctica, the sleeping giant, is waking up. We are shaking the giant through our indiscriminate greenhouse gas pollution of the atmosphere.


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