Australia: The Land Where Time Began
Antarctica – The Threat Beneath
The Australian ice-breaker RSV Aurora Australis had been attempting to batter its way through the heavy sea ice off the coast of East Antarctica for several days and was still trying to get through to their objective, to reach a part of Antarctica that previous attempts had failed to reach.
The attempt was saved when a wind change blew the ice away from the shore, which opened up a path through the pack ice. In January 2015 the ship eventually reached the coast of Antarctica shortly after midnight. Rintoul’s research team were the first scientists to reach the Totten Ice Shelf, a vast floating ice ledge that is the marine extension of the largest glacier in East Antarctica. For more than 12 hours the research team carried out non-stop probing of the water temperature and salinity, the speed and direction of the ocean currents as well as the shape and depth of the seabed. They also deployed automatic instruments that would continue operating after the ship had departed.
The first direct observations made by Rintoul’s team confirmed a fear they had for a long time: that the floating glacier tongue can be melted away from below by the warmer ocean waters surrounding it (Rintoul, S. R. et al., 2016). They suggested this could explain the thinning of the Totten Ice Shelf that had been occurring over the past few decades.
Some of the scary truths about East Antarctica, that vast, flat landmass to the east of the Transantarctic Mountains (see ‘Ice King’), are being revealed by findings such as these . East Antarctica is of a similar size as the entire USA, the majority of which stands on a high plateau that is about 4,093 m above sea level, where the temperature can go as low as -95oC. Researchers believed that the East Antarctic Ice Sheet had been stable in the past and was not likely to change in the future, because it was so cold and isolated, which is in stark contrast to the West Antarctic Ice Sheet, which is much smaller, which has raised alarms because many of its glaciers have been retreating rapidly.
According to Tas van Ommen, a glaciologist with the Australian Antarctic Division in Kingston, near Hobart, almost everything that had been believed to be known about East Antarctica has proven to be wrong, as has been shown by research over the past few years. His team found by flying planes carrying instruments capable of probing beneath the ice that a large part of East Antarctica is well below sea level, and as a result is more vulnerable to the warming ocean than previously believed. Also uncovered were clues that the massive Totten Glacier, which alone holds about as much ice as all of West Antarctica, has shrunk and grown repeatedly in the past (Aitken, A.R.A. et al., 2016) – which is also a sign that it could again retreat in the future.
Though East Antarctica doesn’t appear to be losing much ice at the present, there are indications that it is being affected by the rising temperature of climate change and is changing in measureable ways. According to Oiu this is disconcerting because the East Antarctic Ice Sheet is more than 10 times the size of the West Antarctic Ice Sheet. If just all of the ice below sea level in East Antarctica melted the oceans would be raised by 20 m.
In order to better predict what will happen in the future research is being carried out to gather as much information as possible about East Antarctica. The concern about the East Antarctic Ice Sheet is that it might reach a tipping point over the next few centuries. According to Rintoul once the retreat of the glaciers goes beyond a certain point things might get worse very rapidly which would cause a rapid rise in sea level.
East Antarctica is a region that has been neglected by climate researchers for a long time; Rintoul was among the first scientists who warned about possible trouble in that region. In 2013 his team combined satellite imagery, airborne surveys and climate models to detail the behaviour off ice around the margin of Antarctica. Evidence was found that 6 ice shelves in East Antarctica, including Totten, were melting from below at much higher rates than expected, and some of them were even rivalling fast-retreating glaciers in West Antarctica (Rignot, Jacobs, Mouginot & Scheuchl, 2013).
When the researchers took a closer look at some of those glaciers in East Antarctica there were more surprises. It was shown by satellite imagery and airborne surveys between 1996 and 2013 that the Totten Glacier surface had dropped by 12 m and it’s grounding line, the point on the coast where the tongue of the glacier begins to float on the ocean, had retreated inland by up to 3 km (Li et al., 2015).
Stokes et al. analysed satellite data that had been obtained between 1996 and 2012 covering all of the Antarctic coastal regions. There was no net ice gain or loss in most areas. The Wilkes Land region with an area larger than Greenland, that includes the Totten Glacier, is the only exception (Miles, Stokes & Jamieson, 2016). In this region ¾ of the glaciers retreated between 2000 and 2012. According to Stokes, Wilkes Land may be the weak point in in the East Antarctica Ice Sheet.
Ommen et al. flew over Antarctica to probe the underside of the Totten Glacier. The landscape beneath the ice is of fundamental importance for how the glaciers flow and the ways in which they respond to climate change. When an international initiative, ICECAP (International Collaboration for Exploration of the Cryosphere through Aerogeophysical Profiling) 10 years ago to survey systematically the landscape of East Antarctica nothing was known about the landscape beneath the ice.
Every southern summer the aircraft of ICECAP have been crisscrossing the vast continent using radar as well as gravitational and magnetic sensors to detect the landscape beneath the ice. Appearing to be featureless, the ice sheet is continually changing, with snow dunes sculpted by the wind and the ice simmering in thousands of shades under the unearthly Antarctic light; it has been compared to another planet.
The flights have revealed an astoundingly dramatic landscape that is hidden beneath the ice that is relatively flat. The existence of a 1,100 km long canyon, which is the longest in the world, and almost as deep as the Grand Canyon, has been confirmed by glaciologist Sun Bo at the Polar Research Institute of China in Shanghai. Van Ommen et al. discovered that 21 %m of the catchment of the Totten Glacier is more than 1 km below sea level, in previous flights over Wilkes Land, an area that is 100 times larger than had previously been estimated. It wasn’t expected to be as extensive as has been found.
Underwater troughs were also found that extended from the edge of the Totten Ice Shelf to the grounding line that was 125 km inland, as deep as 2.7 km below sea level (Greenbaum et al., 2015). Warming waters from offshore could be allowed by this deeply contoured landscape to rapidly reach and erode the ice from below.
When RSV Aurora Australis reached Totten in 2015 it was the first chance scientists had to study the fate of that water. Warm water at 0.3oC was detected near the glacier tongue by Rintoul’s team, which was much warmer than the -2oC, which is the freezing point of sea water (Rintoul, S. R. et al., 2016). It is the water temperatures higher than freezing that are driving the rapid melt rates. The automated instruments that were left in place after the expedition moved out of the ice have shown that the elevated water temperatures are present throughout the year. Rintoul et al. suggest that if the channel that has been found recently is used by the sea water to reach the grounding line of the Totten Ice Shelf, they will be at least 3.2oC warmer than the freezing point at that depth, which Rintoul says “would be really bad news”
It has been found that there are lakes beneath the Antarctic ice sheet that periodically send flood waters towards the coast, and these are another threat, originating in the Antarctic interior, to the ice shelves. Lake Cook, that is located beneath the ice sheet in Wilkes Land drained suddenly 10 years ago flushing 5.2 billion m3 of flood water, which was the largest event of this type to have been reported in Antarctica . According to Leigh Stearns, a glaciologist at the University of Kansas in Lawrence, this type of flood could be another destabilising factor, which causes the ice to flow more rapidly leading to more calving of icebergs.
According to researchers these are not just hypothetical scenarios. It has been revealed by studies over the last few years that East Antarctica has lost a lot of ice in the past, and could possibly do so again in the near future.
A 2010 expedition that was supported by the Integrated Ocean Drilling Program, which retrieved seafloor sediments off the coast of East Antarctica, found some evidence for the loss of large amounts of ice in the past. Getting those sediments proved to be a dangerous endeavour, as the ship had to repeatedly stop drilling to dodge massive icebergs. For ocean drilling the waters surrounding Antarctica are some of the challenging environments for ocean drilling.
It was revealed by the drilling that there had been surprising changes in the history of the East Antarctic Ice Sheet. It had long been believed that since the East Antarctic Ice Sheet had formed about 14 million years ago it had remained at its current size. It was believed it is a very large stable ice mass that wasn’t responding to climate change.
It was revealed by the seafloor sediments that at many times between 5.3 Ma and 3.3 Ma, in the Pliocene, a time when atmospheric temperatures were up to 2oC above those of the present, it had expanded and contracted (Cook et al., 2013). A clear was received every time there was a warm period, which suggests that the ice sheet was sensitive to climate warming.
According to the researchers they have some intriguing preliminary results from the most recent interglacial period, between 129,000 years ago and 116,000 years ago, a time when the Earth was as warm as it is at the present. At that time the ice sheet retreated slightly less than it did during the Pliocene, which was much warmer, which was a big surprise to the researchers.
Based on these results it seems a certain amount of ice can be lost quite easily with only a small amount of warming.
Researchers are becoming increasingly concerned about the future, as the vulnerability of East Antarctica comes into focus. Computer models that simulate the ice sheets response to a changing climate are the only way to forecast decades or centuries into the future.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|