The fate of our world’s coasts will be decided in the Antarctic, where huge ice shelves have to date prevented gigantic inland ice masses from pouring into the ocean and raising the sea level. But how long will this brake hold out? In the course of several expeditions, AWI researchers have placed instruments directly below the Filchner-Ronne Ice Shelf, allowing them to now monitor live whether – and, if so, how – heat from the ocean could pose a threat to the largest ‘ice brake‘ in the Antarctic.
In the past, climate researchers weren’t very worried about the Filchner-Ronne Ice Shelf. The massive sheet of ice, which is roughly the size of Sweden, conceals a bay in the far southern reaches of the Weddell Sea – a region in which the average air temperature was a frosty 11.4 degrees below zero in the summer of 2016. From the east, south and west, glaciers drain into the bay. They supply the Filchner-Ronne Ice Shelf with ice from the East and West Antarctic Ice Sheets and are the reason why the largest permanently floating ice shelf in Antarctica measures 1.6 kilometres at its thickest point. And the list of superlatives goes on: the coldest water masses in the world circulate below the ice shelf. With a temperature of minus 2.5 degrees Celsius, they only remain liquid because of the high pressure they’re under. “So far, this extremely salty water has kept the Filchner-Ronne Ice Shelf from melting,” says AWI oceanographer Dr Tore Hattermann. It’s formed in winter, when the surface water directly in front of the ice shelf freezes and brine from the sea ice seeps into the water below. The ocean’s uppermost layer becomes denser, sinks and forms a cold-water barrier on the continental shelf – i.e., before and below the ice shelf – that has kept warm water from the Weddell Sea from flowing under the ice shelf.
What happens when warm water from the Antarctic Circumpolar Current finds its way under ice shelves is something that polar researchers have been observing for the past few decades in the Amundsen Sea. Here, the ice shelves begin melting from below; as a result, they gradually lose contact with the ground, and with it, their buttressing effect on the glaciers pushing from behind. The ice streams of the Amundsen Sea lost over 334 gigatonnes of ice in 2013 alone, i.e., roughly 110 gigatonnes more than in 1994. If we compare the current melting and calving rates with data from 1977, they’re now losing 77 percent more ice than they did 40 years ago. At the same time, nearly all glaciers are now forcing their ice masses out to sea much faster than they did in the 1970s – a fact that also helps explain why this region alone accounts for ten percent of the global sea level rise.
Taken together, the hinterland of the Filchner-Ronne Ice Shelf holds so much ice that the global sea level would rise by twelve metres if this giant brake suddenly disappeared and all the inland ice poured out into the ocean. But will the Filchner-Ronne Ice Shelf share the fate of the ice streams in the Amundsen Sea?
To answer that question, oceanographers at the AWI are combining climate models (see the graphic on p. 20/21) with extensive measurements gathered in the Weddell Sea, both below and even inside the ice shelf. “Our models indicate that the Antarctic will grow warmer over the next few decades. As, this progresses, we’ll likely see less sea ice forming, which could cause the cold-water barrier to collapse in the second half of this century,” says AWI oceanographer Dr Hartmut Hellmer. “If that happens, the warm water could then flow directly under the ice shelf. Whether or not this development has already begun is something we’re investigating right now, on various Polarstern expeditions to the Weddell Sea and with the sensor chains that we’ve placed in and below the ice.”
To successfully install the devices below the ice, in the course of two summer expeditions Tore Hattermann and his German-British team drilled seven holes in the up to 900-metre-thick ice. “Three of our boreholes are to the north, roughly 60 kilometres behind the calving front; another four are located 200 kilometres farther south,” he explains. At all sites, thermistor chains deployed directly in the ice shelf are now documenting how cold the ice is in the different layers.
In the water below the ice shelf, sensors measure the water masses’ temperature, salinity, and their flow speed and direction. The readings are then sent to Tore Hattermann by satellite every night. “Every morning I receive 28 mails from the Filchner Ice Shelf, with the latest readings in the attachments,” says the 33-year-old researcher.
In January 2017 the British daily newspaper "The Guardian" reported on the joint Filchner drilling project of the Alfred Wegener Institute and the British Antarctic Survey.
This simple data transfer belies the logistical masterpiece it took to install all of the sensors. As Hattermann explains, “For our hot-water drilling we needed 13 tonnes of equipment, which had to be transported to the shelf on board the Polarstern and other ships, bit by bit, in the course of several consecutive summers. Plus there was all the fuel we needed for the Twin Otter flights, the snowmobiles and the generators. And the glaciologists took seismic readings on long transects to gain insights into the topography of the seafloor below the ice, which we previously knew nothing about.”
Parallel to the drilling on the ice shelf on board the research icebreaker Polarstern, AWI oceanographers Dr Michael Schröder and Svenja Ryan monitored the currents in the eastern Weddell Sea – roughly 250 kilometres north of the Filchner Ice Shelf. There, at the outlet of the Filchner Trough, a deep undersea trench, the researchers installed three sensor chains that monitor year-round whether or not the 0.8-degree ‘warm‘ water from the Weddell Gyre has made its way onto the continental shelf and into the Filchner Trough. According to Svenja Ryan, “Our data from the first three years shows seasonal variation in the flow. In the Antarctic summer, the warm water masses actually reach the continental shelf. But when winter comes, the colder, heavier ice shelf water becomes dominant again. Then, the warm water masses can’t make it up the steep slope of the continental shelf.”
Further, data gathered by her Norwegian colleagues has confirmed that, in certain phases, the warm-water pulses even reach the ice shelf front. But the AWI experts can’t yet say how long these pulses have been around, or if they’ve grown more intense and now reach below the ice; the Weddell Sea remains one of the least-researched seas in the world. “We’re still lacking in-situ data. Prior to these last two trips, Polarstern hadn’t been in the region since 1995 – and even then, we were only able to gather data for the summer. With the help of our new moorings under the ice shelf and at the edge of the Filchner Trough, we hope to continually record data for at least five years to help us understand the circulation system better and make more accurate forecasts,” explains oceanographer Michael Schröder.
The initial data from the AWI’s moorings under the ice shows that conventional assumptions about the circulation below the ice shelf were too simple. “We had always assumed that the circulation was dominated by cold water masses from the Ronne Ice Shelf cavern. But their influence seems to only affect the southern portion of the Filchner Ice Shelf. Below the northern portion, we mostly see water masses from the upstream Filchner Trough circulating,” says Hartmut Hellmer. As such, accurately predicting the future of the second-largest ice shelf in Antarctica will likely prove to be more difficult than the researchers had hoped.
Text: Sina Löschke
This article has been published in the AWI climate magazine. You can download the magazine as pdf