Week in Review
Drilling into the past to predict the future
Climate change is at the centre of a remarkable international drilling operation into Antarctica’s Ross Ice Shelf this summer. David Williams reports.
If your patient is seriously ill then making radiological scans, and analysing the resulting grainy images, can suggest the source of the problem. But there’s no substitute for surgeons actually opening them up and having a look.
The same applies to Antarctica’s Ross Ice Shelf, the world’s largest floating body of ice.
For several summers, scientists have been assessing what lies in front of the stalled Kamb Ice Stream, about 850 kilometres from Scott Base, near the “grounding line”, where the ice leaves land and starts floating. Radio waves have been fired into the ice and small explosives set off, creating “echoes” in black-and-white images much like those produced in hospitals.
The surveys near Kamb reveal a 600-metre-thick chunk of ice – as deep as Wellington’s Mt Victoria tunnel is long. Washing underneath is a relatively shallow 30 metres of water, part of the world’s least-explored ocean. And, at the bottom, there’s at least 200 metres of sea-floor sediment.
This season, while it’s daylight every hour of the day, a multi-disciplinary group of scientists, led by New Zealand, will set up camp and drill through the ice shelf to reveal the sub-surface secrets. Collaborators include the United States’ space agency NASA, and China’s Jilin University.
There’s something for everyone – engineers working the kilometre-long hot water drilling hose, as well as glaciologists, oceanographers, and paleoclimatologists. It’s a data treasure trove because the area is little explored. The ice shelf is about the size of France and the ocean cavity comparable to the North Sea.
There’s scant understanding of how the ocean water circulates because very few holes have been drilled through the shelf, says University of Otago associate professor Andrew Gorman, a marine seismologist who specialises in sea floor imaging. “It’s not a very good sampling.”
Craig Stevens is a physical oceanographer with Crown research institute National Institute of Water and Atmospheric Research, NIWA. The Wellington-based Australian, who became a New Zealander just this month, says it’s a “pretty weird piece of ocean”. “It’s very different to basically anywhere else in the planet.”
The sea-floor sediments are basically records of past change.
This untold narrative of what’s beneath the Ross Ice Shelf is crucial for the wider story of our warming world.
Floating ice is holding back the land-locked ice sheet, part of an Antarctic system that holds almost 90 percent of the Earth’s ice mass. Should warm ocean water trigger the shelf’s instability, sea level could eventually rise by metres, threatening coastal cities around the world. (For more on sea level rise, read this Eloise Gibson story.)
Nearby Pine Island and Thwaites Glaciers, which flow into the Amundsen Sea, are already thinning, accelerating and receding. Has some instability already been triggered, which will lead to deglaciation of the interior of West Antarctica?
“We don’t know if that’s the case or not,” says University of Otago Professor Christina Hulbe, a glaciologist who leads the Ross Ice Shelf drilling project. “It hasn’t started over here [on the Ross Ice Shelf] yet. But it might.”
The not knowing keeps scientists up at night, she says. “And wanting to fill in the details to improve the projection.”
West Antarctica might be seriously sick, so it’s time to open up the patient and take a look.
Antarctica is divided by the 3200km-long Transantarctic Mountains, with an ice sheet on either side. The larger, thicker, and more stable East Antarctica ice sheet, on which the South Pole sits, has some of the coldest and driest conditions on earth. Because of its shape and elevation, West Antarctica’s ice sheet is more vulnerable to climate change.
Hulbe says satellite maps show that the bed on which ice is grounded in West Antarctica is far below sea level “and that makes this region particularly vulnerable to rapid change”. Evidence from the continental shelf suggests that the response to past climate change hasn’t been uniform. “It’s happened in fits and starts,” she says.
The West Antarctic Ice Sheet is dominated by ice streams, fast-flowing rivers of ice surrounded by more ice. The streams penetrate far into the interior and move the ice relatively quickly (compared to the East) to the coast.
As the ice moves from land to floating on water, going from a “sheet” to a “shelf”, it tends to speed up, stretch out and get thinner, eventually calving off at the ice shelf's imposing front face. The ice sheet rests in a bowl-shaped depression, with its base far below sea level – a shape that inherently causes instability. Hulbe says once the fast retreat begins, the ice will always tend to go afloat as there’s no higher ground to stabilise on.
Victoria University of Wellington’s Nick Golledge, a climate modeller focused on ice sheets and sea level rise, says depending on the simulations, it’s predicted that by the end of the century there could be anything between slight melting of the Ross Ice Shelf and its complete loss. “So, quite a lot of uncertainty.”
Huw Horgan, Golledge’s colleague at Victoria’s Antarctic Research Centre, says 90 percent of the ice that leaves Antarctica does so via ice streams. “So that really tells you if we want to know how ice leaves the continent we need to understand ice streams.”
The big concern, Otago University’s Hulbe says, is a "non-linear" response – what’s called marine ice sheet instability – started by warming and melting at the coast. In a linear response, a little bit of warming gets a little bit of response. With non-linear instability, what starts as a small change is amplified into a runaway response. (The same ice flow physics is at work in both cases, Hulbe says – the difference is the rate of change.)
As Hulbe puts it, the rate of retreat depends on the rate of retreat. “So it just goes faster and you can’t stop it, until the climate changes such that the ice sheet starts to re-grow.”
That puts greater weight on this season’s health check near Kamb Ice Stream.
NIWA’s Stevens says the drill site is “where the rubber hits the road in terms of the whole melting story” – where the under-shelf ocean narrows. This season’s work will be the furthest south Stevens has been, after conducting much of his previous Antarctic research along the northern edge of ice shelves, studying the outflow impacts on sea ice and ocean flows.
“This is the first point at which the warm ocean and the warming ocean contacts the ice and starts melting and starts setting off a train of processes that we’re trying to measure and model and predict how they’ll influence the future ocean.”
Through the drilling, scientists want to know how much melting is happening on the underside of the ice shelf, what ocean temperatures are like, the melt-water outflow from the grounded ice, and, through the sea-bed sediment cores, how that area has responded to past changes.
Gavin Dunbar, also of Victoria’s Antarctic Research Centre, is one of New Zealand’s most experienced geologists at examining Antarctic sediment cores. He first went to the ice as a post-doctoral fellow in 2003, to conduct a site survey for the multi-country, $US30 million ANDRILL project. (What’s being attempted this summer is smaller, less ambitious and, importantly, more affordable.)
“The accumulation of sedimentary rocks, is basically nature’s tape recorder,” Dunbar says. “The sediments accumulate over time and the type of sediment – the micro-fossils within them, the size of the particles, all of those things – indicate what the environment was like at the time this material was deposited.”
ANDRILL records, for example, show that three or four million years ago the earth was about three or four degrees warmer than today. That rock record, collected near Scott Base, shows that where there’s now ice, back then there was open water. “We can use that information to build up a picture of how Antarctica responds to climate change on a large-scale, long timeframe.”
Drillers near the Kamb Ice Stream will use equipment last used in 2017, at a site dubbed Hot Water Drilling Site Two, or HWD2 (even though it was the first drilled), about 350 kilometres from Scott Base, nearer the middle of the Ross Ice Shelf. This season’s drill site is known as HWD1.
A kilometre-long hose uses hot-water jets from a nozzle to “drill” through the ice. That pilot hole is then reamed wider to about 30cm. (“As soon as you make it it starts to freeze closed again,” Otago’s Hulbe says. “It’s a battle against temperature and salinity, as soon as you make the borehole.”)
Then the fun starts, with winches sending down scientific equipment.
As a one-off this season, the team will collaborate with China’s Jilin University, which has designed a specialist sediment coring tube. NASA, meanwhile, in association with US university Georgia Tech, has developed a 3.5m-long, 130kg underwater robot called Icefin which one day might be sent to explore the oceans of Europa, Jupiter’s innermost icy moon.
“Their submarine obviously it comes with lots of NASA tech that we certainly couldn’t afford ourselves,” Dunbar says. “It’ll be a great thing if it works.” (Icefin was meant to be used at HWD2 in 2017 but 12 days of fog meant the plane couldn’t fly there.)
Cores collected from HWD2 were 65cm long, covering about 20,000 years of sedimentary history. The longest corer this season is 3m – which is expected to produce a record over tens of thousands of years rather than millions, but still useful.
“So if we get a good record there we’re confident that we’ll be able to get a much better understanding of what the most vulnerable, large bit of Antarctica was behaving like in the past, which was something that ANDRILL couldn’t do directly,” Dunbar says.
Scientists think the Ross Ice Shelf is fairly close to a state of balance right now, Golledge says. That’s an average – so while some parts, like Thwaites Glacier, are melting relatively quickly, other areas of ice are thought to be thickening. But that potentially good news is based on limited observations.
There are plenty of other important, unanswered questions.
More than 90 percent of the extra heat coming from increased greenhouse gas emissions is being stored in the ocean. Climate modeller Golledge says: “The chances are we’re going to put a lot of extra heat into the ocean, we just don’t really know how much.”
What does that mean for the ocean under the Ross Ice Shelf? That depends on how much heat from the open ocean is flowing into the cavity, and how much extra melting that causes on the ice’s underside.
NIWA’s Stevens believes there is more mixing within the interior of the ocean cavity than perhaps we originally thought. “This may actually be a mechanism for slowing down the melt process. It might be a slightly good news story – but that’s active research.”
On the flipside, there’s the potential effect of ocean mixing on the global thermo-haline circulation – in which cold, salty water sinks near the poles, flows along the ocean bed and slowly mixes back to the surface in the mid-tropics.
“To have that initial kick of salty, cold and oxygenated water, things need to be cold enough and structural enough to be actually growing lots of sea ice,” Stevens says. “If things warm up so much that that process closes down then we’ll change how that heat and salt and oxygen is injected into the planet’s oceans.”
Another Ross Ice Shelf conundrum is why the calving line, the shelf’s dramatic front face in the Ross Sea, has remained fairly static for the last 6000 to 6500 years, despite big temperature swings.
Part of the answer is thought to be pinning points, quirks of geography that can stall ice movement. One such point is Minna Bluff, a rocky promontory south of Ross Island, in the corner of the ice shelf. “It’s kind of like holding the cork in the bottle,” Golledge says
Worryingly, it’s an area known to have high melting. If the ice shelf retreats from that area that could cause huge problems, Golledge says. “Because it’s such a key part then once we lose that pinning point the ice shelf can then retreat a whole lot more quickly from that point on.”
It’s not all doom and gloom.
University of Otago’s Gorman, the marine seismologist, is adamant humans still have time to take action. “Quite often people are thinking it’s too late, I can’t do anything so let’s not even bother trying. But I think it really is worth trying to bring things back to some sort of level that will make things not quite as bad for future generations.”
Gorman’s colleague, Hulbe, says it matters when humans stop pushing the climate. “Even if you’ve committed to getting rid of a lot of the West Antarctica ice sheet, how fast it goes depends on how hard you were pushing when you stopped. In a weird way it’s an optimistic thing, right? It’s always going to matter that we do something.”
Climate modeller Golledge has just returned from France, where he’s helping write the ice sheet, sea level, and ocean chapter of the next big Intergovernmental Panel on Climate Change assessment report, due out in 2021. If his comments can be taken as some sort of medical report, then the planet’s health is heading south.
“Basically if we continue on the temperature emissions trajectory that we’re on, we’re pretty much going to lose those big ice shelves – the Ross and the Filchner-Ronne – probably within the next couple of centuries. That’s pretty bad news for the ice sheet, of course, and then the ice flows into the sea.”
Some studies suggest parts of West Antarctica, particularly in the Amundsen Sea embayment where Thwaites Glacier is, are essentially already undergoing retreat, and even if temperatures are stabilised at present-day values, that retreat won’t stop.
“Unfortunately, I guess it looks as if we’re not going to be in a position to completely prevent some loss of West Antarctica,” Gorman says.
That makes action by governments absolutely urgent. Gorman says the sooner temperatures are under control the more chance there is of slowing the warming process.
“It’s about buying us time. People talk about irreversible retreat and thresholds for collapse and stuff, but these things play out at different rates – and that’s what we have control over.
“It’s not a case of, ‘It’s already happening so there’s nothing we can do’. It’s very much, ‘It’s already happening so we have to do absolutely everything possible to make sure it happens slow enough that the effects can be managed and dealt with’.”