By Finlay MacdonaldMarch 5, 2013
Ice contains a “memory” within its compressed crystals that we can now recover and turn into climate records. From the savage Antarctic comes a team of ice-core drillers — and arriving in New Zealand imminently is their ice, possibly the strongest evidence yet of the vulnerability of great ice sheets to global warming.
Christmas came early to a remote corner of Antarctica last year. After two summers of drilling, the 12-member crew of the Roosevelt Island Climate Evolution Project (RICE) finally made it through nearly 800 metres of solid ice, to bedrock, on December 20, 2012.
In doing so they had, figuratively speaking, travelled back in time more than 100,000 years, to the Earth’s last interglacial period, when the Earth’s ice caps receded and global sea levels rose.
The nine-nation team had spent three months in this remote location on the Ross Ice Shelf — a white expanse roughly the size of France — sleeping in tents, working laboriously in layers of extreme-cold-weather clothing, enduring the regular storms that make even an Antarctic summer a savage environment.
If they toasted their breakthrough, however, it was not without the knowledge that their findings could be cause for anything but celebration.
Because contained within the solid ice cores — which they had painstakingly extracted with the drill rig — lies information that could support a hypothesis that large parts of the frozen continent are more delicate, more dynamic and changing more rapidly than previously thought.
The ice cores are right now locked within two super-refrigerated containers onboard the last ship out of McMurdo Sound before the Antarctic winter closes in, and due to arrive in New Zealand in early March.
It is all precious cargo, but there is one sample that is of particular interest. When the very last ice core was drawn up from a depth of 763 metres, it showed imprints of the sediment on which it had been resting. The team sent the drill down one more time and managed to draw up a little of this sediment — which has the consistency of frozen mud.
The RICE team believes there is a strong probability it will turn out to be marine sediment, most likely from the interglacial period before the last ice age, 105,000 to 130,000 years ago, when temperatures were only a little higher than they are today.
If confirmed, it would mean Roosevelt Island was under water during that interglacial phase. This in turn would mean the Ross Ice Shelf was much smaller than it is today, or had in fact disappeared altogether. The huge West Antarctic ice sheet behind it would have been significantly smaller too.
“Of course,” says ice-core climatologist and RICE project leader Nancy Bertler, “that would mean we are far more susceptible to change than we might think.”
In other words, the pace and scale of Antarctic melting — and therefore the kind of rise in sea levels we all fear — could happen much faster than previously imagined.
COASTAL ICE CORE RESEARCH IS RELATIVELY NEW. Previously, scientists believed the great Antarctic interior, where the ice sheet can reach depths of 4 km, would yield the most useful results.
But in the coastal regions, where the sheet is thinner and the records correspondingly shorter, the cores have provided a complementary picture to those from the interior. “Being at the coast is right at the interface between the ocean and the atmosphere and the ice sheet. This is really where the change is most visible.”
The Roosevelt Island site, then, provides a measure of climatic conditions from the last interglacial period to the present. Within the past 30,000 years of that span, from the end of the last ice age to now, the Earth has experienced a temperature increase of about six degrees. That was enough to melt ice sheets in the northern hemisphere and cause the Ross Ice Shelf to contract up to a thousand kilometres southward to its present perimeter.
As a consequence of this melting, sea levels rose and the big West and East Antarctic ice sheets thinned a little. Throughout this period, Roosevelt Island has been quietly recording a year-by-year snapshot of global warming.
The data extracted from the ice cores will allow the historical CO2 record to be reconstructed, atmospheric and sea-surface temperatures to be calculated, and the sea-ice extent to be estimated. “It allows us now to establish how quickly the Ross Ice Shelf can react to warming and how quickly it retreats because of that warming, both in the ocean and in the atmosphere,” says Bertler.
Roosevelt Island sits at one edge of the Ross Ice Shelf. At the other edge lies Ross Island, home to New Zealand’s Scott Base, which serves as the transport and provisioning hub for the RICE expedition. The $NZ7 million project is a massive logistical operation, involving the transport of some 200 tonnes of cargo 600 kilometres across the ice to the drill site.
At the heart of the site lies a 30-metre-long trench, 10 metres at its deepest point, dug out with chainsaws and shovels. A small complex of caves off to the side serves as storage for the ice cores once they have been hauled up.
Inside the trench and beneath a sheltering tent sits the rig itself, purpose-built for the conditions and with a new hydraulic system designed to break off each ice core sample as cleanly and gently as possible.
According to Bertler, there was some initial skepticism from veteran ice-core drillers about the hydraulic innovation, but it proved ideal for coping with the peculiarities of Antarctic ice.
That is, when under intense pressure beyond a depth of a few hundred metres, Antarctic ice becomes what is known as “brittle ice”. The tiny bubbles of prehistoric atmosphere within — vital for analysing the climatic conditions when they were first formed — react dramatically to even the slightest warming.
Place a sample in your hand, for instance, and it may explode with a radius of several metres. “Imagine when you send an electro-mechanical drill down, then you pull with almost a tonnne of weight on it,” explains Bertler. “Very often this brittle ice comes up in bags rather than cores.”
The ice at Roosevelt Island may be tricky to extract, but it offers a clear window into our planet’s weather history. The island — to the naked eye nothing more than a large rise in the white expanse — is one of the “pinning points” of the Ross Ice Shelf (the other being Ross Island). That is, it anchors the mass of sea ice within the Ross Embayment.
The regular storm tracks that circle Antarctica inevitably penetrate the Ross Sea and the ice shelf, which means Roosevelt Island is a veritable databank of snow precipitation. “And this is our business,” Bertler says. “We read snow. We turn this into climate records.”
THE RICE PROJECT WAS IN PART DESIGNED TO SOLVE A MYSTERY left by a previous New Zealand-led drilling project, known as Andrill. Andrill involved the exploration of Antarctic marine sediment from the mid-Pliocene period, about three to five million years ago, and it showed that at some point during that epoch the entire Ross Ice Shelf had disintegrated.
The mid-Pliocene is important because we have to go that far back to find atmospheric CO2 concentrations equivalent to those of today. At about 400 parts per million (ppm) they were slightly higher than our 397 ppm, and the temperatures were slightly warmer than we’re experiencing now. But human activity is increasing current CO2 levels by about two ppm per year, with temperatures following close behind — suggesting we are rapidly approaching a known tipping point.
As Bertler points out, the collapse of the Ross Ice Shelf would have been dramatic in itself, but because the shelf is made of sea ice it already displaces its own mass and does not affect sea levels. However, what Andrill also showed was that the whole West Antarctic ice sheet had collapsed.
While the West Antarctic ice sheet is smaller than the East Antarctic sheet, it still contains more than two million cubic kilometres of frozen fresh water. The ice mass is so heavy it depresses the underlying rock by up to a kilometre.
When it previously collapsed — in conditions which also caused the northern hemisphere ice sheets to melt, and the margins of the East Antarctic sheet to collapse — global sea levels rose about 20 metres. “That’s very significant, obviously,” says Bertler with admirable understatement.
What Andrill couldn’t show, due to the much lower resolution of sedimentary records, but what RICE is expected to reveal, is how quickly the sea ice retreated or the ice sheet collapsed. It might have taken anywhere between 50 and 500 years — the blink of an eye in geological terms.
Such a broad margin of error poses huge problems for policy makers, who need more accurate predictions when planning for the potentially alarming consequences of multi-metre rises in sea level. “This is where Roosevelt Island comes in,” says Bertler.
As she puts it, ice contains a “memory” within its compressed crystals that we can now recover and interpret, to determine not just what happened in the past, but what will happen in the future.
There are many variables influencing these major climate shifts, from the Earth’s elliptical orbit around the sun to the feedback loops created by ocean warming or ice-sheet accumulation.
And we are only now beginning to understand the true relationship between atmospheric CO2 levels and rising temperatures. As the New York Times recently reported, more sophisticated ice core analysis suggests there is a much closer link than previously argued by some climate change sceptics.
By reconstructing a past that is comparable to our present, Bertler explains, we can “train” our computer climate models to accurately “predict the past”, which in turn will give us more confidence in their ability to forecast future climate conditions.
This is the huge advantage ice cores have over Andrill’s sedimentary record. “We can read the cores like a seasonal diary. So we can tell you exactly what a summer 29,000 years ago looked like in this area.”
The cores’ records aren’t quite as obvious to the eye as the rings of a tree, but are as clear as day to scientists with the right measuring equipment. Bertler: “You see wonderful oscillations, you see warmer and colder temperatures from summers and winters, you see marine air masses dominating during the summer, the sea ice extent that has its maximum sometime in August then reduces to its minimum in January or February. We can see those things for each year, every year, in the ice-core record.”
The RICE research, like the other ice-core projects around Antarctica, exists in the context of a rapidly expanding field of knowledge about the West Antarctic ice sheet’s recent, disconcerting behaviour. Satellite imagery now shows the ice sheet has a negative “mass balance”; that is, the rate of melt is outstripping the rate of snow precipitation, and therefore West Antarctica is losing mass very quickly.
At the inaccessible and inhospitable Amundsen Sea Embayment, the massive Pine Island Glacier, which drains about 10 per cent of West Antarctica’s ice sheet directly into the sea, has been moving faster and thinning out over the past decade, as has the nearby Thwaites Glacier.
Because much of the ice sheet is grounded on rock thousands of metres below sea level, the danger is that sea water will eventually pour in over the lip of the continent and effectively float the ice above it.
“At the moment,” says Bertler, the loss of mass balance “is all due to melt ice being transported into the ocean. But if the water reaches underneath, as soon as you lift that mass, you increase the sea level by the equivalent of that mass.” Such a lifting of the entire West Antarctic sheet “could cause a five- to seven-metre rise in sea levels, but just the Pine Island area [could] raise levels by one to two metres”.
At the same time the Southern Ocean is absorbing more CO2, and warming faster and to a greater depth than previously imagined. Instead of only the first hundred or so metres of water being affected, temperatures have risen at depths of up to 3,000 metres. The increase might be very small (0.2 degrees Celsius in a decade), but the amount of energy required to warm such a vast body of water is huge. The fact that the ocean is absorbing so much of the warming also explains why atmospheric temperatures haven’t risen as steeply as might have been expected.
“In the short term that is really great,” Bertler suggests, “because it buys us a little time. But in the long term it is really bad. The ocean has a very long memory, and the warmth we are currently putting into the ocean will be with us for a very long time. So we are committed to that for many generations to come.”
Pine Island is one indicator of the possible consequences of this warming. Back at Roosevelt Island, the same rules apply for the Ross Ice Shelf. “If you put warm water under an ice shelf, ice shelves don’t like that. They warm rather quickly and melt quickly from underneath. You can’t see that until it collapses.”
In May this year representatives from all the RICE member nations will arrive at the New Zealand Ice Core Research Facility in Wellington to begin analysing the ice cores from Roosevelt Island. Working around the clock and using an ultra-pure nickel disk to melt each sample, they will begin with the most recent cores and work back (or down) towards the past. Hundreds of thousands of measurements will be taken to reveal the history of this surprisingly sensitive and dynamic part of the planet.
But that crucial sample of frozen mud from the surface of the sub-glacial rock will be analysed much more quickly, hopefully by mid-March. If, as the researchers have hypothesised, it turns out to be marine sediment, it will be yet more evidence of the potentially alarming implications of global warming.
And if not?
“Scientifically I would not be so excited,” Bertler concludes. “But personally I would be really pleased.”