Ice core records going back 800,000 years reveal a lot about carbon dioxide and temperature, but climate researchers want to go back further
March 9, 2016 — Somewhere deep below the ice in Antarctica lies a time capsule. It’s the holy grail of climate science and promises to reveal the past and future of Earth’s atmosphere. And right now, scientists are meeting in Hobart to work out a plan to dig it up.
The time capsule is ice that froze 1.5m years ago, capturing tiny bubbles of air, bringing a sample of the ancient atmosphere through time to the present day.
There are already dozens of ice cores from Antarctica and Greenland. They are tubes of ice, sometimes several kilometers long, drilled from the ice sheet, which reveal a timeline of what the atmosphere was like over hundreds of millennia.
Together those cores paint a detailed picture of the history of our atmosphere and climate. They’ve demonstrated the tight relationship that exists between carbon dioxide concentrations and temperature, but also how unusual the levels of carbon dioxide in the atmosphere are now.
They take us back through time to about 800,000 years ago, through ice age after ice age. Although 800 millenia tells us a lot about carbon dioxide and temperature, the ice core record stops at a crucial moment.
We know what temperatures were like on Earth going back millions of years. That’s because the air temperatures leave a unique signature in the sediment at the bottom of the ocean, as well as in other “proxy” systems like ice and tree rings.
Examining that sediment, scientists have found that a strange thing happened about one million years ago. The cycle of ice ages sped up. Rather than happening every 100,000 years, they suddenly started happening every 40,000 years.
“The fact that we can’t fully explain why that change occurred tells us that we still don’t know all that we’d like to know about the climate system,” says Tas van Ommen from the Antarctic Climate and Ecosystems Cooperative Research Center at the University of Tasmania.
“And we think carbon dioxide could well be the key to what drove the change.”
But because the ice core record stops at 800,000 years, there is no way to tell if that’s true.
Enter the search for the million-year ice core. “The million-year ice core is a holy grail for climate science,” says van Ommen, who’s chair of the International Partnerships in Ice Core Sciences (Ipics) conference in Hobart this week.
“The scientific need to try to get an ice core record that pushes back beyond a million years is something we’ve been focussed on for 10 years,” says van Ommen. “We’re pretty sure that there exists ice that is one million years or older towards the bottom of the Antarctic ice sheet, but knowing exactly where is a big exercise.”
“And when you’re talking about a very expensive multi-year program, you want to know where,” he says.
So teams around the world have been busily modelling the flow of ice on Antarctica, to figure out a place where ice is likely to have sat undisturbed for a million years.
Other teams have been flying planes with radar over Antarctica, mapping the bottom of the ice. And yet others have been modelling the past climates of Antarctica, finding a spot where the right amount of snow is likely to have fallen each year, giving clear layers throughout the ice.
At the conference, the teams will be presenting their findings, helping narrow the search. “In the past year or two it’s become apparent there are three or four sites that are promising,” says van Ommen.
Each site is on a “dome” – peaks of ice that have remained particularly high above the land mass. Those are the most promising locations, says van Ommen. “If you think of a big mound of butter on your table on a warm day, and it all melts and oozes out, the butter at the highest point in the middle has always been at that point.”
While the conference papers themselves hash out the science of the project, meetings on the sidelines of the conference nut out the politics: which groups will take on which responsibilities, and what the priorities are for the next year or so.
Van Ommen says it could take until about 2020 for the international collaboration to actually get a drill to break ice in their search for the million-year core.
But no amount of modelling will give the scientists certainty. And so some teams are drilling already.
Yuansheng Li, from the Polar Research Institute of China, will present the results of its drilling at a site near its Kunlun Station on Dome A. During the last drilling season they reached 654m. They plan to reach the bedrock, about 3100m deep, by about 2020.
Van Ommen says there is no certainty the Chinese project will find million-year-old ice. But that is far from failure, he said.
Each ice core carries a swathe of information, adding resolution to the picture we have of the past. Small variations in things such as salt content, for example, can give an indication of local wind conditions, which can then tell scientists what the weather was like over Australia.
And that sort of information forms the basis of one of the other big targets for ice core science in the coming years: the 2000-year array.
Where the million-year-ice-core project aims to tell scientists what happened on the largest climatic timescales, the 2000-year array aims to draw the most detailed picture possible of the past two millennia.
That project is about gathering as many ice cores as possible, and painstakingly combining all the data into one picture.
“If you want to build up a good network of past climate information, you need ice cores from as many locations as you can to build up a spatial picture.”
There are about a hundred ice cores from Antarctica, but most of them only go back a few decades. Going back 2000 years, van Ommen says there’s probably about a dozen ice cores that can be used.
“That’s giving us a baseline for what we’re doing to climate now,” says van Ommen.
“We’re using that information to tell us things like how frequent droughts are in the past two thousand years. What’s the natural range of change and how have we departed from that? Are we prepared?”