Antarctica’s climate change cluesby Rebecca Priestley
Antarctic ice cores are a valuable source of climate change clues.
I’ve travelled to Greenland, Iceland and Antarctica, but the coldest place I’ve been in is a laboratory in Lower Hutt. At the New Zealand Ice Core Research Facility, two storage freezers maintain more than 800m of Antarctic ice cores at a temperature of minus 35°C. Inside the freezer, my skin hurts and my eyeballs ache. Despite my love of cold places, I last about two minutes, then retreat to the next room, a laboratory where scientists work at a more comfortable minus 18°C. Nancy Bertler, of GNS Science and Victoria University’s Antarctic Research Centre, spent the winter months here processing ice cores collected from the Ross Ice Shelf last summer.
The focus of Bertler’s work, like much recent Antarctic science, is climate change. Bertler leads the Roosevelt Island Climate Evolution (Rice) project, an international collaboration that aims to drill down 760m to gather a continuous record of ice deposited over the past 30,000 years. Sediment drilling projects like Andrill, which examine past climate over millions of years, have shown that the Antarctic ice sheet is very dynamic, advancing and retreating – or even collapsing – in response to global air and ocean temperatures. But analysis of sediments is not precise enough to show how fast the ice sheets collapsed. That’s where Rice comes in.
“Andrill showed the ice sheet will collapse; we want to show how quickly it will collapse,” says Bertler. One of Andrill’s most significant findings is that three to five million years ago, when CO2 levels were 400 parts per million, global air temperatures were 3-4°C warmer, the southern oceans were 5-6°C warmer, and the Greenland ice sheet, the West Antarctic ice sheet and some of the East Antarctic ice sheet collapsed. “That caused a global sea level rise of 20m,” says Bertler. This ice sheet collapse is highly significant, because in a few years atmospheric CO2 levels will again reach 400ppm. Her team is trying to work out how fast the ice sheets collapsed and how fast the sea level rose, by looking at ice cores, which record annual changes in the ice. “When you look at ice cores, you can see these beautiful layers. They’re like tree rings. Each year there is a new layer of snow added, so you have a continuous record, and as you go down into the core, you go back in time.”
There are no ice deposits that go back three to five million years. “But there’s another time period when the Ross ice shelf and the West Antarctic ice sheet had rather dramatic changes, and that was during the last glacial maximum, about 20,000 years ago, when temperatures were about 6°C lower than today.” By looking at ice from this time period, Rice will determine annually resolved air temperature, and an approximation of sea surface temperature, and the presence or absence of an ice shelf. “And from that we can reconstruct how sensitive the ice shelf was to temperature increases. Was it a smooth gradual retreat? We’re pretty sure that wasn’t the case. Were there chunks that broke off? And when it went, how quickly did it go?”
Last season, Bertler and her team spent four months on the ice and drilled down through 130m of ice. Bertler’s back in Antarctica now, hoping to reach bedrock at 760m by late January. And she’s happy to be fleeing New Zealand’s spring and summer for the Antarctic chill. “On the ice, early in the season, it will be around minus 30, which is a bit cold. Later in the season it can be as warm as minus five, which is definitely too warm, because it’s so dry and sunny. “Minus 15 is perfect.”
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