Thaw pointby Rebecca Priestley
The last time CO2 levels were as high as they are now was three to five million years ago.
They say for every 10°C drop in temperature, it’s a “different kind of cold”. The -20°C I experienced camping at Friis Hills in Antarctica was certainly new for me, but the geologists I was with seemed unconcerned: their minds were on another Antarctica. They were there to investigate times in our planet’s history when the continent was a lot warmer than it is today.
Friis Hills is a 1300m-high, ice-free plateau in the Dry Valleys region of the Transantarctic Mountains, about 100km from Scott Base. The views – steep-sided mountains, enormous glaciers, a glimpse of the polar plateau – are awesome, but we weren’t there for the scenery. Our field party, led by Professor Tim Naish from Victoria University’s Antarctic Research Centre and Richard Levy from GNS Science, was focused on the sediments and glacial deposits beneath our boots.
Adam Lewis, a glacial geomorphologist from North Dakota State University, had already spent five field seasons at Friis Hills. On his first visit, he and his colleague found “fossil tundra everywhere we looked”. The leaves, insects, pollen and seeds hinted at a climate with summer air temperatures some 20°C warmer than today. Lewis and his students have since identified a sequence of glacial moraines interspersed with life-supporting water bodies – ponds, marshes and small lakes – created by glacial meltwaters.
It’s clear that the landscape was created by a series of advancing and retreating glaciers, but what’s remarkable about the Friis Hills landscape, and what makes it so interesting to geologists, is that it was created 14-20 million years ago and preserved intact since then. After this time, the climate changed: air temperatures became colder, rain stopped falling and the water became locked up in glacial ice – plants could no longer grow here. At least that’s what the geomorphologists believe.
Sedimentologists such as Naish and Levy have found evidence of a warm period three to five million years ago. Ross Sea sediment cores from the Andrill project revealed that “the ocean warmed up, there was no Ross Ice Shelf, the West Antarctic Ice Sheet collapsed and the ocean was too warm to support sea ice”, says Naish.
It’s puzzling that at the same time as the ocean was warming, the mountains seem to have remained cold and frozen. These contradictory pieces of paleoclimate evidence are part of a 30-year debate over what happened to the massive East Antarctic Ice Sheet three to five million years ago: did it collapse or remain intact?
Sediments that preserve information about land-based climate from three to five million years ago are rare – but there are 14-20 million-year-old deposits in both marine sediments and Friis Hills. “That these deposits are so well preserved at Friis Hills implies that much of the high interior East Antarctic Ice Sheet survived the global warmth of the Pliocene, unlike the West Antarctic Ice sheet, which was attacked by warming oceans,” says Levy.
The team working at Friis Hills last December took sediment cores and did acoustic surveys across old lake beds to get a more detailed picture of what was going on. Correlating this land-based evidence with existing drill-core evidence – and determining the likely temperature gradient from mountain to sea – will help reveal what happened to the East Antarctic Ice Sheet over past periods of global warming.
The greenhouse gases we’re pumping into the atmosphere have raised atmospheric CO2 levels to 400 parts per million. The last time CO2 levels were this high was three to five million years ago. Naish says we have a good understanding of what was happening in the oceans at that time, but understanding what was happening on land is “important for getting to the nub of the controversy over how warm Antarctica was three to five million years ago”.
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