The drilling site at Allan Hills in Antarctica where scientists extracted a 3 million year old ice core
Austin Carter
Bubbles in a 3-million-year-old ice core have allowed scientists to measure gases in the atmosphere during the late Pliocene for the first time.
This era, when global temperatures are thought to have been around 1°C warmer than today and sea levels were as much as 25 meters higher, is often taken as a warning for our own time. But the levels of carbon dioxide and methane in the bubbles are much lower than today, which could mean that Earth’s climate is more sensitive than we thought to small atmospheric changes.
In some parts of Antarctica, snow accumulates each year on top of last year’s snow and is pressed into layers of ice that contain air bubbles, giving us a continuous record of the past atmosphere. Last year, the Beyond EPICA group extracted the oldest continuous ice record to date, stretching back over 1 million years.
But researchers have also extracted even older ice in places where younger ice has been eroded by the wind and older “blue” ice is near the surface.
Julia Marks-Peterson of Oregon State University and her colleagues used this ancient Allan Hills ice to provide the first direct measurement of carbon dioxide and methane in the Pliocene atmosphere. The team found unexpectedly low levels of both gases around 3 million years ago: 250 parts per million for carbon dioxide and 507 parts per billion for methane. During the subsequent cooling period, the team found a slight decrease in carbon dioxide and no noticeable changes in methane.
The results are significantly lower than previous estimates based on indirect measurements, which had indicated carbon dioxide levels of 400 parts per million, closer to today. There are no indirect ways of measuring past methane, which today hovers at just under 2,000 parts per billion.
“We were definitely a little surprised,” says Marks-Peterson. If correct, the findings could suggest that even small changes in greenhouse gas levels can trigger large changes in climate. “Perhaps the Earth system is even more sensitive to changes in CO2 than we have understood,” she says. “It’s a bit of a scary thought and something that I’d say our record can’t answer yet.”
“Ultimately, any new data suggesting that Pliocene CO2 levels were lower than previously expected means that future climate change may be worse than previously expected,” says Cristian Proistosescu of the University of Illinois Urbana-Champaign, who was not involved in the study.
But more work is needed before we can use the data to inform climate models, warns Tim Naish at Victoria University of Wellington in New Zealand. “It’s far too early to throw the baby out with the bathwater,” he says. Naish wants to see the record carried further back in time, when the Pliocene was even warmer.
Thomas Chalk, at the European Center for Research and Education in Environmental Geosciences, says he is confident that the study’s low CO2 values are correct. However, he points out that distortion of old ice makes it impossible to know whether the low values represent a cold period, a warm period or an accurate average. “It’s representative of something. We need to know what it’s representative of. Because it doesn’t come with its own little temperature estimate, and it certainly doesn’t come with a global temperature estimate,” he explains.
The team hopes to improve the reliability of their analysis by comparing the results with those of the continuous ice core from the Beyond EPICA group. “It will help us better understand our own record,” says Marks-Peterson. “We’re all on pins and needles waiting to hear what they find.”
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