Ice core reveals low CO2 during warm spell 3 million years ago

Bubbles in a 3-million-year-old ice core allowed researchers to measure for the first time the gases present in the atmosphere at the end of the Pliocene.

This era, when global temperatures are estimated to have been around 1°C higher than today and sea levels were up to 25 meters higher, is often seen as a warning for our times. 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 areas of Antarctica, snow accumulates each year on top of the previous year’s snow and is compressed into layers of ice containing air bubbles, giving us a continuous record of the past atmosphere. Last year, the Beyond EPICA group extracted the oldest continuous record of ice to date, dating back over a million years.

But scientists have also extracted even older ice in places where younger ice has been eroded by wind and older “blue” ice is close to the surface.

Julia Marks-Peterson of Oregon State University and colleagues used this ancient Allan Hills ice to propose the first direct measurement of carbon dioxide and methane in the Pliocene atmosphere. The team found surprisingly 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 saw a slight decrease in carbon dioxide and no noticeable change in methane.

The results are significantly lower than previous estimates based on indirect measurements, which indicated carbon dioxide levels of 400 parts per million, closer to today. There is no indirect way to measure past methane, which today stands at just under 2,000 parts per billion.

“We were definitely a little surprised,” Marks-Peterson says. If these results are correct, they could suggest that even small changes in greenhouse gas levels could trigger major climate changes. “Maybe the Earth system is even more sensitive to changes in CO2 than we think,” she says. “That’s a bit of a scary thought and something I would say our case can’t answer yet.”

“Ultimately, any new data suggesting that Pliocene CO2 levels were lower than previously predicted means that future climate change could be worse than expected,” says Cristian Proistosescu of the University of Illinois at Urbana-Champaign, who was not involved in the study.

But more work is needed before the data can be used to inform climate models, warns Tim Naish of Victoria University of Wellington in New Zealand. “It’s way too early to throw the baby out with the bathwater,” he said. Naish wants the record to be carried back further in time, when the Pliocene was even warmer.

Thomas Chalk, of the European Center for Research and Education in Environmental Geosciences, says he is convinced that the study’s low CO2 values ​​are correct. However, he points out that the distortion of ancient ice makes it impossible to know whether the low values ​​represent a cold period, a warm period or a precise average. “It’s representative of something. We need to know what it’s representative of. Because it doesn’t come with its own little estimate of temperature, and it certainly doesn’t come with an estimate of global temperature,” he explains.

The team hopes to improve the reliability of its analysis by comparing its results with those of the Beyond EPICA group’s continuous ice core. “This will help us better understand our own track record,” says Marks-Peterson. “We’re all on ants and needles waiting to hear what they find. »