A team of U.S. researchers has recovered what may be the oldest sample of Earth’s atmosphere ever directly observed—air sealed inside Antarctic ice dating back more than 5 million years. The findings, published in the Proceedings of the National Academy of Sciences, provide new insight into atmospheric conditions during the late Miocene to early Pliocene, long before the onset of Earth’s modern ice ages.
The breakthrough came from East Antarctica’s Allan Hills, a site already known for preserving ancient ice. There, wind and glacial movement expose deep layers of ice near the surface, offering rare access to prehistoric climate data without the need for deep drilling. The same location yielded a 2.7-million-year-old core in 2017, then the oldest ever analyzed.
That record now appears to have been surpassed. Researchers from Princeton University and the Center for Oldest Ice Exploration (COLDEX) extracted fragments of ice estimated to be between 4.3 and 5.1 million years old, extending the observational record of atmospheric gases further into Earth’s past than previously thought possible.
Direct Access to a Warmer World
The Allan Hills’ exposed ice layers—known as a blue ice area—form when ancient ice flows upward across bedrock ridges while younger snow is stripped away by strong winds. These dynamics allow scientists to collect old ice at shallow depths, bypassing the limits of conventional deep-core drilling, which typically tops out around 800,000 years.
According to the study published in PNAS, samples were retrieved from depths of 100 to 200 meters and dated using gas isotopic methods involving argon and potassium. Though less precise than annual layer counting, the technique can reliably estimate ages well beyond the range of other dating systems.
The bubbles trapped in this ancient ice contain direct samples of Earth’s prehistoric atmosphere, allowing researchers to measure concentrations of gases like carbon dioxide and methane with greater accuracy than sediment-based climate proxies.
Earlier ice recovered from the Allan Hills in 2015 and analyzed in a 2017 study published in Science revealed that atmospheric CO₂ levels 2.7 million years ago stayed below 300 parts per million. That data supports models suggesting that lower CO₂ concentrations were a trigger for the onset of cyclical glaciations in the Northern Hemisphere.
Filling Gaps in the Ice Age Timeline
“This is the only sample of ancient Earth’s atmosphere that we have access to,” said David Shuster, a geochemist at the University of California, Berkeley, who was not involved in the research, in comments to Science. The findings offer a rare point of reference for validating or challenging climate models based on indirect indicators.
Because blue ice areas disrupt the typical annual layering found in central Antarctica, they produce fragmented records rather than continuous timelines. Yet they offer something even deeper: access to time periods that were once considered out of reach.
The Allan Hills site in particular has emerged as a critical location for paleoclimate research, not only because of the exposed old ice but also due to its accessibility for field teams. In the 2015 expedition, harsh winds cut short one drilling attempt, but still yielded ice dating back 1 million years—making it the first sample older than the 800,000-year benchmark set by cores from Dome C in central Antarctica.
“This old ice really makes people stand up and notice,” said Ed Brook, a geochemist at Oregon State University and member of the discovery team, in the 2017 Science article. “We’re just scratching the surface.”
New Clues, New Questions
The new samples align with what climate scientists know about the Pliocene: a period of higher global temperatures, elevated sea levels, and reduced polar ice. Models suggest Earth at that time may have been 3°C to 4°C warmer than preindustrial conditions, with ocean levels up to 20 meters higher than today.
However, some proxy records—such as fossil-based reconstructions from shallow marine environments—have indicated higher atmospheric CO₂ levels than those observed in the ice cores. If the ice data holds up under further scrutiny, some of those proxies may require recalibration.
“We have some work to do,” said Yige Zhang, a paleoclimatologist at Texas A&M University, in the Science report, referring to the discrepancies between models and direct evidence.