At the windswept edge of the East Antarctic ice sheet, in a place called Allan Hills, scientists have been chipping away at what may be the oldest ice ever pulled from our planet — and the tiny bubbles trapped inside it have just rewritten three million years of Earth's climate story.

The discovery, published in two new papers in the journal Nature, comes from a team led by Oregon State University's Center for Oldest Ice Exploration, known as COLDEX. By analysing the ancient air sealed inside the ice — pockets no bigger than a pinhead — researchers have produced the first direct measurements of carbon dioxide, methane and ocean temperature stretching back three million years.

And the picture they've painted is a puzzle.

A planet that cooled without much help from CO2

Scientists have known for over a century that Earth was much warmer three million years ago. Fossilised subtropical forests have turned up in Greenland. Ancient shorelines along the US East Coast sit far inland from today's beaches. Sea levels were dramatically higher.

What no one could fully explain was why the planet then cooled so much over the following millions of years.

The new ice cores offer a surprising clue. Across the entire three-million-year stretch, atmospheric carbon dioxide stayed below 300 parts per million — sitting at around 250 ppm 2.7 million years ago and drifting down by roughly 20 ppm by a million years ago. Methane barely budged from about 500 parts per billion.

In other words, the greenhouse gases that dominate today's climate conversation didn't change very much at all. Yet the oceans cooled by 2 to 2.5 degrees Celsius.

"Our hope is that this work will refine our view of past warmer climates and sharpen our understanding of how different elements of the Earth system interact," said Julia Marks-Peterson, the OSU doctoral student who led the greenhouse gas study.

Snapshots, not a timeline

Allan Hills isn't a normal ice core site. Most ice cores are read like a book — layer by neat layer, year by year. But at Allan Hills, the ice has been pushed and folded by the slow movement of the ice sheet itself, scrambling the chronological order.

The trade-off is extraordinary. Instead of a continuous record, scientists get "snapshots" of the atmosphere from points far deeper in time than any other site can reach.

"Those snapshots extend climate records from ice much further than previously possible," said COLDEX director Ed Brook, a paleoclimatologist at OSU. "These longer records are also now raising new questions about Earth's climate evolution."

The deep ocean tells a different story

A second study, led by Sarah Shackleton — formerly of Princeton, now at Woods Hole Oceanographic Institution — used the noble gases trapped in the same ice to reconstruct ocean temperatures.

The clever bit: noble gases dissolve into seawater at rates that depend on temperature, so the air bubbles in Antarctic ice carry a global signal of how warm the oceans were.

The team found the deep ocean cooled rapidly between 3 and 2 million years ago, lining up with the formation of huge ice sheets in the Northern Hemisphere. Surface waters, by contrast, cooled much more gradually until about a million years ago.

"The noble gases in ice provide a unique way to look at ocean temperature change," Shackleton explained. "Other methods can give you information about ocean temperature at a single site, but this gives a more global view."

More than one climate lever

Together, the studies suggest greenhouse gases alone don't explain Earth's long, slow cool-down. Other forces — shifting ice sheets, changes in the planet's reflectivity, evolving ocean circulation, even the spread of vegetation — must have been pulling powerful levers behind the scenes.

For context, today's atmosphere contains 425 ppm of carbon dioxide and 1,935 ppb of methane, according to NOAA's 2025 figures — far above anything seen in the three-million-year record.

The COLDEX team isn't stopping there. They've already identified ice that may be six million years old at the bottom of one core, and drilling continues across Antarctica in search of older samples still.

The deepest secrets of Earth's climate, it seems, are still frozen — waiting to be read.