Ice Core Study Extends Greenhouse, Ocean History 3M Yrs

New analyses of ancient Antarctic ice reveal insights into Earth's climate over the past 3 million years.

Mar. 19, 2026 at 3:08am

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New analyses of ancient ice from Antarctica and the air contained inside it are extending the history of Earth's climate records and expanding researchers' understanding of how the planet has changed over the last 3 million years. The findings show the long-term cooling of Earth's climate during this period has been accompanied by only a modest decline in heat-trapping greenhouse gases in the atmosphere.

Why it matters

The new research provides a more comprehensive understanding of Earth's climate history, including the role of greenhouse gases and other factors in the planet's cooling over the past 3 million years. This knowledge can help refine models of past warmer climates and improve predictions of future climate change.

The details

The studies, led by researchers at Oregon State University and Princeton University, used recently discovered archives of multi-million-year-old ice from Allan Hills in Antarctica to analyze the ratio of different noble gases in trapped air, which reflects ocean temperature changes. The data shows the average ocean temperature has declined by 2 to 2.5 degrees Celsius over the past 3 million years, with most of the cooling occurring early on as ice sheets formed in the northern hemisphere. The researchers also identified the first direct records of atmospheric carbon dioxide and methane levels over the last 3 million years, showing a modest decline in CO2 and stable methane levels.

  • The studies analyzed ice samples from the last 3 million years.
  • Carbon dioxide levels were 250 parts per million 2.7 million years ago and declined modestly by about 20 parts per million until 1 million years ago.
  • Methane levels remained unchanged at 500 parts per billion over the last 3 million years.

The players

Julia Marks-Peterson

A doctoral student at Oregon State University who led one of the studies.

Sarah Shackleton

A postdoctoral fellow at Princeton University who led one of the studies and is now a professor at Woods Hole Oceanographic Institution.

Ed Brook

The director of the NSF Center for Oldest Ice Exploration (COLDEX) and a paleoclimatologist at Oregon State University.

National Science Foundation Center for Oldest Ice Exploration (COLDEX)

A nationwide collaboration exploring Antarctica for Earth's oldest ice, headquartered at Oregon State University.

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What they’re saying

“Those snapshots extend climate records from ice much further than previously possible. These longer records are also now raising new questions about Earth's climate evolution and how far back in time we might be able to go with ice core data.”

— Ed Brook, Director, NSF Center for Oldest Ice Exploration (COLDEX)

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

— Sarah Shackleton, Professor, Woods Hole Oceanographic Institution

“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.”

— Julia Marks-Peterson, Doctoral Student, Oregon State University

What’s next

COLDEX researchers recently discovered ice as old as 6 million years at the bottom of one of their cores and are currently developing new data from these older samples. Recently completed drilling of new ice cores should access additional old ice. Researchers are also investigating methods to test carbon dioxide reconstruction, studying other gases in the ice cores and developing a deeper understanding of the conditions that lead to preservation of very old ice, which should help identify new targets for drilling.

The takeaway

This research provides a more comprehensive understanding of Earth's climate history over the past 3 million years, revealing that the long-term cooling of the planet has been accompanied by only a modest decline in greenhouse gases. This suggests that other factors, such as changes in Earth's reflectivity, vegetation, ice cover, and ocean circulation, have also played important roles in the planet's cooling.