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, published this week in two papers in the journal Nature, 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.
Scientists have known that Earth was much warmer and sea level much higher as recently as 3 million years ago since the first discoveries more than 100 years ago of temperate and subtropical forest fossils in Alaska and Greenland and ancient stranded beaches stretching from Georgia to Virginia.
But the cause of that period of global warmth and subsequent cooling has remained a mystery, in large part due to the difficulties in accurately reconstructing global temperatures and the levels of heat-trapping atmospheric greenhouse gases.
The new research was led by scientists with the National Science Foundation Center for Oldest Ice Exploration, a nationwide collaboration exploring Antarctica for Earth's oldest ice, headquartered at Oregon State University.
The two studies, led by Julia Marks-Peterson, a doctoral student at OSU, and Sarah Shackleton, then a postdoctoral fellow at Princeton University and now a professor at Woods Hole Oceanographic Institution, use recently discovered archives of multi-million-year-old ice from Allan Hills, at the margin of the East Antarctic ice sheet.
Allan Hills is an unusual setting where ice from the Antarctic interior is stranded in mountain ranges at the edge of the continent. Flow patterns deform the originally horizontal layers, making it difficult to find continuous records of climate. Instead, the data provide "snapshots" that indicate average environmental conditions at discrete time periods.
"Those snapshots extend climate records from ice much further than previously possible," said COLDEX Director Ed Brook, a paleoclimatologist in OSU's College of Earth, Ocean, and Atmospheric Sciences. "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."
Using precise measurements of the ratio of different noble gases in air trapped in the ice, which reflect ocean temperature changes, Shackleton and colleagues showed that the average temperature of the ocean has declined by 2 to 2.5 degrees Celsius over the past 3 million years. Previous studies have extensively examined changes in ocean surface temperatures, which also show cooling over this period, but the new work shows that the timing of cooling is different between the ocean surface and the ocean depths.
"The noble gases in ice provide a unique way to look at ocean temperature change," Shackleton said. "Other methods can give you information about ocean temperature at a single site, but this gives a more global view."
For example, a large fraction of the mean ocean temperature cooling happened early, starting 3 million years ago and continuing for about a million years, during the time that ice sheets began to form in the northern hemisphere. In contrast, surface temperatures cooled gradually until about 1 million years ago. In the paper, Shackleton and her coauthors suggest that these discrepancies involve changes in how heat is transferred between the surface and deep ocean.
Using the same ice core samples, Marks-Peterson and her coauthors identified the first direct records of the levels of two of the most important atmospheric greenhouse gases, carbon dioxide and methane, over the last 3 million years.
The data show that long-term average atmospheric carbon dioxide levels have likely remained below 300 parts per million over this time; measured 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. Long-term average levels of atmospheric methane remained unchanged at 500 parts per billion.
Some previous work using the chemistry of ancient sediments has suggested higher values of carbon dioxide than the new data indicate, but not all such studies agree, underscoring the need for ice core data going back as far as possible, the researchers noted.
Modern levels of carbon dioxide and methane have increased dramatically in the last two centuries, with carbon dioxide averaging 425 parts per million in 2025 and methane averaging 1,935 parts per billion in 2025, according to the National Oceanic and Atmospheric Administration.
The implications of the results are that the cooling of the last 3 million years probably involves, in addition to the key role of heat-trapping greenhouse gases, important contributions from other components of the climate system such as Earth's reflectivity, variations in vegetation and/or ice cover and ocean circulation.
"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 Marks-Peterson.
The work has led to new research questions, many of which are currently being investigated by others in NSF COLDEX, Brook said. 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.
COLDEX is supported by the NSF Office of Polar Programs; the Science and Technology Center Program at the NSF Office of Integrative Activities; and Oregon State University. Fieldwork in Antarctica is supported by the U.S. Antarctic Program and funded by NSF. Ice drilling support is provided by the NSF U.S. Ice Drilling Program and ice sample curation by the NSF Ice Core Facility in Denver, Colorado.
