Scientists studying ancient Antarctic ice are uncovering new details about how Earth’s climate has changed over the past three million years. By analyzing both the ice and the small pockets of air trapped within it, researchers are building a longer-term, more complete record of past climate conditions.
Two new studies have been published in the journal nature Unexpected patterns emerge. During this time, the Earth gradually cooled, but the levels of heat-trapping greenhouse gases in the atmosphere decreased only slightly.
A long-standing climate mystery
For more than a century, scientists have known that the Earth was significantly warmer about 3 million years ago. Evidence includes fossilized temperate and subtropical forests found in places like Alaska and Greenland, as well as ancient coastlines along the U.S. East Coast from Georgia to Virginia, indicating that sea levels were much higher.
However, the reasons for this warm period and subsequent cooling remain unclear. One major challenge is the difficulty in accurately reconstructing both global temperatures and greenhouse gas levels going back far into the past.
Searching for the oldest ice in Antarctica
The new study is from the National Science Foundation’s oldest ice exploration center, known as COLDEX, and is a collaboration led by Oregon State University. The research team is focused on finding and analyzing some of the oldest ice on Earth.
The study was led by Julia Marks Peterson, a doctoral student at OSU, and Sarah Shackleton, who conducted the research as a postdoctoral fellow at Princeton University and is now a professor at Woods Hole Oceanographic Institution. They examined ancient ice recovered from the Arran Hills, a unique region along the edge of the East Antarctic Ice Sheet.
Unlike typical ice core sites, Arran Hills contains ice that has been pushed up and distorted by movement within the ice sheet. This destroys the original hierarchical structure, so instead of a continuous timeline, researchers obtain “snapshots” of climate conditions from different points in the past.
“These snapshots extend the climate record from ice much further than was previously possible,” said COLDEX Director Ed Brook, a paleoclimatologist in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “These long records also raise new questions about the evolution of Earth’s climate and how far back we can go back with ice core data.”
Ocean cooling revealed by trapped gas
One study used measurements of noble gases stored in trapped gas bubbles to estimate changes in ocean temperatures over time. These gases provide global signals about ocean conditions.
The results showed that average ocean temperatures have fallen by about 2 to 2.5 degrees Celsius over the past three million years. Previous studies have recorded cooling of the sea surface, but this study found that the timing of cooling differs between the surface and deep layers.
“Noble gases in ice provide a unique way to observe changes in ocean temperature,” Shackleton said. “Other methods give information about ocean temperatures at a single point, but this method gives us a more holistic view.”
Much of the overall cooling occurred early, starting about 3 million years ago and lasting about 1 million years. This period coincides with the formation of large ice sheets in the Northern Hemisphere. In contrast, ocean surface temperatures declined more slowly until about 1 million years ago. The researchers suggest that this difference may be related to changes in the transfer of heat between the ocean’s surface and depth.
Greenhouse gas levels show only small changes
Marks Peterson and her team used the same ice samples to directly measure carbon dioxide and methane levels over the past three million years for the first time.
Their findings show that carbon dioxide levels generally remained below 300 ppm during this period. About 2.7 million years ago, levels were about 250 ppm, but by 1 million years ago they had decreased slightly to about 20 ppm. Methane levels stabilized at about 500 ppb.
Previous estimates based on ancient sediments suggested higher carbon dioxide levels, but results were mixed. This highlights the importance of looking further back in the ice core record to improve accuracy.
In contrast, today’s greenhouse gas levels are much higher. According to the National Oceanic and Atmospheric Administration, carbon dioxide averaged 425 ppm in 2025, and methane reached 1,935 ppb.
Greenhouse gases are not the only thing that shapes the Earth’s climate
The findings suggest that greenhouse gases alone cannot fully explain the long-term cooling trend. Other factors are also likely to have played an important role, including changes in Earth’s reflectivity, changes in vegetation and ice cover, and variations in ocean circulation.
“Our hope is that this research will refine our view of past warm climates and improve our understanding of how different components of the Earth system interact,” Marks-Peterson said.
Old ice may also contain more answers
This research is already leading to new questions. Scientists participating in COLDEX continue to examine old ice samples to push the climate record further back.
Researchers recently identified ice at the base of one core that may be as old as 6 million years, and are currently analyzing these samples. New drilling efforts are also underway to discover more ancient ice.
Scientists are working to improve ways to reconstruct carbon dioxide levels, study other gases trapped in ice, and improve our understanding of how very old ice is preserved. These efforts could help identify new locations for future drilling and further expand the record of Earth’s climate history.
COLDEX is supported by the NSF Polar Program Office. Science and Technology Centers Program of the NSF Directorate of Integrated 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 management is provided by the NSF Ice Core Facility in Denver, Colorado.
