Scientists studying ancient Antarctic ice are uncovering new details about how Earth’s climate has changed over the past 3 million years. By analyzing both the ice and the tiny pockets of air trapped inside it, researchers are building a longer and more complete record of past climate conditions.
Two new studies published in the journal Nature reveal a surprising pattern. While the planet gradually cooled over this time, levels of heat-trapping greenhouse gases in the atmosphere declined only slightly.
A Long-Standing Climate Mystery
For more than a century, scientists have known that Earth was significantly warmer about 3 million years ago. Evidence includes fossils of temperate and subtropical forests found in places like Alaska and Greenland, as well as ancient shorelines along the U.S. East Coast from Georgia to Virginia, showing that sea levels were much higher.
However, the reason behind this warm period and the cooling that followed has remained unclear. One major challenge has been the difficulty of accurately reconstructing both global temperatures and greenhouse gas levels from so far back in time.
Searching for the Oldest Ice in Antarctica
The new research comes from the National Science Foundation Center for Oldest Ice Exploration, known as COLDEX, a collaborative effort led by Oregon State University. The team focuses on locating and analyzing some of the oldest ice on Earth.
The studies were led by Julia Marks-Peterson, a doctoral student at OSU, and Sarah Shackleton, who conducted the work as a postdoctoral researcher at Princeton University and is now a professor at Woods Hole Oceanographic Institution. They examined ancient ice recovered from Allan Hills, a unique region along the edge of the East Antarctic ice sheet.
Unlike typical ice core sites, Allan Hills contains ice that has been pushed up and distorted by movement within the ice sheet. This disrupts the original layering, so instead of a continuous timeline, researchers get “snapshots” of climate conditions from different points in the past.
“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.”
Ocean Cooling Revealed by Trapped Gases
One study used measurements of noble gases preserved in the trapped air bubbles to estimate changes in ocean temperature over time. These gases provide a global signal of ocean conditions.
The results show that average ocean temperatures have dropped by about 2 to 2.5 degrees Celsius over the past 3 million years. While earlier research has documented cooling at the ocean surface, this study found that the timing of cooling differed between surface waters and deeper layers.
“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.”
Much of the overall cooling occurred early, beginning around 3 million years ago and continuing for about 1 million years. This period coincides with the formation of large ice sheets in the Northern Hemisphere. In contrast, surface ocean temperatures declined more gradually until about 1 million years ago. Researchers suggest this difference may be linked to changes in how heat moves between the ocean’s surface and its depths.
Greenhouse Gas Levels Show Only Modest Change
Using the same ice samples, Marks-Peterson and her team produced the first direct measurements of carbon dioxide and methane levels spanning the past 3 million years.
Their findings indicate that carbon dioxide levels generally stayed below 300 parts per million during this period. Around 2.7 million years ago, levels were about 250 parts per million and then decreased slightly by roughly 20 parts per million by 1 million years ago. Methane levels remained steady at about 500 parts per billion.
Some earlier estimates based on ancient sediments suggested higher carbon dioxide levels, but results have varied. This highlights the importance of extending ice core records further back in time to improve accuracy.
In contrast, greenhouse gas levels today are much higher. According to the National Oceanic and Atmospheric Administration, carbon dioxide averaged 425 parts per million in 2025, while methane reached 1,935 parts per billion.
More Than Greenhouse Gases Shaped Earth’s Climate
The findings suggest that greenhouse gases alone do not fully explain the long-term cooling trend. Other factors likely played significant roles, including changes in Earth’s reflectivity, shifts in vegetation and ice coverage, and variations in 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.
Even Older Ice May Hold More Answers
The research is already leading to new questions. Scientists involved in COLDEX are continuing to explore older ice samples to push the climate record even further back.
Researchers have recently identified ice that may be as old as 6 million years at the base of one core and are now analyzing these samples. New drilling efforts are also underway to locate additional ancient ice.
Scientists are working to improve methods for reconstructing carbon dioxide levels, studying other gases trapped in the ice, and better understanding how very old ice is preserved. These efforts could help identify new sites for future drilling and further expand the record of Earth’s climate history.
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.
