Antarctic ice cores push greenhouse gas records back 3 million years
What made Earth so much warmer 3 million years ago? Forests grew in Alaska. Subtropical vegetation covered Greenland. Ancient beaches stretched from Georgia to Virginia, evidence of sea levels far higher than today's. Scientists have known about this warm period for more than a century, ever since the first fossil discoveries revealed temperate ecosystems in places now locked under ice. But the cause of that warmth - and the subsequent cooling that brought us to the present - has remained stubbornly unclear.
The difficulty was simple and maddening: no one had reliable records of what the atmosphere actually looked like that far back. Until now.
Air from 3 million years ago, sealed in Antarctic ice
Two studies published this week in Nature, led by researchers at the National Science Foundation's Center for Oldest Ice Exploration (COLDEX) headquartered at Oregon State University, have extended the direct record of atmospheric greenhouse gases and ocean temperatures back 3 million years. The data come from an unlikely archive: ancient ice stranded in the Allan Hills at the margin of the East Antarctic ice sheet.
Allan Hills is a peculiar geological setting. Ice from the Antarctic interior flows toward the continent's edge and gets trapped against mountain ranges, where erosion and ablation bring ancient layers to the surface. The ice does not form continuous records like traditional deep ice cores. Instead, it provides what COLDEX Director Ed Brook, a paleoclimatologist at Oregon State, describes as snapshots - discrete windows into average environmental conditions at specific time periods.
But those snapshots contain something invaluable: tiny bubbles of ancient air, sealed inside the ice when it originally formed. That trapped air preserves a direct chemical record of the atmosphere at the time - including concentrations of carbon dioxide and methane, the two most important heat-trapping greenhouse gases.
CO2 stayed below 300 ppm for 3 million years
Julia Marks-Peterson, a doctoral student at Oregon State, led one of the two studies. Her team measured carbon dioxide and methane levels directly from the ancient ice, producing the first ice-core-based records of these gases extending back 3 million years.
The results show that long-term average atmospheric CO2 likely remained below 300 parts per million (ppm) throughout this entire period. Measured values were 250 ppm at 2.7 million years ago and declined modestly - by about 20 ppm - over the next 1.7 million years. Long-term average methane levels held steady at approximately 500 parts per billion (ppb).
For context, modern CO2 levels averaged 425 ppm in 2025 and methane averaged 1,935 ppb, according to NOAA - dramatically higher than anything the ice core record shows for the past 3 million years. The ancient atmosphere, even during its warmer phases, contained far less of both gases than today's.
Some previous reconstructions of ancient CO2 levels, based on the chemistry of ocean sediments rather than direct air measurements, had suggested higher values. But those indirect methods do not always agree with one another, and the new ice core data provide a more direct measurement that may help resolve the discrepancy.
The ocean cooled 2 to 2.5 degrees - but the timing was uneven
The second study, led by Sarah Shackleton - then a postdoctoral fellow at Princeton University and now a professor at Woods Hole Oceanographic Institution - tackled ocean temperature. Rather than using traditional marine proxies that measure temperature at a single location, Shackleton's team exploited a different signal: the ratio of noble gases trapped in the ice.
Noble gases dissolved in seawater in proportions that depend on ocean temperature. When water evaporates and forms snow that eventually becomes ice, the air trapped in that ice carries a chemical fingerprint of the ocean temperature at the time. This method provides a globally integrated view of ocean temperature rather than a site-specific one.
The data show that the mean temperature of the global ocean declined by 2 to 2.5 degrees Celsius over the past 3 million years. But the timing of that cooling was not uniform. A large fraction of the deep ocean cooling happened early - starting 3 million years ago and continuing for about a million years, coinciding with the formation of ice sheets in the northern hemisphere. Surface ocean temperatures, by contrast, cooled more gradually, continuing their decline until about 1 million years ago.
This mismatch between surface and deep ocean cooling is significant. It suggests that changes in how heat is transferred between the ocean's surface and its depths played an important role in Earth's climate evolution - a mechanism that is not yet well understood and that complicates simple narratives about greenhouse gases driving temperature in a straightforward way.
Greenhouse gases alone do not explain the cooling
Perhaps the most striking implication of the two studies taken together is what they do not explain. CO2 declined by only about 20 ppm over nearly 2 million years, yet the ocean cooled by 2 to 2.5 degrees and massive ice sheets formed in the northern hemisphere. A 20 ppm drop in CO2 is modest by any standard - certainly not enough on its own to trigger the dramatic climate shifts of the past 3 million years.
This means other components of the climate system must have contributed substantially to the cooling trend. The researchers point to several candidates: changes in Earth's reflectivity (albedo) as ice sheets expanded, shifts in vegetation cover, alterations in ocean circulation patterns, and feedback loops between ice, atmosphere, and ocean that amplified relatively small initial changes.
Understanding these interactions is not merely an academic exercise. Today's CO2 levels are roughly 70% higher than the highest values recorded in the 3-million-year ice core record. If the ancient climate system was shaped by factors beyond greenhouse gases alone, then predicting the full consequences of modern emissions requires understanding those factors too.
Pushing deeper: ice as old as 6 million years
The work has opened new research directions. COLDEX researchers recently discovered ice as old as 6 million years at the bottom of one of their Antarctic cores - potentially doubling the time horizon accessible through direct atmospheric measurements. New data from these older samples are currently being developed.
Recently completed drilling campaigns should access additional ancient ice. Researchers are also investigating methods to validate CO2 reconstructions from multiple independent approaches, studying other trace gases preserved in the ice, and working to understand the geological conditions that lead to the preservation of very old ice. That knowledge should help identify promising new drilling targets.
What these records cannot yet tell us
The snapshot nature of the Allan Hills ice imposes real limitations. Unlike continuous deep ice cores, which provide year-by-year or century-by-century records, the Allan Hills data offer averages over discrete time windows. Short-term climate variability - the kind that matters most for understanding rapid climate transitions - is not captured.
The deformation of ice layers during their long journey from the Antarctic interior to the margins also introduces uncertainty about the precise age of individual samples. Dating relies on a combination of gas measurements and flow modeling rather than simple layer counting, and the error bars can be substantial for the oldest ice.
The noble gas ocean temperature method, while providing a valuable global average, does not reveal regional temperature patterns. Whether the tropics or the poles cooled more, and how that distribution changed over time, requires complementary evidence from ocean sediment records.
Still, these are the first direct measurements of ancient atmospheric composition extending this far back in time. For a field that has long relied on indirect proxies with their own substantial uncertainties, that directness carries real scientific weight.