Frozen air from Antarctica is giving scientists a longer look at a climate mystery that has lingered for decades: why Earth cooled so much over the past 3 million years even though its greenhouse gas levels seem to have changed only modestly.<\/p>\n
Two new studies in Nature push direct records of ancient atmospheric carbon dioxide<\/a> and methane far beyond the 800,000-year span covered by the oldest continuous ice core. Using ice from Allan Hills, an unusual blue ice area at the edge of East Antarctica, researchers found that average carbon dioxide levels likely stayed below 300 parts per million over the past 3 million years. Methane, meanwhile, appears to have remained roughly steady at about 500 parts per billion.<\/p>\n That is a striking result because the same ice suggests the planet cooled a great deal over the same stretch of time.<\/p>\n \u201cThe noble gases in ice provide a unique way to look at ocean temperature change,\u201d Sarah Shackleton, now a professor at Woods Hole Oceanographic Institution, said in one of the papers<\/a>. \u201cOther methods can give you information about ocean temperature at a single site, but this gives a more global view.\u201d<\/p>\n Characteristics of the greenhouse gas data from the ALHIC1901 ice core. (CREDIT: Nature) Ice that works more like snapshots<\/p>\n Scientists have long known that Earth was warmer around 3 million years ago. Fossils of temperate and subtropical forests found in Alaska and Greenland, along with ancient beaches from Georgia to Virginia, point to a world with much higher sea levels and milder conditions.<\/p>\n What has been much harder to pin down is why that warm period faded.<\/p>\n The new work, led by researchers tied to the National Science Foundation Center for Oldest Ice Exploration, used ice from Allan Hills, where flow patterns and strong winds bring ancient ice to the surface. That makes the site valuable, but messy. The layers are folded, thinned and sometimes broken apart, so the ice does not preserve a clean year-by-year climate record. Instead, the samples act more like snapshots of average conditions from different periods.<\/p>\n Those snapshots still reach much farther back in time than standard ice cores.<\/p>\n \u201cThose snapshots extend climate records from ice much further than previously possible,\u201d said Ed Brook, director of COLDEX and a paleoclimatologist at Oregon State University<\/a>. \u201cThese longer records are also now raising new questions about Earth\u2019s climate evolution and how far back in time we might be able to go with ice core data.\u201d<\/p>\n A cooler planet, but only a small greenhouse gas shift<\/p>\n One study<\/a>, led by Julia Marks-Peterson, a doctoral student at Oregon State University, reconstructed carbon dioxide and methane from trapped air in the ice. The team found carbon dioxide levels around 250 ppm about 2.7 million years ago, followed by a decline of roughly 20 ppm by 1 million years ago. Mean methane levels stayed near 500 ppb.<\/p>\n Bubbles trapped in ancient Antarctic ice. (CREDIT: OSU) <\/p>\n That is not much movement for gases that play a central role in trapping heat.<\/p>\n A second study, led by Shackleton, used noble gases preserved in the same ice to estimate changes in ocean temperature. It found that mean ocean temperature fell by about 2 to 2.5 degrees Celsius over the last 3 million years. The cooling appears to have started early, around 3 million years ago, and continued for about a million years, around the same time large ice sheets<\/a> began growing in the Northern Hemisphere.<\/p>\n Surface ocean temperatures told a somewhat different story. They cooled more gradually until about 1 million years ago. The researchers suggest that this mismatch may reflect changes in how heat moved between the ocean surface and the deep ocean.<\/p>\n Put together, the papers point to a climate system in which greenhouse gases mattered, but were probably not the whole story. The authors say changes in Earth\u2019s reflectivity, vegetation, ice cover and ocean circulation likely also played major parts in the planet\u2019s long cooling trend.<\/p>\n What the ice can, and cannot, say<\/p>\n The Allan Hills archive comes with caveats, and the papers are direct about them. The oldest ice is stratigraphically discontinuous and probably heavily compressed. Some samples may average conditions over one or more glacial cycles rather than capture shorter swings. The site may also preserve interglacial conditions a bit better than glacial ones, which could bias values slightly high.<\/p>\n Julia Marks-Peterson, a doctoral student at Oregon State University. (CREDIT: OSU) <\/p>\n The researchers estimate that this preservation bias is small, about 10 ppm for carbon dioxide and about 30 ppb for methane, but they do not claim the record is perfect. They also found evidence that some deep samples contained metabolic carbon dioxide produced within the ice, forcing them to correct some values and exclude others.<\/p>\n Even with those uncertainties, the authors say the long-term trend remains: carbon dioxide declined only modestly, methane barely changed, and the planet still cooled sharply.<\/p>\n That result also puts the new ice-core data at odds with some earlier carbon dioxide reconstructions from marine sediments, especially boron-based estimates that suggested much higher ancient CO2 levels. Not all sediment-based studies agreed with one another, and the new papers argue that direct ice-core evidence is badly needed to sort out the dispute.<\/p>\n \u201cOur 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,\u201d Marks-Peterson said.<\/p>\n COLDEX<\/a> researchers have already found ice as old as 6 million years in one core, and more drilling has recently been completed.<\/p>\n Three million years of climate records from blue ice.\u2002Around one million years ago, the repeated expansion and retreat cycle of glaciers transitioned from a period of 100,000 years to around 40,000 years, an event known as the Mid-Pleistocene Transition (MPT). (CREDIT: Nature) Practical implications of the research<\/p>\n These studies give scientists a better baseline for comparing natural climate change with what is happening now. The ancient record suggests Earth cooled over millions of years with only a small drop in carbon dioxide and little change in methane<\/a>, meaning other parts of the climate system can strongly shape global conditions. <\/p>\n At the same time, modern greenhouse gas levels are far above those ancient values. According to NOAA, carbon dioxide averaged 425 ppm in 2025 and methane averaged 1,935 ppb in 2025. <\/p>\n That makes today\u2019s atmosphere stand out sharply from the long climate history preserved in Antarctic ice.<\/p>\n