Scientists analyzing ancient ice from Antarctica have found that ocean temperatures dropped by 2 to 2.5 degrees Celsius over the past 3 million years, while carbon dioxide and methane levels changed only modestly. The studies, led by researchers from Oregon State University and others, suggest factors beyond greenhouse gases drove much of Earth's long-term cooling. The findings come from ice cores at Allan Hills in East Antarctica.
Researchers with the National Science Foundation's Center for Oldest Ice Exploration (COLDEX), led by Oregon State University, examined ice from Allan Hills along the East Antarctic ice sheet. This site provides snapshots of past climate due to ice distortion. Two studies published in Nature detail cooling trends and gas levels spanning 3 million years back. Julia Marks-Peterson, a doctoral student at Oregon State University, and Sarah Shackleton, now at Woods Hole Oceanographic Institution, led the work. They measured noble gases in trapped air bubbles to track ocean temperatures and directly assessed carbon dioxide and methane concentrations for the first time over this period. CO2 stayed below 300 parts per million, dropping from about 250 ppm around 2.7 million years ago to roughly 20 ppm lower by 1 million years ago. Methane held steady at around 500 parts per billion. Much ocean cooling happened early, from 3 million to 2 million years ago, aligning with Northern Hemisphere ice sheet growth. Surface waters cooled more gradually until about 1 million years ago, possibly due to shifts in ocean heat distribution. Ed Brook, COLDEX director and paleoclimatologist at Oregon State University, said, 'Those snapshots extend climate records from ice much further than previously possible.' Shackleton noted that noble gases offer a global ocean temperature view unlike site-specific methods. The modest gas changes imply other drivers like ice sheets, ocean circulation, Earth's reflectivity, and vegetation shifts played key roles in cooling. For comparison, 2025 CO2 averaged 425 ppm and methane 1,935 ppb, per NOAA data. Marks-Peterson hopes the work refines understanding of past warm climates and Earth system interactions. COLDEX continues seeking even older ice, up to 6 million years.