Antarctic Ice Snapshots Reveal 3-Million-Year History of Greenhouse Gases and Deep Ocean Cooling

New Oregon State University research uses ancient Antarctic ice to reveal 3 million years of greenhouse gas levels and deep ocean cooling patterns.

By: AXL Media

Published: Mar 18, 2026, 3:01 PM EDT

Source: Information for this report was sourced from Oregon State University

Unlocking the Secrets of the Allan Hills Archive

A nationwide collaboration led by the National Science Foundation Center for Oldest Ice Exploration (COLDEX) has successfully analyzed multi-million-year-old ice trapped at the margins of the East Antarctic ice sheet. Unlike traditional ice cores that provide a continuous timeline, the samples from Allan Hills offer "snapshots" of average environmental conditions from discrete periods in the distant past. These snapshots have allowed scientists to reconstruct atmospheric and oceanic conditions dating back 3 million years, a period when the Earth was significantly warmer and sea levels were much higher. According to COLDEX Director Ed Brook, these records are raising fundamental new questions about the evolution of the planet's climate.

The Divergent Cooling of Surface and Deep Waters

Research led by Sarah Shackleton utilized precise measurements of noble gas ratios trapped within the ice to track changes in global mean ocean temperature. The study found that the deep ocean has cooled by approximately 2 to 2.5 degrees Celsius over the last 3 million years. Interestingly, the timing of this cooling differs from that of the ocean surface; a large portion of the deep-sea cooling occurred early, between 3 million and 2 million years ago, coinciding with the initial formation of Northern Hemisphere ice sheets. This discrepancy suggests a complex shift in how heat is transferred between the surface and the depths of the global ocean system.

Carbon Dioxide Levels Remain Historically Low

In a companion study, doctoral student Julia Marks-Peterson identified the first direct records of atmospheric greenhouse gases over this 3-million-year span. The data indicate that carbon dioxide levels likely remained below 300 parts per million (ppm) throughout this period. Measurements showed CO2 at 250 ppm roughly 2.7 million years ago, with only a modest decline of 20 ppm over the subsequent 1.7 million years. Methane levels remained largely unchanged at 500 parts per billion. These figures stand in stark contrast to modern 2025 averages, which saw carbon dioxide at 425 ppm and methane at 1,935 ppb.