https://news.oregonstate.edu/news/new-ice-core-studies-expand-histories-greenhouse-gases-and-ocean-temperature-3-million-years
Two Nature papers published March 18, 2026 used ice up to about 3 million years old from Allan Hills, Antarctica. The layers are jumbled, so they give snapshots rather than a continuous record. Still, they hold the first direct measurements of air from that far back.
Carbon dioxide was about 250 ppm 2.7 million years ago and fell only about 20 ppm by a million years ago. Methane stayed around 500 ppb. Noble gases in the bubbles show the average ocean cooled by 2–2.5 °C, with much of that cooling early on.
So the long cool-down took only a modest drop in greenhouse gases. Ice, vegetation and ocean circulation probably did a lot of the work. For scale, CO2 averaged about 425 ppm in 2025.
Source: news.oregonstate.edu · Also: Nature: CO2 and CH4 · Nature: ocean heat content