New ice core studies expand histories of greenhouse gases and ocean temperature to 3 million years

[OKIsItJustMe]

March 17, 2026

CORVALLIS, Ore. — New analyses of ancient ice from Antarctica and the air contained inside it are extending the history of Earth’s climate records and expanding researchers’ understanding of how the planet has changed over the last 3 million years.

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Using precise measurements of the ratio of different noble gases in air trapped in the ice, which reflect ocean temperature changes, Shackleton and colleagues showed that the average temperature of the ocean has declined by 2 to 2.5 degrees Celsius over the past 3 million years. Previous studies have extensively examined changes in ocean surface temperatures, which also show cooling over this period, but the new work shows that the timing of cooling is different between the ocean surface and the ocean depths.

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Using the same ice core samples, Marks-Peterson and her coauthors identified the first direct records of the levels of two of the most important atmospheric greenhouse gases, carbon dioxide and methane, over the last 3 million years.

The data show that long-term average atmospheric carbon dioxide levels have likely remained below 300 parts per million over this time; measured carbon dioxide levels were 250 parts per million 2.7 million years ago and declined modestly by about 20 parts per million until 1 million years ago. Long-term average levels of atmospheric methane remained unchanged at 500 parts per billion.

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Shackleton, S., Hishamunda, V., Yan, Y. et al. Global ocean heat content over the past 3 million years. Nature 651, 653–657 (2026). https://doi.org/10.1038/s41586-026-10116-3

Nature 651, 592-593 (2026) https://doi.org/10.1038/d41586-026-00636-3