Researchers analyzed ancient, disrupted Antarctic ice cores from the Allan Hills blue ice area to reconstruct Earth’s climate drivers over the past ~3 million years, extending records farther back than traditional continuous cores.

Key findings from the research:

• Over long timescales, Earth’s climate has undergone major shifts, including the onset of Northern Hemisphere glaciation ~2.6 million years ago (with ~40,000-year glacial-interglacial cycles) and the Mid-Pleistocene Transition (MPT) ~1 million years ago (shifting to ~100,000-year cycles with larger ice sheets).
• Traditional views emphasize greenhouse gases (like CO₂) as a primary driver of these changes, often in feedback loops with orbital variations (Milankovitch cycles), ice-sheet dynamics, and other factors.
• However, the new ice core data (including disrupted, non-chronological layers from old Antarctic sites) suggest that at certain key transition points, changes in ocean temperatures and circulation may have played a larger role than fluctuations in greenhouse gases in driving major climate reorganizations.
• Atmospheric CO₂ levels appear to have remained relatively stable and modest (long-term averages below ~300 ppm, with only slight declines over millions of years), yet significant cooling and ice-sheet changes occurred. This implies other factors—like ocean heat distribution, circulation changes, or ice-sheet sensitivity—were crucial drivers during those periods.
• A commentary in Nature by climatologist Eric Wolff notes this could mean ice sheets were extremely sensitive to small CO₂ changes, or that “something else” (beyond greenhouse gases) drove past climate shifts in ways not fully captured by current models.

This doesn’t overturn the well-established role of greenhouse gases in past glacial-interglacial cycles (where temperature and CO₂ often tracked closely, with CO₂ acting as a feedback amplifier) or in modern human-driven warming (where CO₂ levels now far exceed natural prehistoric ranges due to fossil fuel emissions).

Instead, it highlights the complexity of the climate system: multiple factors interact, and ocean dynamics may have been more dominant during specific ancient transitions.

For context, ice cores remain one of our best archives for past atmospheres (trapping air bubbles) and temperatures (via water isotopes), but interpreting very old, disturbed cores involves challenges like dating and layering disruptions.

The Allan Hills cores (some dating back even ~6 million years, exposed by winds) act as time capsules via air bubbles and isotopes—extending our view of pre-human climate. This helps test models for how the planet responded to past forcings.

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Something Else Used to Drive Climate Changes, Ancient Ice Cores Reveal

The rapid climate change we are experiencing today is mainly driven by the greenhouse gases we humans keep releasing into the air.

But new evidence from ancient Antarctic ice cores suggests this wasn’t always the case for the past three million years of Earth’s changing climate. ScienceAlert has the story.

According to the findings of two new papers published in Nature, at certain transition points ocean temperatures could have had a greater influence over Earth’s climate than greenhouse gases.

Two research teams analyzed ice cores extracted from the Allan Hills, a blue ice region of Antarctica. The Allan Hills cores are samples of some of the world’s oldest ice, with some dating as far back as 6 million years ago.

Blue ice areas like the Allan Hills make up only about 1 percent of the surface area of Antarctica’s ice sheet, and they’re named as such because strong winds blow away any new snow, keeping older, glacial ice exposed at the surface.

The Allan Hills region hasn’t moved horizontally or vertically very much at all, making it a particularly unique site for taking cores of very, very old ice.

Ice cores are some of our best natural ‘archives’ of Earth’s long-term climate.

They don’t necessarily contain a complete, continuous record. The Allan Hills cores, for example, contain layers that are out of chronological order thanks to the way the ice was deposited across the millennia.

But each layer of ice contains a climate snapshot that can tell us a lot about what was going on at the time of freezing, and there are ways of decoding their secrets.

Certain isotopes in the ice hint at ocean temperature. Impurities like volcanic ash and other particles can indicate sources of air pollution.

And, perhaps most importantly to climate scientists, the ice can trap tiny bubbles of the air, revealing the historic gas composition of the atmosphere across millions of years.

Read the full story here.