気候科学：300万年にわたる氷と気候の物語

Key climate shifts in the past 3 million years may have been more heavily influenced by changing ocean temperatures than greenhouse gases, according to analyses of ancient Antarctic ice cores published in two Nature papers. The findings provide new insights into Earth’s past climates.

In the past 3 million years, the global climate has gradually cooled, with increasing glacial cycles, and it has undergone two key shifts. About 2.6 million years ago, ice sheets formed over the Northern Hemisphere and at high latitudes, with glacial cycles that lasted roughly 40,000 years. The cycle period increased to around 100,000 years roughly 1.2 million years ago, which allowed ice sheets to grow larger. However, the factors that drove these changes have been debated. Ice cores provide a record of changing greenhouse gases and elements associated with ocean temperatures. Cores taken from Allan Hills Blue Ice Area, East Antarctica within the past 7 years have extended previous records by at least 2 million years.

In two separate papers, Sarah Shackleton and colleagues, and Julia Marks-Peterson and colleagues, use the Allan Hills ice cores to provide a record of mean ocean temperature and greenhouse gas concentrations, respectively, over the past 3 million years. They find no significant change in methane concentrations and a small decline in carbon dioxide (of around 20 parts per million) between 2.9 and 1.2 million years ago, followed by stable concentrations from 1.2 to 0.8 million years ago. This finding suggests that greenhouse gas concentrations may not have been the main cause of the two climate shifts. However, changes in ocean temperature seem to be associated with climate transitions. Measurements of noble gas concentrations, which provide a proxy for ocean temperature (xenon and krypton dissolve at different temperatures), indicate that there was a pronounced cooling around 2.7 million years ago, and then steady temperatures around 1.2 to 0.8 million years ago.

The new records demonstrate that blue ice areas can extend the reach of ice core records. The authors note that the ice cores are not continuous and therefore only provide snapshots, but they still offer insights into climate evolution over millions of years.
 

• Article
• Published: 18 March 2026

Marks-Peterson, J., Shackleton, S., Higgins, J. et al. Broadly stable atmospheric CO₂ and CH₄ levels over the past 3 million years. Nature 651, 647–652 (2026). https://doi.org/10.1038/s41586-025-10032-y

• Article
• Published: 18 March 2026

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

News & Views: Climate snapshots trapped in ancient ice tell a surprising story
https://www.nature.com/articles/d41586-026-00636-3