Mid-Pleistocene Transition

For about the first million years of the Pleistocene, the Earth's ice sheets grew and shrank on a regular 41,000-year beat, matching changes in the tilt of the planet's axis. Between roughly 1.25 million and 750,000 years ago, the pattern changed. Glacial cycles slowed to about 100,000 years, ice sheets grew larger, and the cold periods became deeper and more severe, separated by shorter warm interglacials. This shift is called the Mid-Pleistocene Transition. It is recorded in ocean sediments, where the oxygen in the shells of tiny sea creatures reveals how much water was locked up in ice.

The cause is still debated. The Earth's orbit has not changed in a way that explains the new 100,000-year rhythm, so scientists look to feedbacks inside the climate system. One leading idea is that falling carbon dioxide levels slowly cooled the planet, while the ice sheets on land, especially in North America, grew thicker as old soft soil was scraped away to bare rock, letting ice survive longer and grow bigger. For humans the effect was dramatic: bigger swings in climate and sea level, drier Africa at times, and a repeated need to adapt. The longer, harsher cycles built toward the Last Glacial Maximum, about 26,000 years ago.