Large shifts in global sea level occurred throughout the last ice age, not just toward its end, according to a new study published this week in the journal Science.

The findings challenge long-standing theories about how the Pleistocene epoch unfolded and suggest that massive ice sheets were forming and melting far earlier and more dramatically than previously believed.

“This is a paradigm shift in our understanding of the history of the ice age,” said Peter Clark, lead author of the study and a paleoclimatologist at Oregon State University. “It forces us to develop new explanations.”

The Pleistocene, which lasted from about 2.6 million to 11,700 years ago, was marked by cycles of glaciation and retreat. Until now, scientists believed that the most significant changes in sea level occurred after a transition period between 1.25 million and 700,000 years ago, when glaciation cycles shifted from 41,000-year intervals to longer, more intense 100,000-year cycles.

But the new reconstruction, which spans the past 4.5 million years, shows that many of the earlier cycles were just as large. That suggests the ice sheets were already substantial and that their growth and decay were driven more by internal climate feedbacks than by external forces such as carbon dioxide levels or orbital changes.

The evidence comes from microscopic marine organisms called foraminifera, preserved in ocean sediment cores. These tiny shells record changes in sea level and temperature, offering a detailed archive of Earth’s climate history.

Clark said the findings challenge the conventional wisdom about the middle Pleistocene transition and open the door to new theories about how ice sheets behave. “We have two big ice sheets today, in Antarctica and Greenland, and it’s important to think about how ice sheets like this can exist under a variety of conditions,” he said.

The research builds on earlier work by Clark and colleagues to reconstruct global temperatures and better understand the links between ice sheets and climate. The international team includes scientists from the United States, Europe, China and the United Kingdom.