CORVALLIS, Ore. — Large changes in global sea level, fueled by fluctuations in ice sheet growth and decay, occurred throughout the last ice age, rather than just toward the end of that period, a study publishing this week in the journal Science has found.
The findings represent a significant change in researchers' understanding of how the Pleistocene – the geological period from about 2.6 million to 11,700 years ago and commonly known as the last ice age – developed, said Peter Clark , a paleoclimatologist at Oregon State University and the study's lead author.
"This is a paradigm shift in our understanding of the history of the ice age," said Clark, a university distinguished professor in OSU's College of Earth, Ocean, and Atmospheric Sciences.
During the last ice age, Earth experienced cycles of dramatic shifts in global sea level caused by the formation and melting of large ice sheets over northern areas of North America and Eurasia. These changes are recorded in the shell remains of microscopic marine organisms called foraminifera, which are found in ocean sediment and collected by drilling cores, giving scientists an important record of past climate history.
When the first reconstruction of global sea level over the last ice age was published nearly 50 years ago, the science suggested there was a transition period about 1.25 million to 700,000 years ago, known as the middle Pleistocene transition, when the size of the ice sheets and the cycle of forming and melting changed.
"Before that transition, the glaciation cycles occurred about every 41,000 years, and after the transition, the cycles were every 100,000 years and were larger in amplitude," said Clark. "All theories developed to explain this transition were focused on an increase in the size of the ice sheets through this transition. Every sea level reconstruction since that initial study produced the same storyline until now."
Researchers had two leading hypotheses to explain why the transition occurred. One suggests that global cooling from decreasing carbon dioxide levels contributed to the cycle change and the other suggested that changes in how ice sheets move played a role.
In the new study, researchers reconstructed sea level changes for the past 4.5 million years. They found that many of the glaciation cycles during the early Pleistocene, when the cycles were 41,000 years in duration, were as large as the more recent cycles.
"Having those large ice sheets present throughout that time means that their formation and decay were likely influenced by internal feedbacks in the climate system, rather than external dynamics," Clark said. "This finding challenges the conventional wisdom on the middle Pleistocene transition and forces us to develop new explanations."
The research builds on previous work by Clark and colleagues to reconstruct global atmospheric temperatures and mean ocean temperatures, a project that began in 2017 to further understand past climate dynamics.
"Our ability to understand the past gives us a greater understanding of ice sheet-climate interactions and provides context for what we might experience in the future," Clark said. "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."
Co-authors of the study are Steven W. Hostetler and Nicklas G. Pisias of Oregon State University; Jeremy D. Shakun of Boston College; Yair Rosenthal of Rutgers University; David Pollard of Pennsylvania State University; Peter Kohler of the Alfred-Wegener Institute of Germany; Patrick J. Bartlein of University of Oregon; Jonathan M. Gregory of University of Reading of the United Kingdom; Chenyu Zhu of the Chinese Academy of Sciences; Daniel P. Schrag of Harvard University; and Zhengyu Liu of Ohio State University.
