A groundbreaking international study has revealed that dramatic shifts in global sea level occurred throughout the last ice age, not just toward its end – upending decades of scientific consensus and reshaping our understanding of Earth’s climate history.
Published in Science, the research shows that massive ice sheets in the Northern Hemisphere grew and shrank repeatedly over the past 2.5 million years, driving sea level changes of up to 150 meters. These fluctuations were previously thought to intensify only during the Middle Pleistocene Transition (MPT), a period between 1.2 and 0.62 million years ago.
Lead author Peter Clark, a paleo-climatologist and Ulster University Professor of Quaternary Science in the School of Geography and Environmental Sciences said:
“This is a paradigm shift in our understanding of the ice age. We now know that large ice sheets were present and dynamic much earlier than we thought, and that their behaviour was shaped by internal climate feedbacks—not just changes in Earth’s orbit.”
Using a novel reconstruction of seawater oxygen isotopes, the team mapped global mean sea level (GMSL) over the past 4.5 million years. Their findings challenge the long-held belief that glaciation cycles only became large and irregular after the MPT. Instead, the study shows that many earlier cycles, occurring every 41,000 years, were just as intense as the later 100,000-year cycles.
Professor Clark, who is also a professor in Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences, continued:
“This forces us to rethink the mechanisms behind ice sheet growth and collapse, not just about orbital forcing. The Southern Ocean carbon cycle and atmospheric CO₂ variability played a much bigger role than previously recognized.”
The implications are profound. The study suggests that ice sheets can remain stable under warmer global temperatures if they reach a critical height, a finding that could inform predictions about the future of Antarctica and Greenland in a warming world.
“This research gives us a deeper understanding of how ice sheets interact with climate. It’s essential context for anticipating how today’s ice sheets might respond to ongoing climate change.”
Professor Liam Maguire, Pro Vice Chancellor for research at Ulster University, said:
“This is a landmark moment in climate science, and I want to warmly congratulate Professor Peter Clark on leading such a transformative study. His work challenges long-standing assumptions about the last ice age and opens up new avenues for understanding how ice sheets and sea levels respond to climate change. At Ulster University, we are delighted to see one of our professors contributing to such globally significant research, and we commend the international team for this remarkable breakthrough.”
The study builds on nearly a decade of work by Clark and colleagues to reconstruct past global temperatures and sea levels. It involved researchers from 12 institutions across the U.S., Europe, and Asia, including Ulster University, Boston College, Rutgers University, and the Chinese Academy of Sciences.
