Sea levels saw large fluctuations throughout last ice age

Large changes in global sea level, driven by ice sheet growth and decay, occurred throughout the last 2.6 million years rather than just toward the end of that period, a study published this week in the journal Science has found.  

The findings represent a significant change in researchers’ understanding of the development of the Pleistocene, the geological period which started about 2.6 million years ago. It ended 11,700 years ago, once all the ice sheets had vanished except the ones which remain on Greenland and Antarctica today.

During the Pleistocene Period, 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, including Scotland. 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 Pleistocene 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. 

Professor Peter Clark, a paleoclimatologist at Oregon State University and the study’s lead author, said: “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. 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.” 

'Understanding the past helps us predict the future'

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,” Professor Clark said. “This finding challenges the conventional wisdom on the middle Pleistocene transition and forces us to develop new explanations.”  

Professor Jonathan Gregory, co-author from the National Centre for Atmospheric Science at the University of Reading’s Department of Meteorology, said: "The chemical signatures preserved in ocean sediments tell us about ocean temperature changes and ice volume changes at the same time. We’ve realised that we’d been overestimating the variations of temperature, and underestimating how large and dynamic the early ice sheets really were. The evidence hasn’t changed, but our interpretation has improved. This matters because understanding the past helps us to predict the future."

Read the article in Science.

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