For decades, scientists assumed the Northern Hemisphere was the puppet master behind one of Earth's most consequential climate shifts. But new research reveals a different conductor entirely—one much further south.
University of Delaware geoscientist Chandranath Basak and his team have uncovered evidence that the Southern Hemisphere, specifically changes connected to Antarctica, triggered the Mid-Pleistocene Transition—a fundamental reorganization of Earth's ice age rhythms that occurred roughly one million years ago. Their findings, published in Science Advances, upend a long-standing assumption about what drives our planet's climate machinery.
"What happened in and around Antarctica played an important role in controlling global deep ocean circulation," said Basak's doctoral student Kapuge, who co-authored the study alongside graduate Emily Symes. "It is the same deep ocean circulation system we're actively monitoring today for signs of modern weakening. So, in that regard, this study has modern relevance."
The discovery came from an unlikely source: fossil fish teeth. Basak's team analyzed traces of neodymium—a rare-earth element—preserved in the enamel of ancient fish recovered from sediment cores drilled in the central South Pacific during the International Ocean Discovery Program Expedition 383 in 2019. These microscopic time capsules carry chemical signatures that reveal how deep ocean currents flowed millions of years ago.
Before the Mid-Pleistocene Transition, Earth cycled through glacial and warm periods every 41,000 years. Then, between 1.2 million and 700,000 years ago, that rhythm stretched dramatically to 100,000-year cycles—a shift that fundamentally altered how our planet handles ice ages. Scientists have long debated what sparked this change.
The answer, it turns out, was hiding in the southernmost waters. Basak's data show that the transition began in the Southern Hemisphere and was later reinforced by changes in the north—meaning Antarctica's freeze and the ocean currents around it acted as a climate domino that reshaped the entire globe.
For Basak, the implications extend far beyond ancient geology. As ice sheets melt in both polar regions due to rising temperatures, understanding their historical interplay becomes crucial for predicting what comes next.
"Every bit of past observations that we make helps us to understand how these different climatological factors interact with each other," Basak said. "Whatever changes are happening in these records are completely driven by natural changes. That helps us better parameterize climate models to eventually give us insight into future changes that we can expect moving forward in a rapidly changing world."
In other words, to know where Earth's climate is heading, scientists now know to look south.