For decades, scientists have grappled with a perplexing climatic enigma: the Mid-Pleistocene Transition (MPT). Roughly 1.5 million years ago, Earth experienced a dramatic shift, transitioning from 40,000-year ice ages to much longer, colder 100,000-year cycles. The cause of this shift remained shrouded in mystery, despite researchers pointing towards the Earth's orbital wobbles and the role of massive ice sheets.
However, a groundbreaking new study published in Science sheds light on the culprit behind this drastic shift: carbon dioxide. This research, compiling global temperature data spanning the past 4.5 million years, pins the blame on a phenomenon occurring in the Southern Ocean.
The study proposes that a significant strengthening of the ocean pump around Antarctica played a pivotal role in the MPT. As vast swathes of sea ice formed in the Southern Ocean, it acted like a sieve, extracting freshwater. This left behind denser, saltier water that plunged towards the ocean floor, carrying dissolved carbon dioxide with it. This underwater "plunge" effectively sequestered carbon dioxide from the atmosphere, leading to global cooling and intensifying the ice ages.
This process created a self-perpetuating cycle. As the planet cooled, sea ice formation intensified, further strengthening the ocean pump and accelerating the decline in global temperatures. This cycle continued until the collapse of northern ice sheets, injecting freshwater into the oceans and disrupting the pump, leading to a rise in global temperatures and a temporary halt to the extended ice ages.
This new research challenges the previously dominant theory, known as the regolith hypothesis, which attributed the MPT solely to the influence of ice sheets. This theory postulated that once glaciers in the Northern Hemisphere grew large enough, their weight triggered their collapse, extending the duration of ice ages. However, the new temperature record suggests that ice sheets played a minimal role in driving global temperatures.
The study not only unveils the cause of the MPT but also raises concerns about potential underestimations of climate sensitivity in current models. The newly reconstructed temperature record reveals a 3°C greater cooling over the past 4.5 million years than predicted by existing models. This signifies that the climate system might be more susceptible to changes in carbon dioxide than previously thought, with potentially far-reaching consequences for future climate projections.
While further work is needed to extend ice core data and refine understanding, this groundbreaking record already serves as a cornerstone for future research. Scientists are now looking to explore how changing weather patterns and cloud responses across different regions may be influenced by the Southern Ocean's dynamics.
The study not only unravels a long-standing climate mystery but also highlights the critical role of the Southern Ocean. As sea ice in this region rapidly diminishes due to climate change, researchers are actively investigating the potential ramifications of this shift on the global climate and carbon storage mechanisms. This research serves as a stark reminder of the delicate balance of our planet's climate system and the urgency of addressing factors contributing to its disruption.
