Why did glacial cycles intensify a million years ago?

glacial cycles intensify

A major event seems to have impacted our Planet about one million years ago, during an episode called “mid-Pleistocene transition”: before this period, the cycles between glacial (colder) and interglacial (warmer) periods followed each other every 41,000 years, but they then intensified, sometimes giving rise to glacial periods lasting over 100,000 years. Paleoclimatologists have long debated the precise origin of this abrupt climate change.

Extending from 2.58 million years to 11,700 years before the present, the Pleistocene is a geological period covering the majority of recent known glaciations. The climate is characterized by intense cycles of glaciation, allowing the ice to cover more than 30% of the Earth’s surface during the maximum of glaciation: we find imposing glaciers in New Zealand, in the Ethiopian mountains or in the Atlas Mountains, in North Africa.

But these glacial cycles, characterized by a period of 41,000 years more than a million years ago, seem to have been largely impacted by a still unknown phenomenon to give the longer cycles we observe today.

Milankovitch cycles at the origin of these variations?

Scientists have been hesitating for a long time about the origin of this phenomenon. The most likely reason currently accepted is the Milankovitch cycles, cyclical variations in the Earth’s orbit that affect the amount of energy absorbed by the Earth.

On a geological scale, this is indeed the main natural driver for the alternation of warm and cold periods over several million years. However, some researchers argue that if the Milankovitch cycles did not undergo major changes a million years ago, another internal phenomenon was probably at work.

The role of the North Atlantic

Researchers at Columbia University’s Earth Institute analyzed sediment cores from the North and South Atlantic, looking specifically at the concentrations of the isotope Neodymium, a chemical element used as a tool for studying past ocean currents.

They found that a major Atlantic current system, the AMOC (Atlantic Meridional Circulation), underwent a severe weakening at a time coinciding with the “Mid-Pleistocene Transition” in the northern ocean. The researchers in the study link this change in ocean current to the greater presence of glaciers in the northern hemisphere, creating a more intense global cooling.

This discovery sheds light on the origin of a recent climate transition that is still unknown, and highlights the importance of the North Atlantic and ocean currents in past and future climate variations.