A new study suggests that a million years ago, glaciers stuck to bedrock more easily, contributing to global cooling and longer ice ages, which is valuable to know for climate scientists trying to understand climate change.

A million years ago, Earth faced a major change that led to more intense glacial periods that formed ice sheets in the Northern Hemisphere. The event is called the Mid-Pleistocene Transition (MPT). Before then, colder (glacial) and warmer (interglacial) periods occurred every 41,000 years. After this, glacial periods intensified enough to form ice sheets in the Northern Hemisphere lasting 100,000 years.

What caused the transition has been debated. One root cause could have been Milankovitch cycles, where changes in Earth’s orbit and orientation in relation to the Sun determine the energy the planet absorbs. However, these cycles have driven warm and cold periods for millions of years but research shows they did not change much a million years ago, so something else must be responsible.

Coincidentally, during the MPT, a large system of ocean currents moving heat across the globe weakened dramatically. The slow down of the system, named the Meridional Overturning Circulation (AMOC) may have been linked to the shift.

As a result of an international collaboration of researchers, a new paper was published in the journal Proceedings of the National Academy of Sciences that sheds some light. The co-authors analysed deep-sea sediment cores in the North and South Atlantic, finding evidence that ice sheets in the Northern Hemisphere stuck to bedrock more easily, causing thicker glaciers which contributed to global cooling, leading to more intense ice ages and the MPT.

Fundamentally, these findings emphasise the important role of the North Atlantic and ocean circulation for climate change, not only in the past, but the present and future.