For the past 1.2 million years (during the Late Pleistocene period), ice ages have occurred in cycles lasting roughly 120,000 years. Before this period (during the Early Pleistocene period), these cycles lasted only about 41,000 years. The cause of the change in ice age duration, known as the Mid-Pleistocene Transition, is unknown. A recent article published in Reviews of Geophysics examines possible explanations for the Mid-Pleistocene Transition. We asked the authors about the Mid-Pleistocene Transition and possible explanations for this period.
What makes the Mid-Pleistocene Transition (MPT) particularly fascinating to study?
Studying glacial cycles, and particularly the MPT, requires you to think about all the possible ways the different components of the Earth’s climate can interact. A growing ice sheet will reflect more sunlight back into space, cooling down the climate. A colder climate means colder oceans, which absorb more CO2 from the atmosphere. Ice sheets cause erosion, which creates airborne dust and influences the flow of the ice. Some of this dust can rain down on top of the ice sheet, creating dirty snow that absorbs more sunlight, or it can be blown into the oceans, where it can fertilize algae growth and increase CO2 drawdown.
These interactions between the ice sheets, the global climate, the oceans, the carbon cycle, and even the solid Earth, are fascinating. A change in any one of these physical systems will affect all others, which is a profound realization when thinking about current climate change.
How did the cycle of ice ages differ between the Early Pleistocene and Late Pleistocene?
The Pleistocene (the last 2.8 million years of Earth’s geological history) is distinguished by the presence of glacial cycles (“ice ages”): periodic, dramatic climate changes that caused vast ice sheets to appear and disappear over large parts of North America and Europe.
During the Early Pleistocene, these glacial cycles occurred roughly every 41,000 years. This makes sense, because these cycles are caused by small changes in Earth’s orbit, which also occur every 41,000 years. The MPT marks the transition to the Late Pleistocene, where the glacial cycles took much longer (about 100,000 on average). Understanding how a 41,000-year change in Earth’s orbit can lead to a 100,000-year climate response is one question we tried to answer; the other is why it only did so during the Late, and not during the Early Pleistocene.
What are the various explanations for the MPT?
There are two groups of theories. The first is the “global cooling plus non-linear feedbacks” group, which says that ice sheets respond non-linearly to changes in climate. The larger ice sheets of the cold Late Pleistocene created their own cold climate environment, making them more resistant to climate warming. This allowed them to survive some of the warm interglacial periods, growing even larger during the next cold phase. The reason why this didn’t happen during the Early Pleistocene is because the world was warmer then, so that the ice sheets never reached the required size to survive a warm period.
The second group are the “erosion” theories. Ice sheets slowly grind away the land underneath them, scraping away the soil until nothing remains but bare rock. Ice slides more easily over soil than over rock, so that soil-based ice sheets tend to “flatten out” when compared to rock-based ice sheets. Also, as mentioned before, soil-based ice sheets create airborne dust which can lead to dirty snow (which absorbs more sunlight), and oceanic algae fertilization (which draws CO2 out of the atmosphere). In this theory, the MPT marks the moment when the last soil was eroded away in northern North America and Europe, and these different processed ceased.
Does the available evidence appear to more strongly support one explanation over the others?
It’s likely that all of these mechanisms at least played a role, but determining how much of a role is tricky due to the lack of detailed and conclusive data. Since the erosive action of ice sheets tends to remove evidence of earlier ice sheets, it’s difficult to figure out what the older ones looked like. Data on the state of the Earth’s climate or the composition of the atmosphere is also limited. Our most valuable source of information is the ice core record, but that only extends to 800,000 years ago – not long enough to cover the MPT.
What different research approaches could be used to resolve the question of the MPT’s cause?
Currently, a team of scientists is drilling into the Antarctic ice sheet to produce a core that, if all goes well, should go back well over a million years. We’re very excited to see what comes out of that project, particularly in terms of CO2 concentration.
At the same time, the ice-sheet modelling community is working on improving the physics of sliding and oceanic melting. Although most of the focus is on near-future ice-sheet retreat, the outcomes are also important for the MPT, since these processes were important in those times as well. And the more accurate our models can reproduce ice-sheet evolution in the past, the better they are at predicting the future.