The mid-Pleistocene transition (MPT), occurring between 1.25 million and 700,000 years ago, marks a turning point in Earth’s recent climate history. This period saw a shift in the rhythm and intensity of the planet’s ice age cycles. Before the MPT, glacial periods were less severe and more frequent. Afterward, the planet settled into a new pattern of longer and more extreme ice ages. This event was not caused by a change in external forces, but by internal changes within Earth’s climate system. Understanding this transformation helps scientists comprehend the interactions that govern our planet’s long-term climate.
The 41,000-Year World
During the early Pleistocene epoch, Earth’s ice ages cycled approximately every 41,000 years. These glacial periods were moderate and short-lived, with thinner ice sheets that grew and melted frequently. This rhythm was paced by a regular variation in Earth’s axial tilt, known as obliquity. The planet’s tilt wobbles between 22.1 and 24.5 degrees over a 41,000-year period, altering the distribution of solar energy across the globe. A greater tilt leads to more extreme seasons, and these changes in solar radiation at the poles were the main driver of the glacial-interglacial cycles.
The Shift to a 100,000-Year Cycle
The mid-Pleistocene transition marked a departure from this established climate rhythm. The 41,000-year cycle gave way to a new beat with a period of roughly 100,000 years. This new regime was characterized by ice ages that were longer, colder, and more severe. During these extended glacial periods, large continental ice sheets grew to cover vast portions of North America and Eurasia, locking up enormous quantities of water.
The transition was not merely a change in timing but also in character. The new cycles became asymmetrical, featuring long, slow periods of cooling and ice sheet growth, followed by rapid terminations that shifted the planet into a warm interglacial state. This change presents a puzzle for climatologists, called the “100-kyr problem.” The 100,000-year cycle does not align with a correspondingly strong astronomical driver, unlike the previous cycles that matched the 41,000-year obliquity forcing.
The astronomical cycle with a similar frequency is the change in Earth’s orbit shape, known as eccentricity. However, the effect of eccentricity on solar radiation is much weaker than that of obliquity. The climate system began to amplify this weak signal into the dominant beat of the ice ages. This indicates that internal feedbacks within Earth’s systems became the primary drivers of the planet’s climate state.
Proposed Causes of the Transition
The cause of the MPT is likely a combination of interacting factors that pushed Earth’s climate system across a threshold. One hypothesis centers on a gradual decrease in atmospheric carbon dioxide (CO2). As CO2 levels dropped over millions of years, the planet’s baseline temperature cooled, making it easier for ice sheets to form and persist. This slow decline in CO2 may have been a precondition for the shift, priming the climate system for a change.
Another theory involves the internal dynamics of the ice sheets, related to the material beneath them, called the “regolith hypothesis.” In earlier glaciations, ice sheets formed on a thick layer of loose sediment and soil, or regolith. This soft bed allowed the ice to slide and melt more easily. As successive glaciers advanced, they scraped away this loose material, exposing hard bedrock. Ice sheets on this rigid base were less slippery and could grow thicker, making them more stable and longer-lasting.
These two mechanisms likely worked in concert. The lowering of atmospheric CO2 created a climate cold enough to support larger ice sheets. Once established, their erosive power changed the landscape by removing the regolith. This combination of a cooler climate and a stable bedrock foundation allowed ice sheets to grow to a larger size. These ice sheets could then persist through weaker warm periods, locking the climate into a longer, 100,000-year rhythm.
Global Consequences of the New Climate Rhythm
The shift to longer and more intense ice ages had lasting consequences for the planet. The growth of large ice sheets reshaped the Northern Hemisphere, scouring landscapes, carving deep valleys, and displacing river systems. The weight of the ice depressed Earth’s crust. The volume of water locked within them caused global sea levels to drop by more than 100 meters, exposing continental shelves and creating land bridges.
This new climate regime also triggered changes in global ocean circulation and atmospheric patterns. For instance, the Indian Summer Monsoon’s strength fluctuated, and dust from expanded dry regions may have fertilized parts of the ocean, altering marine ecosystems. The environmental shifts exerted evolutionary pressure on flora and fauna, forcing species to migrate, adapt, or face extinction. This period of climate instability is also thought to have influenced the evolution and migration of early humans, presenting challenges and opportunities that shaped our species’ history.