The Mid-Pleistocene Climate Transition

The timing of ice ages over the past ∼2,600 thousand years (kyr) follows pacing by cyclical changes in three aspects of Earth's orbit that influence the solar energy received as a function of latitude and season. Explaining the large magnitude of the climate changes is challenging, particularly so across the period of time from ∼1,250 to 750 ka—the Mid-Pleistocene Transition or MPT. The average repeat time of ice age cycles changed from an earlier 41-kyr rhythm to longer and more intense glaciations at a spacing of about 100 kyr. Explaining this change is very difficult because there was no corresponding change in the orbital pacing that would trigger a change in timing. While the first generation of hypotheses looked largely to changes in the behavior of Northern Hemisphere ice sheets, more recent work integrates ice behavior with new data capturing the evolution of other important aspects of past climate. A full explanation is still lacking, but attention increasingly focuses on the ocean carbon cycle and atmospheric CO 2 levels as the crucial agents involved in the MPT. ▪ The pattern of climate changes connected to the ice ages of the past few million years changed radically between about 1,250 and 750 thousand years ago, a time known as the Mid-Pleistocene Transition or MPT. ▪ While the glacial cycles were ultimately triggered by cyclical changes in Earth's orbit, the changes across the MPT came from changes in the Earth system itself, most likely in the form of a change in the carbon cycle. ▪ The dramatic change in many essential aspects of climate—ice volume, temperature, rainfall on land, and many others—in the absence of an external change suggests how important feedbacks are to the climate system. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.