Mid-Pleistocene transition

Ice ages once followed a rhythm of about 41,000 years. Around a million years ago, much longer cycles became dominant. Earth’s orbital variations continued, so the puzzle is what changed in the climate system’s response to that pacing.[1] [3]

Around a million years ago, glaciations became longer even though Earth’s orbital variations continued. The open question is which change in ice sheets or the rest of the climate system altered their response.

In the PACCO model, quarrying removes loose regolith beneath the ice faster than it is replenished. With less soft material to slide over, ice discharge falls and thicker ice can persist through more summers. Snowfall adds ice, ablation removes it, and flow carries it away. This is a physical way to change the rhythm without imposing a new orbital period, but it is a modeled scenario rather than an identified historical cause.[1] [3]

Removing loose material beneath ice could let thicker ice sheets survive more summers. Carbon-cycle changes could also alter the response. Models demonstrate possibilities, but the records have not selected one explanation for the transition.[1] [3]

That mechanism is not uniquely identified by the simulation. Different snowfall sensitivities can also change the rhythm. Revised sea-level interpretations and accumulation-weighted blue-ice gas means constrain different quantities; neither should be treated as an unambiguous continuous measure of ice volume or orbital-scale carbon variability.[1] [2] [3] [4] [5]

A regular external push can produce a different rhythm when the system being pushed changes. In proposed explanations, changes within ice sheets and climate can let ice survive a summer-sunlight increase that once would have ended a glaciation. Missing that melting opportunity can lengthen the interval until major ice loss. This links the astronomical pacing of ice ages to feedbacks inside the climate system: the timing of sunlight alone need not determine the timing of the response. The models illustrate that connection without selecting a unique historical cause.[1] [3]

This history asks how internal feedbacks can turn continuing orbital forcing into a different ice-age rhythm. If a changing bed made ice harder to remove, ice dynamics would help explain the shift; if carbon or snowfall changes were decisive, a different part of the climate system would carry that explanation. Sea-level reconstructions and gas samples matter because they constrain those alternatives in different ways, not because a fitted threshold by itself reveals the mechanism.[3] [4]