A “glacial cycle” is defined as a major global climatic oscillation of the order of 105 yr, developed within an “ice age” sensu lato which may last 106–107 yr and which recurs at widely spaced intervals in geologic time (ca. 2 × 108 yr). The ice age situation thus only persists during 5–10% of all geologic time and is preconditioned by geotectonics and not by extraterrestrial controls.
The repetitive nature of the glacial cycle is demonstrated by independent records from (a) glacigenic, ice-melt deposits, i.e., tills and fluvioglacial terrace accumulations; (b) eolian by-products, i.e., loess; (c) marine terraces and beach deposits, eustasy inversely reflecting the ice volumes; and (d) deep-sea deposits, biologically and lithologically reflecting the climatic events. Only the marine section offers a continuous, uninterrupted record. Different chronometric techniques confirm a major cycle of the order of 105 yr (eight repetitions in the last 700,000 yr), but there are multiple types of modulation, in part related to self-accelerating feedback, either negative (retardation) or positive, leading to acceleration.
Within any given cycle there is an Interglacial, Anaglacial, Pleniglacial and Kataglacial phase, characteristics of which are repeated on a small scale in minor cycles. Their timing is variable in a latitudinal sense, apparently steered by radiation changes that first affect the tropics.
Interglacials are defined in their classical stratotype areas of NW Europe, by sedimentary sequences characterized by the pollen of deciduous forests, pointing to climates at least as warm as those of the present time. The present cycle began ca. 10,000 YBP, with the start of the Holocene epoch, and the contemporary warm phase is seen as a typical interglacial stage. Such warm peaks characteristically last about 10,000 yr. Symptoms of the expected ensuing glaciation range from a global fall in temperature since mid Holocene, to tropical desiccation (growth of deserts) and high latitude retreat of tree lines.