This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Environmental Science. Please check back later for the full article.
The Quaternary period of Earth history, which commenced ca. 2.6 Ma ago, is noted for a series of dramatic shifts in global climate between long, cool (“icehouse”) and short, temperate (“greenhouse”) stages. This also coincides with the extinction of later Australopithecine hominins and evolution of modern Homo sapiens.
Wide recognition of a fourth, Quaternary, order of geologic time emerged in Europe between ca. 1760–1830 and became closely identified with the concept of an ice age. This most recent episode in Earth history is also the best preserved in stratigraphic and landscape records. Indeed, much of its character and processes continue in present time, which prompted early geologists’ recognition of the concept of uniformitarianism—the present is the key to the past.
Quaternary time was quickly divided into a dominant Pleistocene (“most recent”) epoch, characterized by cyclical growth and decay of major continental ice sheets and peripheral permafrost. Disappearance of most of these ice sheets, except in Antarctica and Greenland today, ushered in the Holocene (“wholly modern”) epoch, once thought to terminate the Ice Age but now seen as the current interglacial or temperate stage, commencing ca. 11.7 ka ago. Covering 30–50% of Earth’s land surface at their maxima, ice sheets and permafrost squeezed remaining biomes into a narrower circum-equatorial zone, where research indicated the former occurrence of pluvial and desiccation events. Early efforts to correlate them with mid-high latitude glacials and interglacials revealed the complex and often asynchronous Pleistocene record.
Nineteenth-century recognition of just four glaciations reflected a reliance on geomorphology and short terrestrial stratigraphic records, concentrated in northern hemisphere mid- and high-latitudes, until the 1970s. Correlation of δ16-18 O isotope signals from seafloor sediments (from ocean drilling programs after the 1960s) with polar ice core signals from the 1980s onward has revolutionized our understanding of the Quaternary, facilitating a sophisticated, time-constrained record of events and environmental reconstructions from regional to global scales. Records from oceans and ice sheets, some spanning 105–106 years, are augmented by similar long records from loess, lake sediments, and speleothems (cave sediments). Their collective value is enhanced by innovative analytical and dating tools.
Over 100 Marine Isotope Stages (MIS) are now recognized in the Quaternary, with dramatic climate shifts at decadal and centennial timescales—with the magnitude of 22 MIS in the past 900,000 years considered to reflect significant ice sheet accumulation and decay. Each cycle between temperate and cool conditions (odd- and even-numbered MIS respectively) is time-asymmetric, with progressive cooling over 80,000 to 100,000 years, followed by an abrupt termination then rapid return to temperate conditions for a few thousand years.
The search for causes of Quaternary climate and environmental change embraces all strands of Earth System Science. Strong correlation between orbital forcing and major climate changes (summarized as the Milankovitch mechanism) is displacing earlier emphasis on radiative (direct solar) forcing, but uncertainty remains over how the orbital signal is amplified or modulated. Tectonic forcing (ocean-continent distributions, tectonic uplift, and volcanic outgassing), atmosphere-biogeochemical and greenhouse gas exchange, ocean-land surface albedo and deep- and surface-ocean circulation are all contenders and important agents in their own right.
Modern understanding of Quaternary environments and processes feeds an exponential growth of multidisciplinary research, numerical modeling, and applications. Climate modeling exploits mutual benefits to science and society of “hindcasting,” using paleoclimate data to aid understanding of the past and increasing confidence in modeling forecasts. Pursuit of more detailed and sophisticated understanding of ocean-atmosphere-cryosphere-biosphere interaction proceeds apace.
The Quaternary is also the stage on which human evolution plays. And the essential distinction between natural climate variability and human forcing is now recognized as designating, in present time, a potential new Anthropocene epoch. Quaternary past and present are major keys to its future.