in the north polar area during the Pleistocene Period The modern
continents were essentially at their present positions during the Pleistocene, the
plates upon which they sit probably having moved no more than relative to each other since the beginning of the period. In glacial periods, the sea level would drop by up to lower than today during peak glaciation, exposing large areas of the present
continental shelf as dry land. According to
Mark Lynas (through collected data), the Pleistocene's overall climate could be characterised as a continuous
El Niño with
trade winds in the south
Pacific weakening or heading east, warm air rising near
Peru, warm water spreading from the west Pacific and the
Indian Ocean to the east Pacific, and other El Niño markers.
Glacial features Pleistocene climate was marked by repeated glacial cycles in which
continental glaciers pushed to the 40th
parallel in some places. It is estimated that, at maximum glacial extent, 30% of the Earth's surface was covered by ice. In addition, a zone of
permafrost stretched southward from the edge of the glacial sheet, a few hundred kilometres in
North America, and several hundred in
Eurasia. The mean annual temperature at the edge of the ice was ; at the edge of the permafrost, . Each glacial advance tied up huge volumes of water in continental ice sheets thick, resulting in temporary sea-level drops of or more over the entire surface of the Earth. During interglacial times, such as at present,
drowned coastlines were common, mitigated by isostatic or other emergent motion of some regions. The effects of glaciation were global.
Antarctica was ice-bound throughout the Pleistocene as well as the preceding Pliocene. The
Andes were covered in the south by the
Patagonian ice cap. There were glaciers in
New Zealand and
Tasmania. The current decaying glaciers of
Mount Kenya,
Mount Kilimanjaro, and the
Ruwenzori Range in east and central Africa were larger. Glaciers existed in the mountains of
Ethiopia and to the west in the
Atlas Mountains. In the northern hemisphere, many glaciers fused into one. The
Cordilleran Ice Sheet covered the North American northwest; the east was covered by the
Laurentide. The
Fenno-Scandian ice sheet rested on
northern Europe, including much of Great Britain; the
Alpine ice sheet on the
Alps. Scattered domes stretched across
Siberia and the Arctic shelf. The northern seas were ice-covered. South of the ice sheets large lakes accumulated because outlets were blocked and the cooler air slowed evaporation. When the Laurentide Ice Sheet retreated, north-central North America was completely covered by
Lake Agassiz. Over a hundred basins, now dry or nearly so, were overflowing in the North American west.
Lake Bonneville, for example, stood where
Great Salt Lake now does. In Eurasia, large lakes developed as a result of the runoff from the glaciers. Rivers were larger, had a more copious flow, and were
braided. African lakes were fuller, apparently from decreased evaporation. Deserts, on the other hand, were drier and more extensive. Rainfall was lower because of the decreases in oceanic and other evaporation. It has been estimated that during the Pleistocene, the
East Antarctic Ice Sheet thinned by at least 500 metres, and that thinning since the
Last Glacial Maximum is less than 50 metres and probably started after c. 14 ka.
Major events , stored in the bubbles from glacial ice of
Antarctica During the 2.5 million years of the Pleistocene, numerous cold phases called
glacials (
Quaternary ice age), or significant advances of continental ice sheets, in Europe and North America, occurred at intervals of approximately 40,000 to 100,000 years. The long glacial periods were separated by more temperate and shorter
interglacials which lasted about 10,000–15,000 years. The last cold episode of the
last glacial period ended about 10,000 years ago. Over 11 major glacial events have been identified, as well as many minor glacial events. A major glacial event is a general glacial excursion, termed a "glacial". Glacials are separated by "interglacials". During a glacial, the glacier experiences minor advances and retreats. The minor excursion is a "stadial"; times between stadials are "interstadials". These events are defined differently in different regions of the glacial range, which have their own glacial history depending on latitude, terrain and climate. There is a general correspondence between glacials in different regions. Investigators often interchange the names if the glacial geology of a region is in the process of being defined. However, it is generally incorrect to apply the name of a glacial in one region to another. For most of the 20th century, only a few regions had been studied and the names were relatively few. Today the geologists of different nations are taking more of an interest in Pleistocene glaciology. As a consequence, the number of names is expanding rapidly and will continue to expand. Many of the advances and stadials remain unnamed. Also, the terrestrial evidence for some of them has been erased or obscured by larger ones, but evidence remains from the study of cyclical climate changes. The glacials in the following tables show
historical usages, are a simplification of a much more complex cycle of variation in climate and terrain, and are generally no longer used. The headings "Glacial 1" to "Glacial 4" are designations indicating the four most recent glacials, with "Glacial 4" being the most recent. These names have been abandoned in favour of numeric data because many of the correlations were found to be either inexact or incorrect and more than four major glacials have been recognised since the historical terminology was established. Corresponding to the terms glacial and interglacial, the terms pluvial and interpluvial are in use (Latin:
pluvia, rain). A pluvial is a warmer period of increased rainfall; an interpluvial is of decreased rainfall. Formerly a pluvial was thought to correspond to a glacial in regions not iced, and in some cases it does. Rainfall is cyclical also. Pluvials and interpluvials are widespread. There is no systematic correspondence between pluvials to glacials, however. Moreover, regional pluvials do not correspond to each other globally. For example, some have used the term "Riss pluvial" in Egyptian contexts. Any coincidence is an accident of regional factors. Only a few of the names for pluvials in restricted regions have been stratigraphically defined.
Palaeocycles The sum of transient factors acting at the Earth's surface is cyclical: climate, ocean currents and other movements, wind currents, temperature, etc. The waveform response comes from the underlying cyclical motions of the planet, which eventually drag all the transients into harmony with them. The repeated glaciations of the Pleistocene were caused by the same factors. The
Mid-Pleistocene Transition, approximately one million years ago, saw a change from low-amplitude glacial cycles with a dominant periodicity of 41,000 years to asymmetric high-amplitude cycles dominated by a periodicity of 100,000 years. However, a 2020 study concluded that ice age terminations might have been influenced by
obliquity since the Mid-Pleistocene Transition, which caused stronger summers in the
Northern Hemisphere.
Milankovitch cycles Glaciation in the Pleistocene was a series of glacials and interglacials, stadials and interstadials, mirroring periodic climate changes. The main factor at work in climate cycling is now believed to be
Milankovitch cycles. These are periodic variations in regional and planetary solar radiation reaching the Earth caused by several repeating changes in the Earth's motion. The effects of Milankovitch cycles were enhanced by various positive feedbacks related to increases in atmospheric carbon dioxide concentrations and Earth's
albedo. Milankovitch cycles cannot be the sole factor responsible for the variations in climate since they explain neither the long-term cooling trend over the Plio-Pleistocene nor the millennial variations in the Greenland Ice Cores known as
Dansgaard-Oeschger events and
Heinrich events. Milankovitch pacing seems to best explain glaciation events with periodicity of 100,000, 40,000, and 20,000 years. Such a pattern seems to fit the information on climate change found in oxygen isotope cores.
Oxygen isotope ratio cycles In
oxygen isotope ratio analysis, variations in the ratio of to (two
isotopes of
oxygen) by
mass (measured by a
mass spectrometer) present in the
calcite of oceanic
core samples is used as a diagnostic of ancient ocean temperature change and therefore of climate change. Cold oceans are richer in , which is included in the tests of the microorganisms (
foraminifera) contributing the calcite. A more recent version of the sampling process makes use of modern glacial ice cores. Although less rich in than seawater, the snow that fell on the glacier year by year nevertheless contained and in a ratio that depended on the mean annual temperature. Temperature and climate change are cyclical when plotted on a graph of temperature versus time. Temperature coordinates are given in the form of a deviation from today's annual mean temperature, taken as zero. This sort of graph is based on another isotope ratio versus time. Ratios are converted to a percentage difference from the ratio found in standard mean ocean water (SMOW). The graph in either form appears as a
waveform with
overtones. One half of a period is a
Marine isotopic stage (MIS). It indicates a glacial (below zero) or an interglacial (above zero). Overtones are stadials or interstadials. According to this evidence, Earth experienced 102 MIS stages beginning at about 2.588
Ma BP in the Early Pleistocene
Gelasian. Early Pleistocene stages were shallow and frequent. The latest were the most intense and most widely spaced. By convention, stages are numbered from the Holocene, which is MIS1. Glacials receive an even number and interglacials receive an odd number. The first major glacial was MIS2-4 at about 85–11 ka BP. The largest glacials were 2, 6, 12, and 16. The warmest interglacials were 1, 5, 9 and 11. For matching of MIS numbers to named stages, see under the articles for those names. ==Fauna==