Pleistocene Epoch (2.59-0.012 mya), Ice Age
About ten percent of Earth’s land surface is covered by ice, much of which moves downwards from ice peaks. Over the short term the ice melts at its edges at about the same rate as it advances and thus the total ice area remains about the same.
When the amount of snow that falls exceeds the amount that melts, the snow at the bottom compacts into ice. If the accumulating snow ice becomes so heavy that gravity starts to pull the frozen mass downhill it becomes a glacier.
The term ‘ice age’ implies the presence of intensive ice sheets in the northern and southern hemispheres. Within a long-term ice age there are individual pulses of cold climate (41, 100 ky) named glacials, and the warming period between two glacials is called an interglacial. An ice age can therefore be described as a succession of glacials and interglacials. We are currently living in the warm Holocene interglacial period that followed the Wurm glacial period (115-11.7 kya), the latest in the succession of glacial and interglacial periods that make up the Quaternary ice age (2580 kya to present).
During glacials there are colder periods of insufficient duration to be considered as glacials and these are known as stadials, and the relatively warmer period between two stadials is called an interstadial.
In the 1920s Milutin Milankovitch (1879-1958) showed that the variations of the heat from the Sun (the Milankovitch variations) due to positional changes (tilt, inclination, orbit) corresponded to these glacial and interglacial changes. However, many other internal factors, such as fluctuations in the amount of volcanic dust in the atmosphere, and interactions between the oceanic and atmospheric circulations could also be involved
There have been five periods during which a large part of Earth’s surface was glaciated: Early Proterozoic, Late Proterozoic, Late Ordovician to Early Silurian, Carboniferous, and the Quaternary (known as the Ice Age).
The Pleistocene (‘most new’ life) is marked by at least four glacial events, during which ice sheets spread southwards over most of Asia, northern Europe and North America. In the south the Antarctic ice expanded and covered the tips of Africa and South America.
The severe climatic changes during the Pleistocene had major impacts on the fauna and flora. With each advance of ice, plants and animals moved out of its path and large areas of the continents became depopulated. Towards the end of the epoch a major extinction began of the large mammals (megafauna), including mammoths, mastodons and sabre-toothed cats such as Smilodon (‘chisel tooth’)
Holocene Epoch (0.012 mya – present): Current Interglacial
At the end of the nineteenth century it was realised that during the Holocene (‘wholly recent’ life) there had been a series of changes in vegetation. It was thought that the remains of different plants should correspond with species immigration following the last glaciation, and that the sequence of these plant remains reflected past climatic changes. The epoch has five stages: Preboreal (9700-8200 BC), Boreal (Boreas; 8200-6300 BC), Atlantic (6300-3800 BC), Subboreal (3800-600 BC) and Subatlantic (600-00-present).
The start of the Holocene is marked by rapid increase in temperature. During the Preboreal (birch) and Boreal (hazel, pine), the ice sheets and glaciers shrank, and the midlatitudes and tropics received hugely increased rainfall. As lakes and rivers rose, moist grasslands invaded the Sahara and other regions of the world. The Atlantic Stage (oak) was warmer than today in northern lands. Vegetation spread worldwide except up to the highest mountaintops. Forest ousted much of the northern tundra. The Subboreal (oak) brought cooler drier winters to the midlatitudes. Steppe and prairie plants and animals multiplied. The Subatlantic (grasses, beech, pine) began the climatically fluctuating stage that we live in now.
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