Article 35: During the present Ice Age, temperate zones are alternately covered by glaciers, during glacial periods, and uncovered during interglacial periods, when the glaciers retreat, like the period we are living in right now.
It is during the Pleistocene that the most recent episodes of global cooling or ice ages take place.
An age is called an Ice Age when at least one permanent large ice sheet, f.e. Antarctica, exists continuously. No completely satisfactory theory has been proposed to account for Earth’s history of glaciation. The cause of glaciation may be related to several, simultaneously occurring factors, such as astronomical cycles, called Milankovitch cycles, atmospheric composition, plate tectonics and ocean currents.
The three main orbital cycles:
Eccentricity: changes in the shape of the Earth’s orbit (100.000 year)
Obliquity: changes in the tilt of the Earth’s rotational axis (41.000 year)
Precession: wobbles in the Earth’s rotational axis (26.000 year)
The combined effect of these three orbital cycles causes long term changes in the amount of sunlight hitting the earth at different seasons, particularly at high latitudes.
The Pleistocene Ice Age is the last of five Ice Ages during the Earth’s history. The other four are the Huronian, from 2400 – 2100 million years ago, the Cryogenian, from 720 – 635 million years ago, the Andean-Saharan, from 450 – 420 million years ago and the Karoo Ice Age, from 360 – 260 million years ago.
The coldest periods during the Pleistocene Ice Age itself are the Pretiglian, from 2,6 to 2,45 million years ago; the Eburonian, from 1,8 to 1,5 million years ago; the Menapian, from 1,2 to 1,1 million years ago; the Linge and the Dorst glacials during the Bavelian, from 1,07 million to 850.000 years ago; the Elsterian, from 475.000 to 410.000 years ago; the Saalian, from 370.000 to 130.000 years ago and the Weichselian, from 115.000 to 12.000 years ago.
When the Earth is in its “Ice House” climate mode, there is ice at least at one of the poles. The polar ice sheet expands and contracts because of variations in the Earth’s orbit (Milankovitch cycles). The last expansion of the polar ice sheets took place about 18.000 years ago.
During an Ice Age, much of the world’s temperate zones are alternately covered by glaciers during cool or glacial periods, and uncovered during the warmer or interglacial periods, when the glaciers retreat. During interglacial times, such as at present, drowned coastlines are common.
At one point during the Pleistocene Ice Age, sheets of ice covered all of Antarctica, large parts of Europe, North America and South America, and small areas in Asia.
In North America they stretched over Greenland and Canada and parts of the northern United States. The remains of these glaciers can still be seen in parts of the world, including Greenland and Antarctica. 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 kilometers in North America and several hundred in Eurasia.
The mean annual temperature at the edge of the ice was −6 °C (21 °F), at the edge of the permafrost 0 °C (32 °F). Each glacial advance tied up huge volumes of water in continental ice sheets, 1500 to 3000 meters (4900 – 9800 ft) thick, resulting in temporary sea-level drops of 100 meters (300 ft) or more over the entire surface of the Earth.
The current (2016) decaying glaciers of Mount Kenya, Mount Kilimanjaro (next picture) and the Rwenzori Mountains 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-Scandinavian Ice Sheet rested on Northern Europe, including Great Britain, the Alpine Ice Sheet rested on the Alps. Scattered domes stretched across Siberia and the Arctic shelf. The Northern Seas were ice-covered.
Europe during its last glaciation, about 70.000 to 20.000 years BP, in Northern Europe called Weichselian Glaciation. The extent of glaciation, sea and lakes have been painted freehand. Credit to European Geosciences Union, Creative Commons, user Ulam
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.
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 totally 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 does now.
In Eurasia large lakes developed as a result of the runoff from Mount Kilimanjaro. 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 decrease in oceanic and other evaporation.
While Homo sapiens evolved, many vertebrates, especially large mammals, succumbed to the harsh climate conditions of this period. In addition to the woolly mammoth, mammals such as saber-toothed cats (Smilodon), giant ground sloths (Megatherium) and mastodons roamed the Earth during this period. Other mammals that thrived during this period included moonrats, tenrecs (hedgehog-like creatures) and macrauchenia (similar to llamas and camels).
Although many vertebrates became extinct during this period, mammals that are familiar to us today — including apes, cattle, deer, rabbits, kangaroos, wallabies, bears and members of the canine and feline families — could be found during this time. Birds flourished during this period, including members of the duck, geese, hawk and eagle families. There were also some flightless birds such as ostriches, rheas and moas. The flightless birds did not fare as well, as they had to compete with mammals and other creatures for limited supplies of food and water, as a good portion of the water was frozen. Crocodiles, lizards, turtles, pythons and other reptiles also thrived during this period.
As for vegetation, it was fairly limited in many areas. There were some scattered conifers, including pines, cypress and yews, along with some broadleaf trees, such as beeches and oaks. On the ground, there were prairie grasses as well as members of the lilly, orchid and rose families.
Currently, the Earth is in an interglacial period, the Holocene epoch. This period may persist for another 50.000 years, if CO2 levels increase to 750 parts per million. The present atmospheric concentration of CO2 is about 400 ppm by volume, but is rising rapidly as humans continue to burn fossil fuels. If CO2 drops instead to 210 ppm, then the next glacial period may only be 15.000 years away.
Many paleontologists study Pleistocene fossils in order to understand the climates of the past. The Pleistocene is not only a time during which climates and temperatures shift dramatically; Pleistocene fossils are often abundant, well-preserved and can be dated very precisely. Some, such as diatoms, foraminifera and plant pollen, are both abundant and highly informative about paleo-climates.
Today, there is concern about future climate change, the global warming, and how it will affect us. Paleontologists who work on Pleistocene fossils are providing a growing amount of data on the effect of climate change on the Earth’s biota, making it possible to understand the effects of future climate change.
Thanks to the University of California, Berkeley, Museum of Paleontology, to Wikipedia, Pleistocene, to Christopher R. Scotese and to LiveScience