The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began around 2.58 million years ago and is ongoing. Although geologists describe this entire period up to the present as an "ice age", in popular culture this term usually refers to the most recent glacial period, or to the Pleistocene epoch in general. Since Earth still has polar ice sheets, geologists consider the Quaternary glaciation to be ongoing, though currently in an interglacial period.
During the Quaternary glaciation, ice sheets appeared, expanding during glacial periods and contracting during interglacial periods. Since the end of the last glacial period, only the Antarctic and Greenland ice sheets have survived, while other sheets formed during glacial periods, such as the Laurentide Ice Sheet, have completely melted.
The major effects of the Quaternary glaciation have been the continental erosion of land and the deposition of material; the modification of river systems; the formation of millions of lakes, including the development of pluvial lakes far from the ice margins; changes in sea level; the isostatic adjustment of the Earth's crust; flooding; and abnormal winds. The ice sheets, by raising the albedo (the ratio of solar radiant energy reflected from Earth back into space), generated significant feedback to further cool the climate. These effects have shaped land and ocean environments and biological communities.
Long before the Quaternary glaciation, land-based ice appeared and then disappeared during at least four other ice ages. The Quaternary glaciation can be considered a part of a Late Cenozoic Ice Age that began around 33.9 million years and is ongoing.
Discovery
Evidence for the Quaternary glaciation was first understood in the 18th and 19th centuries as part of the Scientific Revolution. Over the last century, extensive field observations have provided evidence that continental glaciers covered large parts of Europe, North America, and Siberia. Maps of glacial features were compiled after many years of fieldwork by hundreds of geologists who mapped the location and orientation of drumlins, eskers, moraines, striations, and glacial stream channels to reveal the extent of the ice sheets, the direction of their flow, and the systems of meltwater channels. They also allowed scientists to decipher a history of multiple advances and retreats of the ice. Even before the theory of worldwide glaciation was generally accepted, many observers recognized that more than a single advance and retreat of the ice had occurred.
Description
To geologists, an ice age is defined by the presence of large amounts of land-based ice. Prior to the Quaternary glaciation, land-based ice formed during at least four earlier geologic periods: the late Paleozoic (360–260 Ma), Andean-Saharan (450–420 Ma), Cryogenian (720–635 Ma) and Huronian (2,400–2,100 Ma).
Within the Quaternary ice age, there were also periodic fluctuations of the total volume of land ice, the sea level, and global temperatures. During the colder episodes (referred to as glacial periods or glacials) large ice sheets at least thick at their maximum covered parts of Europe, North America, and Siberia. The shorter warm intervals between glacials, when continental glaciers retreated, are referred to as interglacials. These are evidenced by buried soil profiles, peat beds, and lake and stream deposits separating the unsorted, unstratified deposits of glacial debris.
Initially the glacial/interglacial cycle length was about 41,000 years, but following the Mid-Pleistocene Transition about 1 Ma, it slowed to about 100,000 years, as evidenced most clearly by ice cores for the past 800,000 years and marine sediment cores for the earlier period. Over the past 740,000 years there have been eight glacial cycles.
The entire Quaternary period, starting 2.58 Ma, is referred to as an ice age because at least one permanent large ice sheet—the Antarctic ice sheet—has existed continuously. There is uncertainty over how much of Greenland was covered by ice during each interglacial. Currently, Earth is in an interglacial period, the Holocene epoch beginning 11,700 years ago; this has caused the ice sheets from the Last Glacial Period to slowly melt. The remaining glaciers, now occupying about 10% of the world's land surface, cover Greenland, Antarctica and some mountainous regions. During the glacial periods, the present (i.e., interglacial) hydrologic system was completely interrupted throughout large areas of the world and was considerably modified in others. The volume of ice on land resulted in a sea level about lower than present.
Causes
Earth's history of glaciation is a product of the internal variability of Earth's climate system (e.g., ocean currents, carbon cycle), interacting with external forcing by phenomena outside the climate system (e.g., changes in Earth's orbit, volcanism, and changes in solar output).
Astronomical cycles
The role of Earth's orbital changes in controlling climate was first advanced by James Croll in the late 19th century.Discovery of the Ice Age Later, the Serbian geophysicist Milutin Milanković elaborated on the theory and calculated that these irregularities in Earth's orbit could cause the climatic cycles now known as Milankovitch cycles.EO Library: Milutin Milankovitch They are the result of the additive behavior of several types of cyclical changes in Earth's orbital properties.
Firstly, changes in the orbital eccentricity of Earth occur on a cycle of about 100,000 years. Secondly, the inclination or tilt of Earth's axis varies between 22° and 24.5° in a cycle 41,000 years long. The tilt of Earth's axis is responsible for the seasons; the greater the tilt, the greater the contrast between summer and winter temperatures. Thirdly, precession of the equinoxes, or wobbles in the Earth's rotation axis, have a periodicity of 26,000 years. According to the Milankovitch theory, these factors cause a periodic cooling of Earth, with the coldest part in the cycle occurring about every 40,000 years. The main effect of the Milankovitch cycles is to change the contrast between the seasons, not the annual amount of solar heat Earth receives. The result is less ice melting than accumulating, and glaciers build up.
Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that a sufficiently long and detailed chronology of the Quaternary temperature changes was worked out to test the theory adequately.EO Library: Milutin Milankovitch Page 3 Studies of deep-sea cores and their fossils indicate that the fluctuation of climate during the last few hundred thousand years is remarkably close to that predicted by Milankovitch.
Atmospheric composition One theory holds that decreases in atmospheric , an important greenhouse gas, started the long-term cooling trend that eventually led to the formation of continental ice sheets in the Arctic. Geological evidence indicates a decrease of more than 90% in atmospheric since the middle of the Mesozoic Era. An analysis of reconstructions from alkenone records shows that in the atmosphere declined before and during Antarctic glaciation, and supports a substantial decrease as the primary cause of Antarctic glaciation. Decreasing carbon dioxide levels during the late Pliocene may have contributed substantially to global cooling and the onset of Northern Hemisphere glaciation. This decrease in atmospheric carbon dioxide concentrations may have come about by way of the decreasing ventilation of deep water in the Southern Ocean.
levels also play an important role in the transitions between interglacials and glacials. High contents correspond to warm interglacial periods, and low to glacial periods. However, studies indicate that may not be the primary cause of the interglacial-glacial transitions, but instead acts as a feedback. The explanation for this observed variation "remains a difficult attribution problem".
Plate tectonics and ocean currents
An important component in the development of long-term ice ages is the positions of the continents.Glaciers and Glaciation These can control the circulation of the oceans and the atmosphere, affecting how ocean currents carry heat to high latitudes. Throughout most of geologic time, the North Pole appears to have been in a broad, open ocean that allowed major ocean currents to move unabated. Equatorial waters flowed into the polar regions, warming them. This produced mild, uniform climates that persisted throughout most of geologic time.
But during the Cenozoic Era, the large North American and South American continental plates drifted westward from the Eurasian Plate. This interlocked with the development of the Atlantic Ocean, running north–south, with the North Pole in the small, nearly landlocked basin of the Arctic Ocean. The Drake Passage opened 33.9 million years ago (the Eocene-Oligocene transition), severing Antarctica from South America. The Antarctic Circumpolar Current could then flow through it, isolating Antarctica from warm waters and triggering the formation of its huge ice sheets. The weakening of the North Atlantic Current (NAC) around 3.65 to 3.5 million years ago resulted in cooling and freshening of the Arctic Ocean, nurturing the development of Arctic sea ice and preconditioning the formation of continental glaciers later in the Pliocene. A dinoflagellate cyst turnover in the eastern North Atlantic approximately ~2.60 Ma, during MIS 104, has been cited as evidence that the NAC shifted significantly to the south at this time, causing an abrupt cooling of the North Sea and northwestern Europe by reducing heat transport to high latitude waters of the North Atlantic. The Isthmus of Panama developed at a convergent plate margin about 2.6 million years ago and further separated oceanic circulation, closing the last strait, outside the polar regions, that had connected the Pacific and Atlantic Oceans.EO Newsroom: New Images – Panama: Isthmus that Changed the World This increased poleward salt and heat transport, strengthening the North Atlantic thermohaline circulation, which supplied enough moisture to Arctic latitudes to initiate the Northern Hemisphere glaciation. The change in the biogeography of the nannofossil Coccolithus pelagicus around 2.74 Ma is believed to reflect this onset of glaciation. However, model simulations suggest reduced ice volume due to increased ablation at the edge of the ice sheet under warmer conditions.
Collapse of permanent El Niño A permanent El Niño state existed in the early-mid-Pliocene. Warmer temperature in the eastern equatorial Pacific caused an increased water vapor greenhouse effect and reduced the area covered by highly reflective stratus clouds, thus decreasing the albedo of the planet. Propagation of the El Niño effect through planetary waves may have warmed the polar region and delayed the onset of glaciation in the Northern Hemisphere. Therefore, the appearance of cold surface water in the east equatorial Pacific around 3 million years ago may have contributed to global cooling and modified the global climate's response to Milankovitch cycles.
Rise of mountains