question: How gradual do ice ages happen?
comment: Well, there were [certain processes which removed carbon dioxide from the atmosphere starting about 50 million years ago]( URL_0 ) in order to get us to the current ice age state. Antarctica started building ice sheets about 30 million years ago and we weren’t in a proper glaciation until 2.6 million years ago. So when transitioning from a ‘hot-house’ Earth (which definitely existed prior to 50 million years ago) to an ‘ice-house’ one, it takes millions of years. If you meant ‘how long do the cycles of glacials-interglacials take since 2.6 million years ago when we entered the current glaciation?’ then that happens on much shorter timescales. Cycling between glacials (maximum ice cover) and interglacials (slightly warmer period within a glaciation when ice cover is at a minimum) is controlled by [minor variations in the Earth’s orbit]( URL_1 ) and has occurred most recently on a roughly 100,000 year cycle. We are currently in the Holocene interglacial and have been for the last 11,700 years.
passages: Section::::Dating.
   William Ruddiman has proposed the early anthropocene hypothesis, according to which the anthropocene era, as some people call the most recent period in the earth's history when the activities of the human species first began to have a significant global impact on the earth's climate and ecosystems, did not begin in the 18th century with the advent of the Industrial Era, but dates back to 8,000 years ago, due to intense farming activities of our early agrarian ancestors. It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of the Milankovitch cycles. In his overdue-glaciation hypothesis Ruddiman states that an incipient glacial would probably have begun several thousand years ago, but the arrival of that scheduled glacial was forestalled by the activities of early farmers.
   Section::::Geophysical and social impact by region.:Africa.
  
  In Ethiopia and North Africa, permanent snow was reported on mountain peaks at levels where it does not occur today. Timbuktu, an important city on the trans-Saharan caravan route, was flooded at least 13 times by the Niger River; there are no records of similar flooding before or since.
   There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.
   Ice ages can be further divided by location and time; for example, the names "Riss" (180,000–130,000 years bp) and "Würm" (70,000–10,000 years bp) refer specifically to glaciation in the Alpine region. The maximum extent of the ice is not maintained for the full interval. The scouring action of each glaciation tends to remove most of the evidence of prior ice sheets almost completely, except in regions where the later sheet does not achieve full coverage.
  
  Section::::Glacials and interglacials.
   Being in an interglacial, there is less ice than there was during the last glacial period. However, the last glacial period was just one part of the ice age that still continues today. Even though Earth is in an interglacial, there is still more ice than times outside of ice ages. There are also currently ice sheets in the Northern Hemisphere, which means that there is more ice on Earth than there was during the first 31 million years of the Late Cenozoic Ice Age. During that time, only the Antarctic ice sheets existed. Currently (as of 2012), about 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice according to the USGS. The current sea level (as of 2009) is 70 m (230 ft) lower than it would be without the ice sheets of Antarctica and Greenland.
   During this last glacial period there were alternating episodes of glacier advance and retreat. Within the last glacial period the Last Glacial Maximum was approximately 22,000 years ago. While the general pattern of global cooling and glacier advance was similar, local differences in the development of glacier advance and retreat make it difficult to compare the details from continent to continent (see picture of ice core data below for differences). Approximately 13,000 years ago, the Late Glacial Maximum began. The end of the Younger Dryas about 11,700 years ago marked the beginning of the Holocene geological epoch, which includes the Holocene glacial retreat.
   Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively. The Earth is currently in such an interglacial period of the Quaternary glaciation, with the last glacial period of the Quaternary having ended approximately 11,700 years ago, the current interglacial being known as the Holocene epoch. 
  
  Based on climate proxies, paleoclimatologists study the different climate states originating from glaciation.
  
  Section::::Known ice ages.
  
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  Section::::Descriptions.
   There have been at least five major ice ages in the Earth's history (the Huronian, Cryogenian, Andean-Saharan, late Paleozoic, and the latest Quaternary Ice Age). Outside these ages, the Earth seems to have been ice free even in high latitudes.
   In the last three million years, glaciations have spread to the northern hemisphere. It commenced with Greenland becoming increasingly covered by an ice sheet in late Pliocene (2.9-2.58 Ma ago) During the Pleistocene Epoch (starting 2.58 Ma ago), the Pleistocene Glaciation developed with decreasing mean temperatures and increasing amplitudes between glacials and interglacials. During the glacial periods of the Pleistocene, large areas of northern North America and northern Eurasia have been covered by ice sheets.
  
  Section::::History of Discovery and Naming.
   German naturalist, Karl Friedrich Schimper coined the term meaning "ice age" in 1837. For a long time, the term only referred to glacial periods. At some point later, this developed into the concept that they were all part of a much longer ice age.
  
  The concept that the earth is currently in an ice age that began around 30 million years ago can be dated back to at least 1966.
  
  As a geologic time period, the Late Cenozoic Ice Age was used at least as early as 1973.
  
  Section::::The Climate before the Polar Ice Caps.
   The Quaternary Glaciation / Quaternary Ice Age started about 2.58 million years ago at the beginning of the Quaternary Period when the spread of ice sheets in the Northern Hemisphere began. Since then, the world has seen cycles of glaciation with ice sheets advancing and retreating on 40,000- and 100,000-year time scales called glacial periods, glacials or glacial advances, and interglacial periods, interglacials or glacial retreats. The earth is currently in an interglacial, and the last glacial period ended about 10,000 years ago. All that remains of the continental ice sheets are the Greenland and Antarctic ice sheets and smaller glaciers such as on Baffin Island.
   Despite the difficulties, analysis of ice core and ocean sediment cores has shown periods of glacials and interglacials over the past few million years. These also confirm the linkage between ice ages and continental crust phenomena such as glacial moraines, drumlins, and glacial erratics. Hence the continental crust phenomena are accepted as good evidence of earlier ice ages when they are found in layers created much earlier than the time range for which ice cores and ocean sediment cores are available.
  
  Section::::Major ice ages.
   Volcanic eruptions may have contributed to the inception and/or the end of ice age periods. At times during the paleoclimate, carbon dioxide levels were two or three times greater than today. Volcanoes and movements in continental plates contributed to high amounts of CO in the atmosphere. Carbon dioxide from volcanoes probably contributed to periods with highest overall temperatures. One suggested explanation of the Paleocene-Eocene Thermal Maximum is that undersea volcanoes released methane from clathrates and thus caused a large and rapid increase in the greenhouse effect. There appears to be no geological evidence for such eruptions at the right time, but this does not prove they did not happen.
   In the Himalaya and the Tibetan Plateau, glaciers advanced considerably, particularly between 45,000 and 25,000 BC, but these datings are controversial. The formation of a contiguous ice sheet on the Tibetan Plateau is controversial.
   Bill Hart and his son, Nelson drive his daughter Julia to the airport. Bill contacts Dr. Divya who informs Bill a team of scientists he works with has left Iceland. Divya is able to tell Bill an entire range of volcanoes has erupted and a 200-mile long ice shelf is breaking apart before he is killed by the volcanic eruptions. While Bill is distracted, Julia heads into the airport.
   Section::::Overdue-glaciation hypothesis.
  
  In his overdue-glaciation hypothesis Ruddiman claims that an incipient ice age would probably have begun several thousand years ago, but the arrival of that scheduled ice age was forestalled by intense farming and deforestation by early farmers that began raising the level of greenhouse gases eight thousand years ago.
   Maureen Raymo, William Ruddiman and others propose that the Tibetan and Colorado Plateaus are immense "scrubbers" with a capacity to remove enough from the global atmosphere to be a significant causal factor of the 40 million year Cenozoic Cooling trend. They further claim that approximately half of their uplift (and "scrubbing" capacity) occurred in the past 10 million years.
  
  Section::::Causes.:Changes in Earth's atmosphere.
   Ice age
  
  An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth is currently in the Quaternary glaciation, known in popular terminology as the Ice Age. Individual pulses of cold climate are termed "glacial periods" (or, alternatively, "glacials", "glaciations", "glacial stages", "stadials", "stades", or colloquially, "ice ages"), and intermittent warm periods are called "interglacials" or "interstadials" with both climatic pulses part of the Quaternary or other periods in Earth's history.
   At least two major periods of glaciation have been discovered:
  
  BULLET::::- The first glacial period was associated with the Mississippian subperiod (359.2–318.1 Mya): ice sheets expanded from a core in southern Africa and South America.
  
  BULLET::::- The second glacial period was associated with the Pennsylvanian subperiod (318.1–299 Mya); ice sheets expanded from a core in Australia and India.
  
  Section::::Late Paleozoic Glaciations.
   The major ice sheets – Antarctica and Greenland – store ice for very long periods. Ice from Antarctica has been reliably dated to 800,000 years before present, though the average residence time is shorter.
   2.58 million years ago was the official start of the Quaternary glaciation, the current phase of the Late Cenozoic Ice Age. Throughout the Pleistocene, there have been glacial periods (cold periods with extended glaciation) and interglacial periods (warm periods with less glaciation).
  
  The term stadial is another word for glacial period, and interstadial is another word for interglacial period.
  
  The oscillation between glacial and interglacial periods is due to the Milankovitch cycles. These are cycles that have to do with Earth's axial tilt and orbital eccentricity.
   In Southern Africa, sediment cores retrieved from Lake Malawi show colder conditions between 1570 and 1820, suggesting the Lake Malawi records "further support, and extend, the global expanse of the Little Ice Age." A novel 3,000-year temperature reconstruction method, based on the rate of stalagmite growth in a cold cave in South Africa, further suggests a cold period from 1500 to 1800 "characterizing the South African Little Ice age." Periglacial features in the eastern Lesotho Highlands might have been reactivated by the Little Ice Age.
  
  Section::::Geophysical and social impact by region.:Antarctica.
   Section::::Causes.:Uplift of the Tibetan plateau and surrounding mountain areas above the snowline.
   Archaeological evidence of settlement and resettlement in certain areas by humans also serve as proxy measurements. Towards the end of OIS 2 in which the Last Glacial Maximum occurs, the deep-sea sediment cores indicate a gradually warming climate, and the reappearance of certain warm-weather flora and fauna remains throughout the Northern Hemisphere correlate with that trend. The Late Glacial was interrupted by a severe decline in temperatures during the Younger Dryas from c.12,890 to c.11,650 BP.
  
  Section::::Western Europe and the North European Plain.
  