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Ice age

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Variations in temperature, CO2, and dust from the Vostok ice core over
   the last 400,000 years
   Enlarge
   Variations in temperature, CO[2], and dust from the Vostok ice core
   over the last 400,000 years

   An ice age is a period of long-term downturn in the temperature of
   Earth's climate, resulting in an expansion of the continental ice
   sheets, polar ice sheets and mountain glaciers (" glaciation").
   Glaciologically, ice age is often used to mean a period of ice sheets
   in the northern and southern hemispheres; by this definition we are
   still in an ice age (because the Greenland and Antarctic ice sheets
   still exist). More colloquially, when speaking of the last few million
   years, ice age is used to refer to colder periods with extensive ice
   sheets over the North American and Eurasian continents: in this sense,
   the last ice age ended about 10,000 years ago. This article will use
   the term ice age in the former, glaciological, sense; and use the term
   glacial periods for colder periods during ice ages and interglacial for
   the warmer periods.

   Many glacial periods have occurred during the last few million years,
   initially at 40,000-year frequency but more recently at 100,000-year
   frequencies. These are the best studied. There have been four major ice
   ages in the further past.

Origin of ice age theory

   The idea that, in the past, glaciers had been far more extensive was
   folk knowledge in some alpine regions of Europe: Imbrie and Imbrie
   quote a woodcutter telling Jean de Charpentier of the former extent of
   the Swiss Grimsel glacier. No single person invented the idea. Between
   1825 and 1833, Charpentier assembled evidence in support of the
   concept. In 1836 Charpentier convinced Louis Agassiz of the theory, and
   Agassiz published it in his book Étude sur les glaciers (Study of
   Glaciers) of 1840. e. g.: North American review. / Volume 145, Issue
   368, July 1887

   At this early stage of knowledge, what was being studied were the
   glacial periods within the past few hundred thousand years, during the
   current ice age. The existence of ancient ice ages was as yet
   unsuspected.

Evidence for ice ages

   There are three main types of evidence for ice ages: geological,
   chemical, and paleontological.

   Geological evidence for ice ages comes in various forms, including rock
   scouring and scratching, glacial moraines, drumlins, valley cutting,
   and the deposition of till or tillites and glacial erratics. Successive
   glaciations tend to distort and erase the geological evidence, making
   it difficult to interpret. It took some time for the current theory to
   be worked out.

   The chemical evidence mainly consists of variations in the ratios of
   isotopes in sedimentary rocks, ocean sediment cores, and for the most
   recent glacial periods, ice cores. This evidence is also difficult to
   interpret since other factors can change isotope ratios. For example a
   major mass extinction increases the proportion of lighter isotopes in
   sediments and ice because biological processes preferentially use
   lighter isotopes and a reduction in biological processes makes larger
   quantities of lighter isotopes available for deposition.

   The paleontological evidence consists of changes in the geographical
   distribution of fossils - during a glacial period cold-adapted
   organisms spread into lower latitudes, and organisms that prefer warmer
   conditions become extinct or are squeezed into lower latitudes. This
   evidence is also difficult to interpret because it requires: (1)
   sequences of sediments which cover a long time-span and wide range of
   latitudes and are easily correlated, (2) ancient organisms which
   survive for several million years without change and whose temperature
   preferences are easily diagnosed, and (3) the finding of the relevant
   fossils, which requires a lot of luck.

   Despite the difficulties, analyses of ice cores and ocean sediment
   cores unambiguously show the record 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.

Major ice ages

   There have been at least four major ice ages in the Earth's past.

   The earliest hypothesized ice age is believed to have occurred around
   2.7 to 2.3 billion (10^9) years ago during the early Proterozoic Age.

   The earliest well-documented ice age, and probably the most severe of
   the last 1 billion years, occurred from 800 to 600 million years ago
   (the Cryogenian period) and it has been suggested that it produced a
   Snowball Earth in which permanent sea ice extended to or very near the
   equator. It has been suggested that the end of this ice age was
   responsible for the subsequent Cambrian Explosion, though this theory
   is recent and controversial.

   A minor ice age occurred from 460 to 430 million years ago, during the
   Late Ordovician Period.

   There were extensive polar ice caps at intervals from 350 to 260
   million years ago, during the Carboniferous and early Permian Periods,
   associated with the Karoo Ice Age.
   Sediment records showing the fluctuating sequences of glacials and
   interglacials during the last several million years.
   Enlarge
   Sediment records showing the fluctuating sequences of glacials and
   interglacials during the last several million years.

   The present ice age began 40 million years ago with the growth of an
   ice sheet in Antarctica, but intensified during the Pleistocene
   (starting around 3 million years ago) with the spread of ice sheets in
   the Northern Hemisphere. Since then, the world has seen cycles of
   glaciation with ice sheets advancing and retreating on 40,000 and
   100,000 year time scales. The last glacial period ended about ten
   thousand years ago.

Interglacials

   Shows the pattern of temperature and ice volume changes associated with
   recent glacials and interglacials
   Enlarge
   Shows the pattern of temperature and ice volume changes associated with
   recent glacials and interglacials

   In between ice ages, there are multi-million year periods of more
   temperate, almost tropical, climate, but also within the ice ages (or
   at least within the last one), temperate and severe periods occur. The
   colder periods are called 'glacial periods', the warmer periods
   'interglacials', such as the Eemian interglacial era.

   The Earth is in an interglacial period now, the last retreat ending
   about 10,000 years ago. There appears to be a conventional wisdom that
   "the typical interglacial period lasts ~12,000 years" but this is hard
   to substantiate from the evidence of ice core records. For example, an
   article in Nature argues that the current interglacial might be most
   analogous to a previous interglacial that lasted 28,000 years.
   Nonetheless, fear of a new glacial period starting soon does exist
   (See: global cooling). However, many now believe that anthropogenic
   (manmade) forcing from increased " greenhouse gases" would outweigh any
   Milankovitch (orbital) forcing; and some recent considerations of the
   orbital forcing have even argued that in the absence of human
   perturbations the present interglacial could potentially last 50,000
   years.

Causes of ice ages

   The causes of ice ages remain controversial for both the large-scale
   ice age periods and the smaller ebb and flow of glacial/interglacial
   periods within an ice age. The consensus is that several factors are
   important: atmospheric composition (the concentrations of water vapor,
   carbon dioxide, methane, sulfur dioxide, and various other gases and
   particulates in the atmosphere); changes in the Earth's orbit around
   the Sun known as Milankovitch cycles (and possibly the Sun's orbit
   around the galaxy); the motion of tectonic plates resulting in changes
   in the relative location and amount of continental and oceanic crust on
   the Earth's surface; variations in solar output; the orbital dynamics
   of the Earth-Moon system; and the impact of relatively large
   meteorites, and eruptions of supervolcanoes.

   Some of these factors are causally related to each other. For example,
   changes in Earth's atmospheric composition (especially the
   concentrations of greenhouse gases) may alter the climate, while
   climate change itself can change the atmospheric composition (for
   example by changing the rate at which weathering removes CO[2]).

   Discussions of causes are complicated by the tendency for scientists to
   emphasize their own disciplinary specializations; e.g., climatologists
   may emphasize changes in the Earth's atmosphere and geologists may
   emphasize the positions of the continents.

Changes in Earth's atmosphere

   The most relevant change is in the quantity of greenhouse gases in the
   atmosphere. 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 vulcanism.

   The "Snowball Earth" hypothesis maintains that the severe freezing in
   the late Proterozoic was ended by an increase in CO[2] levels in the
   atmosphere and some supporters of "Snowball Earth" argue that it was
   caused by a reduction in atmospheric CO[2].

Position of the continents

   The geological record appears to show that ice ages start when the
   continents are in positions which block or reduce the flow of warm
   water from the equator to the poles and thus allow ice sheets to form.
   The ice sheets increase the Earth's reflectivity and thus reduce the
   absorption of solar radiation. With less radiation absorbed the
   atmosphere cools; the cooling allows the ice sheets to grow, which
   further reduces reflectivity in a positive feedback loop. The ice age
   contunes until the reduction in weathering causes an increase in the
   greenhouse effect.

   There are three known configurations of the continents which block or
   reduce the flow of warm water from the equator to the poles:
     * A continent sits on top of a pole, as Antartica does today.
     * A polar sea is almost land-locked, as the Arctic Ocean is today.
     * A supercontinent covers most of the equator, as Rodinia did during
       the Cryogenian period.

   Since today's Earth has a continent over the South Pole and an almost
   land-locked ocean over the North Pole, geologists believe that Earth is
   likely to experience further glacial periods in the geologically near
   future. Estimates of the timing vary widely, from 2,000 to 50,000 years
   depending on other factors.

   Some scientists believe that the Himalayas are a major factor in the
   current ice age, because these mountains have increased Earth's total
   rainfall and therefore the rate at which CO[2] is washed out of the
   atmosphere, decreasing the greenhouse effect. The Himalayas' formation
   started about 70 million years ago when the Indo-Australian Plate
   collided with the Eurasian Plate, and the Himalayas are still rising by
   about 5mm per year because the Indo-Australian plate is still moving at
   67 mm/year. The history of the Himalayas broadly fits the long-term
   decrease in Earth's average temperature since the Paleocene-Eocene
   Thermal Maximum.

Variations in Earth's orbit (Milankovitch cycles)

   The Milankovitch cycles are a set of cyclic variations in
   characteristics of the Earth's orbit around the sun. Each cycle has a
   different length, so at some times their effects reinforce each other
   and at other times they (partially) cancel each other.

   It is very unlikely that the Milankovitch cycles can start or end an
   ice age (series of glacial periods):
     * Even when their effects reinforce each other they are not strong
       enough.
     * The "peaks" (effects reinforce each other) and "troughs" (effects
       cancel each other) are much more regular and much more frequent
       than the observed ice ages.

   In contrast, there is strong evidence that the Milankovitch cycles
   affect the occurrence of glacial and inter-glacial periods within an
   ice age. The present ice ages are the most studied and best understood,
   particularly the last 400,000 years, since this is the period covered
   by ice cores that record atmospheric composition and proxies for
   temperature and ice volume. Within this period, the match of
   glacial/interglacial frequencies to the Milanković orbital forcing
   periods is so close that orbital forcing is generally accepted. The
   combined effects of the changing distance to the Sun, the precession of
   the Earth's axis, and the changing tilt of the Earth's axis
   redistribute the sunlight received by the Earth. Of particular
   importance are changes in the tilt of the Earth's axis, which affect
   the intensity of seasons. For example, the amount of solar influx in
   July at 65 degrees north latitude varies by as much as 25% (from 400
   W/m^2 to 500 W/m^2, see graph at ). It is widely believed that ice
   sheets advance when summers become too cool to melt all of the
   accumulated snowfall from the previous winter. Some workers believe
   that the strength of the orbital forcing is too small to trigger
   glaciations, but feedback mechanisms like CO[2] may explain this
   mismatch.

   While Milankovitch forcing predicts that cyclic changes in the Earth's
   orbital parameters can be expressed in the glaciation record,
   additional explanations are necessary to explain which cycles are
   observed to be most important in the timing of glacial/interglacial
   periods. In particular, during the last 800,000 years, the dominant
   period of glacial-interglacial oscillation has been 100,000 years,
   which corresponds to changes in Earth's eccentricity and orbital
   inclination. Yet this is by far the weakest of the three frequencies
   predicted by Milankovitch. During the period 3.0 - 0.8 million years
   ago, the dominant pattern of glaciation corresponded to the 41,000 year
   period of changes in Earth's obliquity (tilt of the axis). The reasons
   for dominance of one frequency versus another are poorly understood and
   an active area of current research, but the answer probably relates to
   some form of resonance in the Earth's climate system.

   The "traditional" Milankovitch explanation struggles to explain the
   dominance of the 100,000-year cycle over the last 8 cycles. Richard A.
   Muller and Gordon J. MacDonald and others have pointed out that those
   calculations are for a two-dimensional orbit of Earth but the
   three-dimensional orbit also has a 100 thousand year cycle of orbital
   inclination. They proposed that these variations in orbital inclination
   lead to variations in insolation, as the earth moves in and out of
   known dust bands in the solar system. Although this is a different
   mechanism to the traditional view, the "predicted" periods over the
   last 400,000 years are nearly the same. The Muller and MacDonald
   theory, in turn, has been challenged by Rial .

   Another worker, Ruddiman, has suggested a plausible model that explains
   the 100,000-year cycle by the modulating effect of eccentricity (weak
   100,000 year cycle) on precession (23,000 year cycle) combined with
   greenhouse gas feedbacks in the 41,000 and 23,000-year cycles. Yet
   another theory has been advanced by Peter Huybers who argued that the
   41,000-year cycle has always been dominant, but that the Earth has
   entered a mode of climate behaviour where only the 2nd or 3rd cycle
   triggers an ice age. This would imply that the 100,000-year periodicity
   is really an illusion created by averaging together cycles lasting
   80,000 and 120,000 years. This theory is consistent with the existing
   uncertainties in dating, but not widely accepted at present (Nature
   434, 2005, ).

Variations in the sun's energy output

   There at least 2 types of variation in the sun's energy output:
     * In the very long term, astrophysicists believe that the sun's
       output increases by about 10% per billion (10^9) years. In about 1
       billion years the additional 10% will be enough to cause a runaway
       greenhouse effect on Earth - rising temperatures produce more water
       vapour, water vapour is a greenhouse gas (much weaker than CO[2],
       but there will eventually be vastly more water vapour), the
       temperature rises, more water vapour is produced, etc.
     * Shorter-term variations, some possibly caused by "hunting". Since
       the sun is huge, the effects of imbalances and negative feedback
       processes take a long time to propagate through it, so these
       processes overshoot and cause further imbalances, etc. - "long
       time" in this context means thousands to millions of years.

   The long-term increase in the Sun's output cannot be a cause of ice
   ages.

   The best known shorter-term variations are sunspot cycles, especially
   the Maunder minimum, which is associated with the coldest part of the
   Little Ice Age. Like the Milankovitch cycles, sunspot cycles effects'
   are too weak and too frequent to explain the start and end of ice ages
   but very probably help to explain temperature variations within them.

Vulcanism

   The largest known volcanic events, the flood basalt events which
   produced the Siberian Traps and Deccan traps and are both associated
   with mass extinctions, are not associated with ice ages. At first sight
   this implies that vulcanism cannot have produced ice ages.

   But 70% of Earth's surface is covered by sea, and the theory of plate
   tectonics predicts that all of the Earth's oceanic crust is completely
   replaced about every 200 million years. Hence it is impossible to find
   evidence of submarine flood basalts or other extremely large undersea
   volcanic events more than 200 million years old, and evidence of more
   recent extremely large undersea volcanic events may already have been
   erased. In other words, our failure to find evidence of other extremely
   large volcanic events does not prove that they did not happen.

   It is theoretically possible that undersea volcanos could end an ice
   age by causing global warming. 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.

   It is harder to see how vulcanism could cause an ice age, since its
   cooling effects would have to be stronger than and to outlast its
   warming effects. This would require dust and aerosol clouds which would
   stay in the upper atmosphere blocking the sun for thousands of years,
   which seems very unlikely. And undersea volcanos could not produce this
   effect because the dust and aerosols would be absorbed by the sea
   before they reached the atmosphere.

Recent glacial and interglacial phases

Glaciation in North America

   Northern hemisphere glaciation during the last ice ages. The set up of
   3 to 4 km thick ice sheets caused a sea level lowering of about 120 m.
   Enlarge
   Northern hemisphere glaciation during the last ice ages. The set up of
   3 to 4 km thick ice sheets caused a sea level lowering of about 120 m.

   During the most recent North American glaciation, the Wisconsin
   glaciation (70,000 to 10,000 years ago), ice sheets extended to about
   45 degrees north latitude.

   This Wisconsinian glaciation left widespread impacts on the North
   American landscape. The Great Lakes and the Finger Lakes were carved by
   ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin
   were gouged out by glaciers and later filled with glacial meltwaters.
   The old Teays River drainage system was radically altered and largely
   reshaped into the Ohio River drainage system. Other rivers were dammed
   and diverted to new channels, such as the Niagara, which formed a
   dramatic waterfall and gorge, when the waterflow encountered a
   limestone escarpment. Another similar waterfall near Syracuse, New York
   is now dry.

   Long Island was formed from glacial till, and the watersheds of Canada
   were so severely disrupted that they are still sorting themselves out —
   the plethora of lakes on the Canadian Shield in northern Canada can be
   almost entirely attributed to the action of the ice. As the ice
   retreated and the rock dust dried, winds carried the material hundreds
   of miles, forming beds of loess many dozens of feet thick in the
   Missouri Valley. Isostatic rebound continues to reshape the Great Lakes
   and other areas formerly under the weight of the ice sheets.

   The Driftless Zone, a portion of western and southwestern Wisconsin
   along with parts of adjacent Minnesota, Iowa, and Illinois was not
   covered by glaciers.
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