Milankovitch cycles

Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term is named for Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession of the Earth's orbit resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.

Similar astronomical hypotheses had been advanced in the 19th century by Joseph Adhemar, James Croll and others, but verification was difficult because there was no reliably dated evidence, and because it was unclear which periods were important.

Now, materials on Earth that have been unchanged for millennia (obtained via ice, rock, and deep ocean cores) are being studied to indicate the history of Earth's climate. Though they are consistent with the Milankovitch hypothesis, there are still several observations that the hypothesis does not explain.

MilankovitchCyclesOrbitandCores
Past and future Milankovitch cycles. VSOP allows prediction of past and future orbital parameters with great accuracy.
∤The graphic shows variations in these five orbital elements:
  Obliquity (axial tilt) (ε).
  Eccentricity (e).
  Longitude of perihelion (sin(ϖ) ).
  Precession index (e sin(ϖ) ), which together with obliquity, controls the seasonal cycle of insolation.[1]
  Calculated daily-averaged insolation at the top of the atmosphere,
( ), on the day of the summer solstice at 65° N latitude.
∤ Data from cores of ocean sediment and Antarctic ice are two distinct proxies for global sea levels and temperatures of the past:
  Benthic forams
  Vostok ice core
∤The vertical gray line shows current conditions (at year 2000 A.D.)

Earth's movements

The Earth's rotation around its axis, and revolution around the Sun, evolve over time due to gravitational interactions with other bodies in the solar system. The variations are complex, but a few cycles are dominant.[2]

Eccentricity zero
Circular orbit, no eccentricity
Eccentricity half
Orbit with 0.5 eccentricity, exaggerated for illustration; Earth's orbit is only slightly eccentric

The Earth's orbit varies between nearly circular and mildly elliptical (its eccentricity varies). When the orbit is more elongated, there is more variation in the distance between the Earth and the Sun, and in the amount of solar radiation, at different times in the year. In addition, the rotational tilt of the Earth (its obliquity) changes slightly. A greater tilt makes the seasons more extreme. Finally, the direction in the fixed stars pointed to by the Earth's axis changes (axial precession), while the Earth's elliptical orbit around the Sun rotates (apsidal precession). The combined effect is that proximity to the Sun occurs during different astronomical seasons.

Milankovitch studied changes in these movements of the Earth, which alter the amount and location of solar radiation reaching the Earth. This is known as solar forcing (an example of radiative forcing). Milankovitch emphasized the changes experienced at 65° north due to the great amount of land at that latitude. Land masses change temperature more quickly than oceans, because of the mixing of surface and deep water and the fact that soil has a lower volumetric heat capacity than water.

Orbital shape (eccentricity)

The Earth's orbit approximates an ellipse. Eccentricity measures the departure of this ellipse from circularity. The shape of the Earth's orbit varies between nearly circular (with the lowest eccentricity of 0.000055) and mildly elliptical (highest eccentricity of 0.0679)[3] Its geometric or logarithmic mean is 0.0019. The major component of these variations occurs with a period of 413,000 years (eccentricity variation of ±0.012). Other components have 95,000-year and 125,000-year cycles (with a beat period of 400,000 years). They loosely combine into a 100,000-year cycle (variation of −0.03 to +0.02). The present eccentricity is 0.017 and decreasing.

Eccentricity varies primarily due to the gravitational pull of Jupiter and Saturn. However, the semi-major axis of the orbital ellipse remains unchanged; according to perturbation theory, which computes the evolution of the orbit, the semi-major axis is invariant. The orbital period (the length of a sidereal year) is also invariant, because according to Kepler's third law, it is determined by the semi-major axis.

Effect on temperature

The semi-major axis is a constant. Therefore, when Earth's orbit becomes more eccentric, the semi-minor axis shortens. This increases the magnitude of seasonal changes.[4]

Season durations[5]
Year Northern
Hemisphere
Southern
Hemisphere
Date: GMT Season
duration
2005 Winter solstice Summer solstice 21 December 2005 18:35 88.99 days
2006 Spring equinox Autumn equinox 20 March 2006 18:26 92.75 days
2006 Summer solstice Winter solstice 21 June 2006 12:26 93.65 days
2006 Autumn equinox Spring equinox 23 September 2006 4:03 89.85 days
2006 Winter solstice Summer solstice 22 December 2006 0:22 88.99 days
2007 Spring equinox Autumn equinox 21 March 2007 0:07 92.75 days
2007 Summer solstice Winter solstice 21 June 2007 18:06 93.66 days
2007 Autumn equinox Spring equinox 23 September 2007 9:51 89.85 days
2007 Winter solstice Summer solstice 22 December 2007 06:08  

The relative increase in solar irradiation at closest approach to the Sun (perihelion) compared to the irradiation at the furthest distance (aphelion) is slightly larger than four times the eccentricity. For Earth's current orbital eccentricity, incoming solar radiation varies by about 6.8%, while the distance from the Sun currently varies by only 3.4% (5.1 million km). Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is at its most eccentric, the amount of solar radiation at perihelion will be about 23% more than at aphelion. However, the Earth's eccentricity is always so small that the variation in solar irradiation is a minor factor in seasonal climate variation, compared to axial tilt and even compared to the relative ease of heating the larger land masses of the northern hemisphere.

Effect on lengths of seasons

The seasons are quadrants of the Earth's orbit, marked by the two solstices and the two equinoxes. Kepler's second law states that a body in orbit traces equal areas over equal times; its orbital velocity is highest around perihelion and lowest around aphelion. The Earth spends less time near perihelion and more time near aphelion. This means that the lengths of the seasons vary.

Perihelion currently occurs around January 3, so the Earth's greater velocity shortens winter and autumn in the northern hemisphere. Summer in the northern hemisphere is 4.66 days longer than winter, and spring is 2.9 days longer than autumn.

Greater eccentricity increases the variation in the Earth's orbital velocity. However, currently, the Earth's orbit is becoming less eccentric (more nearly circular). This will make the seasons more similar in length.

Earth obliquity range
22.1–24.5° range of Earth's obliquity

Axial tilt (obliquity)

The angle of the Earth's axial tilt with respect to the orbital plane (the obliquity of the ecliptic) varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.44°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE.

Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.

The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend. Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the onset of an ice age for two reasons: There is less overall summer insolation, and also less insolation at higher latitudes, which melts less of the previous winter's snow and ice.

Axial precession

Earth precession
Precessional movement

Axial precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of 25,771.5 years. This motion means that eventually Polaris will no longer be the north pole star. It is caused by the tidal forces exerted by the Sun and the Moon on the solid Earth; both contribute roughly equally to this effect.

Precessing Kepler orbit 280frames e0.6 smaller
Planets orbiting the Sun follow elliptical (oval) orbits that rotate gradually over time (apsidal precession). The eccentricity of this ellipse, as well as the rate of precession, is exaggerated for visualization.

Currently, perihelion occurs during the southern hemisphere's summer. This means that solar radiation due to (1) axial tilt inclining the southern hemisphere toward the Sun and (2) the Earth's proximity to the Sun, both reach maximum during the summer and both reach minimum during the winter. Their effects on heating are additive, which means that seasonal variation in irradiation of the southern hemisphere is more extreme. In the northern hemisphere, these two factors reach maximum at opposite times of the year: The north is tilted toward the Sun when the Earth is furthest from the Sun. The two effects work in opposite directions, resulting in less extreme variations in insolation.

In about 13,000 years, the north pole will be tilted toward the Sun when the Earth is at perihelion. Axial tilt and orbital eccentricity will both contribute their maximum increase in solar radiation during the northern hemisphere's summer. Axial precession will promote more extreme variation in irradiation of the northern hemisphere and less extreme variation in the south.

When the Earth's axis is aligned such that aphelion and perihelion occur near the equinoxes, axial tilt will not be aligned with or against eccentricity.

Apsidal precession

In addition, the orbital ellipse itself precesses in space, in an irregular fashion, completing a full cycle every 112,000 years relative to the fixed stars.[6] Apsidal precession occurs in the plane of the ecliptic and alters the orientation of the Earth's orbit relative to the ecliptic. This happens primarily as a result of interactions with Jupiter and Saturn. Smaller contributions are also made by the sun's oblateness and by the effects of general relativity that are well known for Mercury.

Apsidal precession combines with the 25,771.5-year cycle of axial precession (see above) to vary the position in the year that the Earth reaches perihelion. Apsidal precession shortens this period to 23,000 years on average (varying between 20,800 and 29,000 years).[6]

Precession and seasons
Effects of precession on the seasons (using the Northern Hemisphere terms).

As the orientation of Earth's orbit changes, each season will gradually start earlier in the year. Precession means the Earth's nonuniform motion (see above) will affect different seasons. Winter, for instance, will be in a different section of the orbit. When the Earth's apsides are aligned with the equinoxes, the length of spring and summer combined will equal that of autumn and winter. When they are aligned with the solstices, the difference in the length of these seasons will be greatest.

Orbital inclination

The inclination of Earth's orbit drifts up and down relative to its present orbit. This three-dimensional movement is known as "precession of the ecliptic" or "planetary precession". Earth's current inclination relative to the invariable plane (the plane that represents the angular momentum of the Solar System, approximately the orbital plane of Jupiter) is 1.57°.

Milankovitch did not study apsidal precession. It was discovered more recently and measured, relative to Earth's orbit, to have a period of about 70,000 years. However, when measured independently of Earth's orbit, but relative to the invariable plane, precession has a period of about 100,000 years. This period is very similar to the 100,000-year eccentricity period. Both periods closely match the 100,000-year pattern of glacial events.[7]

Problems

Cyclic deposits
The nature of sediments can vary in a cyclic fashion, and these cycles can be displayed in the sedimentary record. Here, cycles can be observed in the colouration and resistance of different strata.

Artifacts taken from the Earth have been studied to infer the cycles of past climate. A study of the chronology of Antarctic ice cores using oxygen-nitrogen ratios in air bubbles trapped in the ice, which appear to respond directly to the local insolation, concluded that the climatic response documented in the ice cores was driven by northern hemisphere insolation as proposed by the Milankovitch hypothesis.[8] Analysis of deep-ocean cores, analysis of lake depths,[9][10] and a seminal paper by Hays, Imbrie, and Shackleton[11] provide additional validation through physical artifacts. Climate records contained in a 1,700 ft (520 m) core of rock drilled in Arizona show a pattern synchronized with Earth's eccentricity, and cores drilled in New England match it, going back 215 million years.[12]

These studies fit so well with the orbital periods that they supported Milankovitch's hypothesis that variations in the Earth's orbit influence climate. However, the fit was not perfect, and problems remained reconciling hypothesis with observation.

100,000-year problem

Of all the orbital cycles, Milankovitch believed that obliquity had the greatest effect on climate, and that it did so by varying the summer insolation in northern high latitudes. Therefore, he deduced a 41,000-year period for ice ages.[13][14] However, subsequent research[11][15][16] has shown that ice age cycles of the Quaternary glaciation over the last million years have been at a 100,000-year period, which matches the eccentricity cycle.

Various explanations for this discrepancy have been proposed, including frequency modulation[17] or various feedbacks (from carbon dioxide, cosmic rays, or from ice sheet dynamics). Some models can reproduce the 100,000-year cycles as a result of non-linear interactions between small changes in the Earth's orbit and internal oscillations of the climate system.[18][19]

Jung-Eun Lee of Brown University proposes that precession changes the amount of energy that Earth absorbs, because the southern hemisphere's greater ability to grow sea ice reflects more energy away from Earth. Moreover, Lee says, "Precession only matters when eccentricity is large. That's why we see a stronger 100,000-year pace than a 21,000-year pace."[20][21]

Some have argued that the length of the climate record is insufficient to establish a statistically significant relationship between climate and eccentricity variations.[22]

Transition problem

Five Myr Climate Change
Variations of cycle times, curves determined from ocean sediments

In fact, from 1–3 million years ago, climate cycles did match the 41,000-year cycle in obliquity. After 1 million years ago, the Mid-Pleistocene Transition (MPT) occurred with switch to the 100,000-year cycle matching eccentricity. The transition problem refers to the need to explain what changed 1 million years ago.[23] The MPT can now be reproduced in numerical simulations that include a decreasing trend in carbon dioxide and glacially induced removal of regolith, as explained in more detail in the article Mid-Pleistocene Transition.[24]

Unsplit peak problem

Even the well-dated climate records of the last million years do not exactly match the shape of the eccentricity curve. Eccentricity has component cycles of 95,000 and 125,000 years. However, some researchers say the records do not show these peaks, but only show a single cycle of 100,000 years.[25]

Stage 5 problem

Deep-sea core samples show that the interglacial interval known as marine isotope stage 5 began 130,000 years ago. This is 10,000 years before the solar forcing that the Milankovitch hypothesis predicts. (This is also known as the causality problem, because the effect precedes the putative cause.)[26]

Effect exceeds cause

Vostok 420ky 4curves insolation
420,000 years of ice core data from Vostok, Antarctica research station, with more recent times on the left

Artifacts show that the variation in Earth's climate is much more extreme than the variation in the intensity of solar radiation calculated as the Earth's orbit evolves. If orbital forcing causes climate change, science needs to explain why the observed effect is amplified compared to the theoretical effect.

Some climate systems exhibit amplification (positive feedback) and damping responses (negative feedback). An example of amplification would be if, with the land masses around 65° north covered in year-round ice, solar energy were reflected away. Amplification would mean that an ice age induces changes that impede orbital forcing from ending the ice age.

The Earth's current orbital inclination is 1.57° (see above). Earth presently moves through the invariable plane around January 9 and July 9. At these times, there is an increase in meteors and noctilucent clouds. If this is because there is a disk of dust and debris in the invariable plane, then when the Earth's orbital inclination is near 0° and it is orbiting through this dust, materials could be accreted into the atmosphere. This process could explain the narrowness of the 100,000-year climate cycle.[27][28]

Present and future conditions

InsolationSummerSolstice65N
Past and future of daily average insolation at top of the atmosphere on the day of the summer solstice, at 65 N latitude. The green curve is with eccentricity e hypothetically set to 0. The red curve uses the actual (predicted) value of e. Blue dot is current conditions, at 2 ky A.D.

Since orbital variations are predictable,[29] any model that relates orbital variations to climate can be run forward to predict future climate, with two caveats: the mechanism by which orbital forcing influences climate is not definitive; and non-orbital effects can be important (for example, Human impact on the environment principally increases in greenhouse gases result in a warmer climate[30][31][32]).

An often-cited 1980 orbital model by Imbrie predicted "the long-term cooling trend that began some 6,000 years ago will continue for the next 23,000 years."[33] More recent work suggests that orbital variations should gradually increase 65° N summer insolation over the next 25,000 years.[34] Earth's orbit will become less eccentric for about the next 100,000 years, so changes in this insolation will be dominated by changes in obliquity, and should not decline enough to permit a new glacial period in the next 50,000 years.[35][36]

Effects beyond Earth

Other bodies in the Solar System undergo orbital fluctuations like the Milankovitch cycles. Any geological effects would not be as pronounced as climate change on the Earth, but might cause the movement of elements in the solid state:

Mars

Mars has no moon large enough to stabilize its obliquity, which has varied from 10 to 70 degrees. This would explain recent observations of its surface compared to evidence of different conditions in its past, such as the extent of its polar caps.[37][38]

Outer planets

Saturn's moon Titan has a cycle of approximately 60,000 years that could change the location of the methane lakes.[39][40] Neptune's moon Triton has a variation similar to Titan's, which could cause its solid nitrogen deposits to migrate over long time scales.[41]

Exoplanets

Scientists using computer models to study extreme axial tilts have concluded that high obliquity would cause climate extremes that would threaten Earth-like life. They noted that high obliquity would not likely sterilize a planet completely, but would make it harder for warm-blooded, land-based life to thrive.[42] Although the obliquity they studied is more extreme than Earth ever experiences, there are scenarios 1.5 to 4.5 billion years from now, as the Moon's stabilizing effect lessens, where obliquity could leave its current range and the poles could eventually point almost directly at the Sun.[43]

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Further reading

External links

Media related to Milankovitch cycles at Wikimedia Commons

Milankovitch cycles at Wikibooks

Circa

Circa (from Latin, meaning 'around, about') – frequently abbreviated c., ca. or ca and less frequently circ. or cca. – signifies "approximately" in several European languages and as a loanword in English, usually in reference to a date. Circa is widely used in historical writing when the dates of events are not accurately known.

When used in date ranges, circa is applied before each approximate date, while dates without circa immediately preceding them are generally assumed to be known with certainty.

Examples:

1732–1799: Both years are known precisely.

c. 1732 – 1799: The beginning year is approximate; the end year is known precisely.

1732 – c. 1799: The beginning year is known precisely ; the end year is approximate.

c. 1732 – c. 1799: Both years are approximate.

Cyclothems

In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. Historically, the term was defined by the European coal geologists who worked in coal basins formed during the Carboniferous and earliest Permian periods. The cyclothems consist of repeated sequences, each typically several meters thick, of sandstone resting upon an erosional surface, passing upwards to pelites (finer-grained than sandstone) and topped by coal.

Depositional sequences have been thoroughly studied by oil geologists using geophysical profiles of continental and marine basins. A general theory of basin-scale deposition has been formalized under the name of sequence stratigraphy.Some cyclothems might have formed as a result of marine regressions and transgressions related to growth and decay of ice sheets, respectively, as the Carboniferous was a time of widespread glaciation in the southern hemisphere. A more general interpretation of sequences invokes Milankovitch cycles.

Early anthropocene

The Early Anthropocene Hypothesis (sometimes called Early Anthropogenic) was proposed by William Ruddiman. It posits that the Anthropocene era, as some scientists call the most recent period in Earth's history when the activities of the human race first began to have a significant global impact on Earth's climate and ecosystems, did not begin in the eighteenth century with advent of coal-burning factories and power plants of the industrial era, as was commonly assumed, but dates back to 8,000 years ago, triggered by intense farming activities after agriculture became widespread. It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of rises and falls that had accurately characterized their past long-term behavior, a pattern that is explained by natural variations in Earth's orbit known as Milankovitch cycles.

F. E. Zeuner

Frederick Everard Zeuner, FZS (8 March 1905 – 5 November 1963) was a German palaeontologist and geological archaeologist who was a contemporary of Gordon Childe at the Institute of Archaeology of the University of London. Zeuner proposed a detailed scheme of correlation and dating of European climatic and prehistoric cultural events on the basis of Milankovitch cycles.Zeuner was born in Berlin, Germany, and received his Ph.D. from the University of Breslau in 1927. After working as a Privatdozent at the University of Breslau from 1927-1930 and a lecturer in geology at the University of Freiburg from 1931-34 he emigrated to England where he worked as a research associate at the British Museum (Natural History) from 1934-36. Zeuner was lecturer in geochronology at the University of London's Institute of Archaeology from 1936 to 1945 and received his D. Sc. from the university in 1942. From 1946 to 1963 he was professor and head of environmental archaeology at the University of London's Institute of Archaeology, where postgraduate students included Andrée Rosenfeld. He was a member of the Geologische Vereinigung in Germany and was admitted to the German Academy of Natural Scientists Leopoldina (1952). He was also a member of the Royal Anthropological Institute of Great Britain and Ireland.

Flandrian interglacial

The Flandrian interglacial or stage is the name given by geologists and archaeologists in the British Isles to the first, and so far only, stage of the Holocene epoch (the present geological period), covering the period from around 12,000 years ago, at the end of the last glacial period to the present day. As such, it is in practice identical in span to the Holocene. Present climatological theory (based on analysis of Milankovitch cycles) forecasts that the present Flandrian climate should decline in temperature towards a global climate similar to that of the ice age. Less orbital eccentricity may have the effect of moderating this temperature downturn.The Flandrian began as the relatively short-lived Younger Dryas climate downturn came to an end. This formed the last gasp of the Devensian glaciation, the final stage of the Pleistocene epoch and is traditionally seen as the latest warm interglacial in a series that has been occurring throughout the Quaternary geological period.

The first part of the Flandrian, known as the Younger Atlantic, was a period of fairly rapid sea level rise, known as the Flandrian transgression and associated with the melting of the Fenno-Scandian, Scottish, Laurentide and Cordilleran glaciers.

Fjords were formed during the Flandrian transgression when U-shaped glaciated valleys were inundated with water.

Holocene climatic optimum

The Holocene Climate Optimum (HCO) was a warm period during roughly the interval 9,000 to 5,000 years BP, with a thermal maximum around 8000 years BP. It has also been known by many other names, such as Altithermal, Climatic Optimum, Holocene Megathermal, Holocene Optimum, Holocene Thermal Maximum, Hypsithermal, and Mid-Holocene Warm Period.

This warm period was followed by a gradual decline until about two millennia ago.

For other temperature fluctuations, see temperature record.

For other past climate fluctuation, see paleoclimatology.

For the pollen zone and Blytt-Sernander period, associated with the climate optimum, see Atlantic (period).

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", or "glacial stages", or colloquially, "ice ages"), and intermittent warm periods are called "interglacials", with both climatic pulses part of the Quaternary or other periods in Earth's history.In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene. The amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, and likely more glacial cycles.

James Hays

James D. Hays is a professor of Earth and environmental sciences at Columbia University's Lamont-Doherty Earth Observatory. Hays founded and led the CLIMAP project, which collected sea floor sediment data to study surface sea temperatures and paleoclimatological conditions 18,000 years ago.Hays is probably best known as a co-author of the 1976 paper in Science, "Variations in the Earth's orbit: Pacemaker of the ice ages." Using ocean sediment cores, the Science paper verified the theories of Milutin Milanković that oscillations in climate can be correlated with Earth's orbital variations of eccentricity, axial tilt, and precession around the Sun (see Milankovitch cycles).

He graduated from Columbia University with a Ph.D. in 1964, Ohio State University with a Master of Science in 1960 and Harvard University with a Bachelor of Arts in 1956. He became a recipient of the Milutin Milankovic Medal in 2010.

John Imbrie

John Imbrie (July 4, 1925 – May 13, 2016) was an American paleoceanographer best known for his work on the theory of ice ages. He was the grandson of William Imbrie, an American missionary to Japan.

After serving with the 10th Mountain Division in Italy during World War II, Imbrie earned his bachelor's degree from Princeton University. He then went on to receive a Ph.D. from Yale University in 1951. He was elected to the National Academy of Sciences in 1978 and was the recipient of a MacArthur Fellowship in 1981. He was awarded the Maurice Ewing Medal in 1986 by the AGU and the William H. Twenhofel Medal by the Society for Sedimentary Geology in 1991, the only time the Society has awarded it to a non-member. Imbrie was on the faculty of the Geological Sciences Department at Brown University from 1967, where he held the Henry L. Doherty chair of Oceanography. He later served as Professor Emeritus at Brown.Imbrie is probably best known as a co-author of the paper in Science in 1976, 'Variations in the Earth's orbit: Pacemaker of the ice ages'. Using ocean sediment cores, the Science paper verified the theories of Milutin Milanković that oscillations in climate over the past few million years are correlated with Earth's orbital variations of eccentricity, axial tilt, and precession around the Sun. These changes are now called the Milankovitch cycles. He became a recipient of the Milutin Milankovic Medal with George Kukla in 2003.John Imbrie was featured in the video documentary The Last Ridge: The Uphill Battles of the 10th Mountain Division.He died in Providence, Rhode Island, in 2016 at the age of 90.

List of cycles

This is a list of recurring cycles. See also Index of wave articles, Time, and Pattern.

Milutin Milanković

Milutin Milanković (Serbian Cyrillic: Милутин Миланковић [milǔtin milǎːnkɔʋitɕ]; 28 May 1879 – 12 December 1958) was a Serbian mathematician, astronomer, climatologist, geophysicist, civil engineer and popularizer of science.

Milanković gave two fundamental contributions to global science. The first contribution is the "Canon of the Earth’s Insolation", which characterizes the climates of all the planets of the Solar system. The second contribution is the explanation of Earth's long-term climate changes caused by changes in the position of the Earth in comparison to the Sun, now known as Milankovitch cycles. This explained the ice ages occurring in the geological past of the Earth, as well as the climate changes on the Earth which can be expected in the future.

He founded planetary climatology by calculating temperatures of the upper layers of the Earth's atmosphere as well as the temperature conditions on planets of the inner Solar system, Mercury, Venus, Mars, and the Moon, as well as the depth of the atmosphere of the outer planets. He demonstrated the interrelatedness of celestial mechanics and the Earth sciences, and enabled consistent transition from celestial mechanics to the Earth sciences and transformation of descriptive sciences into exact ones.

Orbital forcing

Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the orbit (see Milankovitch cycles). These orbital changes change the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes (from 400 to 500 Wm−2 at latitudes of 60 degrees). In this context, the term "forcing" signifies a physical process that affects the Earth's climate.

This mechanism is believed to be responsible for the timing of the ice age cycles. A strict application of the Milankovitch theory does not allow the prediction of a "sudden" ice age (sudden being anything under a century or two), since the fastest orbital period is about 20,000 years. The timing of past glacial periods coincides very well with the predictions of the Milankovitch theory, and these effects can be calculated into the future.

Orbital tuning

Orbital tuning refers to the process of adjusting the time scale of a geologic or climate record so that the observed fluctuations correspond to the Milankovitch cycles in the Earth's orbital motion. Because changes in the Earth's orbit affect the amount and distribution of sunlight the Earth receives, such changes are expected to introduce periodic climate changes on time scales of 20-100 kyr. Long records of sedimentation or climate should record such variations; however, such records often have poorly constrained age scales. As a result, scientists will sometimes adjust the timing of the features in their records to match the predictions of orbital theory in the hopes of improving the dating accuracy. However, "overtuning" can result in apparent features that have no basis in the real data, such as occurred with the original SPECMAP record (Muller & MacDonald 2000).

Pleistocene

The Pleistocene ( , often colloquially referred to as the Ice Age) is the geological epoch which lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciations. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology.

The Pleistocene is the first epoch of the Quaternary Period or sixth epoch of the Cenozoic Era. In the ICS timescale, the Pleistocene is divided into four stages or ages, the Gelasian, Calabrian, Middle Pleistocene (unofficially the 'Chibanian') and Upper Pleistocene (unofficially the 'Tarantian'). In addition to this international subdivision, various regional subdivisions are often used.

Before a change finally confirmed in 2009 by the International Union of Geological Sciences, the time boundary between the Pleistocene and the preceding Pliocene was regarded as being at 1.806 million years Before Present (BP), as opposed to the currently accepted 2.588 million years BP: publications from the preceding years may use either definition of the period.

Quaternary

Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.588 ± 0.005 million years ago to the present. The Quaternary Period is divided into two epochs: the Pleistocene (2.588 million years ago to 11.7 thousand years ago) and the Holocene (11.7 thousand years ago to today). The informal term "Late Quaternary" refers to the past 0.5–1.0 million years.The Quaternary Period is typically defined by the cyclic growth and decay of continental ice sheets associated with Milankovitch cycles and the associated climate and environmental changes that occurred.

Red Crag Formation

The Red Crag Formation is a geological formation in England. It outcrops in south-eastern Suffolk and north-eastern Essex. The name derives from its iron-stained reddish colour and crag which is an East Anglian word for shells. It is part of the Crag Group, a series of notably marine strata which belong to a period when Britain was connected to continental Europe by the Weald–Artois Anticline, and the area in which the Crag Group was deposited was a tidally dominated marine bay. This bay would have been subjected to enlargement and contraction brought about by transgressions and regressions driven by the 40,000-year Milankovitch cycles.

The sediment in the outcrops mainly consists of coarse-grained and shelly sands that were deposited in sand waves (megaripples) that migrated parallel to the shore in a south-westward direction. The most common fossils are bivalves and gastropods that were often worn by the abrasive environment. The most extensive exposure is found at Bawdsey Cliff, which is designated a Site of Special Scientific Interest (SSSI); here a width of around 2 kilometres (1.2 mi) of Crag is exposed. At the coastline by Walton-on-the-Naze, remains of Megalodon were found.The Red Crag Formation at depth in eastern Suffolk clearly has one member, the Sizewell Member, a coarse shelly sand with thin beds of clay and silt. It was interpreted as having been deposited in large scale sand waves where the sea bed was deeper. The overlying Thorpeness Member, was provisionally assigned to the Red Crag based on its lithology but there is more evidence to suggest that it is part of the Norwich Crag Formation.

It has been proposed that the Red Crag started in the late Pliocene and to have possibly extended up into the early Pleistocene, but there is disagreement on more precise dating. According to the British Geological Survey, the Red Crag sits within a segment of time from about 3.3 to 2.5 mya. It is considered that the Red Crag at Walton-on-the–Naze is the oldest and that it was deposited in only a few decades at some time between 2.9 and 2.6 mya. This has led to the UK stratigraphic stage name Waltonian, which is usually correlated with the final Pliocene Reuverian Stage in the Netherlands. There are difficulties in reconciling how the Red Crag equates with international chronological stages. In particular, the start and end dates are poorly defined due to the general paucity of age-diagnostic stratigraphic indicators and the fragmentary nature of the geology. It can also be difficult to separate the Red Crag from the overlying Norwich Crag Formation.

Stadial

Stadials and interstadials are phases dividing the Quaternary period, or the last 2.6 million years. Stadials are periods of colder climate while interstadials are periods of warmer climate.

Each Quaternary climate phase is associated with a Marine Isotope Stage (MIS) number, which describe alternation between warmer and cooler temperatures as measured by oxygen isotope data. Stadials have even MIS numbers and interstadials odd MIS numbers. The current Holocene interstadial is MIS 1 and the Last glacial maximum stadial is MIS 2.

Marine Isotope Stages are sometimes further subdivided into stadials and interstadials by minor climate fluctuations within the overall stadial or interstadial regime, which are indicated by letters. The odd-numbered interstadial MIS 5, also known as the Sangamonian interglacial, contains two periods of relative cooling, and so is subdivided into three interstadials (5a, 5c, 5e) and two stadials (5b, 5d). A stadial isotope stage like MIS 6 would be subdivided by periods of relative warming, and so in that case the first and last subdivisions would be stadials; MIS 6a, 6c and 6e are stadials while 6b and 6d are interstadials.

Supercontinent

In geology, a supercontinent is the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass. However, many earth scientists use a different definition: "a clustering of nearly all continents", which leaves room for interpretation and is easier to apply to Precambrian times.Supercontinents have assembled and dispersed multiple times in the geologic past (see table). According to the modern definitions, a supercontinent does not exist today. The supercontinent Pangaea is the collective name describing all of the continental landmasses when they were most recently near to one another. The positions of continents have been accurately determined back to the early Jurassic, shortly before the breakup of Pangaea (see animated image). The earlier continent Gondwana is not considered a supercontinent under the first definition, since the landmasses of Baltica, Laurentia and Siberia were separate at the time.

Tiglian

The Tiglian, also referred to as the Tegelen, is a temporal stage in the glacial history of Northern Europe. It is preceded by the Pre-Tiglian, Praetiglian or Pre-Tegelen stage. The stage was introduced by Zagwijn in 1957 based on geological formations in Tegelen in southern Netherlands. Originally, it was thought to be part of a sequence of glacials and interglacials, namely Pre-Tiglian (cold), Tiglian (warm), Eburonian (cold), Waalian (warm), Menapian (cold), and Bavelian (warm).The Pre-Tiglian and Tiglian are today regarded as corresponding to the Biber stage in the glacial history of the Alps and to the Gelasian (2.6-1.8 million years ago) in the global division of the Quaternary period. Deep sea core samples have identified approximately 40 marine isotope stages (MIS 103 – MIS 64) during the Gelasian. Thus, there have probably been about 20 glacial cycles of varying intensity during Pre-Tiglian and Tiglian. The dominant trigger is believed to be the 41 000 year Milankovitch cycles of axial tilt.The Gelasian of Northern Europe has subsequently been subdivided as follows:

Pre-Tiglian

Tiglian A

Tiglian B

Tiglian C3

Tiglian C4

Tiglian C5

Key topics
Calendars
Astronomic time
Geologic time
Chronological
dating
Genetic methods
Linguistic methods
Related topics
Climate oscillations

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