Milanković cycles

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Diagram of the Milanković cycles, as calculated for the last 1 million years, with the cycles of precession  ( precession ) * and the inclination of the earth's axis  ( obliquity ) as well as the eccentricity of the earth's orbit  ( excentricity ). In addition, the calculated fluctuations in the intensity of solar radiation  ( solar forcing ) as well as the change in cold and warm periods in the younger Pleistocene  ( Stages of Glaciation ) determined from geological climate proxies are plotted .
* Here shown in the form of so-called Präzessionsindex that the variations in the eccentricity of the orbit as a factor includes
Animation of the Milanković cycles

The Milankovitch cycles (after the Serbian mathematician Milutin Milankovitch , 1879-1958) is long-period changes of on the Earth incoming solar radiation beyond the annual variation. The earth's orbit around the sun, the precession of the earth 's axis of rotation and the inclination of the earth's axis and thus the changing angles of incidence of solar radiation on the northern and southern hemisphere are subject to different cycles with a duration of 25,800 to around 100,000 or 405,000 years. They partly explain the natural climate changes , especially during the Quaternary, and are therefore of great importance in the context of climatology and paleoclimatology . Milanković's basic idea was that the astronomical variability of solar radiation in the northern hemisphere had a major influence on the growth and melting of large ice sheets and thus - with the help of the ice-albedo feedback - exercised a control function for the beginning and end of the various phases of the cold period. Thus, with the Milanković theory, a generally accepted explanatory model for the cause of the Pleistocene icing processes emerged for the first time .

The Milanković cycles are often used in paleoclimatological research for climatic reconstructions of the Cenozoic era , such as the formation of the Antarctic Ice Sheet and the associated beginning of the Cenozoic Ice Age 33.9 million years ago. Recently, they have also been increasingly used to analyze significant climatic changes in the Middle Ages ( Mesozoic ) and the ancient times ( Paleozoic ). However, her focus is still on research into recent geological history, especially the Quaternary.

history

Front cover of the monograph Canon of Earth Radiation and its Application to the Ice Age Problem (Milanković, 1941)
James Croll (photography)

The theory developed by Milanković was based on preparatory work by James Croll (1821–1890), a British self-taught person who, in the mid-19th century, examined the gravitational influence of other planets in the solar system on the Earth's orbit parameters, including the associated ice age problem, and his conclusions in 1864 in Philosophical Magazine published. However, Croll's ideas were decades ahead of their time. Only the work of Milanković created the prerequisite that the possible causal relationship between the cold-time cycles of the past and Earth orbit was discussed on a broad scientific basis. Milankovitch extended not only Croll calculations but supplemented his theory to an essential component, namely the special constellation when a hemisphere has so little sunlight that even in summer snow melt occurs the snow cover only partially or not at all. On the basis of this knowledge, he subsequently expanded his calculations.

Milanković was actually an engineer specializing in concrete processing who had filed some patents in this regard. After his appointment to Belgrade University in 1909, he turned to mathematical questions from meteorology , spherical astronomy , celestial mechanics and theoretical physics. After moving from Vienna to “provincial” Belgrade, this was made easier for him by now teaching at a university that had a non-specialized holistic tradition. Here he realized that no general explanation of the Ice Age problem had been attempted before him:

“The reason for this is that you had to get to the root of the problem, solve a number of rather complicated component problems in different branches of science that are sharply separated from each other. ... Therefore this question has not yet been answered, it was within the triangle of spherical astronomy, celestial mechanics and theoretical physics. The university chair enabled me to unite these three sciences that separate other universities. Therefore it was possible for me to recognize the cosmic problem and its meaning and to start with its solution. "

- Milutin Milanković, memoir

During his preventive imprisonment in Austria-Hungary during World War I, Milanković devoted himself to astronomical issues first in Osijek and then after his transfer to Budapest . In Budapest he was given access to the libraries of the Hungarian Academy of Sciences and the Hungarian Institute of Meteorology. Here he delved into the topic of the climate and the ice ages. As others before him recognized, the albedo of snow surfaces has a major impact on the reflection of sunlight, which helps to reinforce the ice albedo feedback . Under these conditions, a snow field could grow into a continental ice field over the course of centuries. Encouraged by the German climatologist Wladimir Köppen , Milanković examined the regions rich in glacial deposits between the 55th and 65th degrees north latitude, since this is where the edges of the former continental ice sheets lay.

When Alfred Wegener , Köppen's father-in-law, wanted to give a palaeoclimatological lecture during the Congress of German Naturalists and Physicists in Innsbruck on September 25, 1925 , Milanković also traveled there. The work "Climates of Prehistoric Geology", completed by Köppen and Wegener in 1924, was presented to the public for the first time. Wegener wrote an article introducing Milanković's method and calculation of solar radiation. After Köppen and Wegener gave a chapter to Milanković's theory in their book, he became well known in research circles. Milanković calculated different solar angles for the climate-sensitive subpolar and boreal zones, especially for the summer months. Köppen compared these calculations with the chronology of the past glacial phases and found good agreement.

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Diagram with “Milanković curves” for the past 650,000 years from Wegener and Köppen's book Klimate der Geologische Vorzeit (1924). The upper part of the diagram shows the reconstructed fluctuations in solar radiation hitting the earth's surface at higher northern latitudes. The degrees on the ordinate stand for "latitude equivalents" (ie for a "shift" of the geographic latitude in relation to the present time corresponding to the change in irradiation). The lower part of the diagram shows the fluctuations in the Earth's orbit parameters calculated for this period.

In the 1940s, Milanković's assumptions were accepted by a number of scientists and also published in several climate textbooks as an explanation for the periodicity of the ice ages. The theory was supported by varve analyzes of moors and fossil lakes as well as clay drill cores from recent waters, which confirmed the 21-ky cycle of the earth axis precession. Nevertheless, most geoscientists continued to reject the existence of a cosmic “clock generator” for earthly climate fluctuations. They justified their view with the sequences of the terminal moraines in numerous regions of the world and with the synchronous occurrence of glaciations in the southern and northern hemisphere. In contrast, Milanković postulated a decrease in solar radiation in the northern hemisphere with a simultaneous increase in the southern hemisphere and vice versa. In addition, it was doubted that relatively small changes in insolation would be able to produce such large effects.

Milanković wrote the final synthesis of his theory in Canon of Earth Radiation and its application to the Ice Age problem in German. In his work he described the variable solar radiation and the emergence of the cold ages over a period from the Middle Pleistocene to the present (about one million years). He handed the manuscript to the printer in Belgrade on April 2, 1941. After the German invasion of Yugoslavia on April 6, 1941, the printing plant was destroyed and most of the manuscripts were destroyed except for a completely preserved copy. It was thanks to Milanković's reputation that two Wehrmacht soldiers, who requested access to his house on May 15, 1941 , wanted to send him greetings from their professor of geology, Wolfgang Soergel in Freiburg, a proponent of Milanković's theses. Milanković handed the intact book manuscript to the soldiers for forwarding to Soergel. The reception of the Milanković cycles found its first expression in the German Meteorological Journal in early 1944 with the positive foreword by the geographer Carl Troll . In the September 1944 issue of the Geologische Rundschau, Walter Wundt wrote a detailed treatise on Milanković's theory that was understandable even for non-mathematicians. In the same edition of the Geologische Rundschau, Wilhelm Meinardus emphatically supported Milanković's assumption of varying solar radiation. Meinardus was the first Quaternary geomorphologist and geographer to be committed to Milanković's ideas. Since the group of geographers had been the sharpest critics of his theory to date, it was a certain progress that a broad discussion began about the mechanisms behind the formation of the Ice Ages.

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“Milankovic curves” of summer irradiation in the higher northern latitudes (“polar dome”, north pole up to 45 ° N) for the past 600,000 years. The numbers on the ordinate correspond to canonical units of radiation, a unit created by Milanković that is mathematically closely related to the solar constant.

Nevertheless, Milanković was denied general acceptance of his theory until the end of his life. Some important geologists such as Albrecht Penck and Richard Foster Flint were irritated by Milanković's ideas, with Penck proving to be one of the main opponents and not changing his mind until 1938. Also Jovan Cvijić , Rector of Belgrade University and Penck Viennese students did not understand the background of Milankovićs time-consuming calculations. Flint also stuck to his rejection for decades. He interrupted Milanković as chairman during the 4th INQUA Congress in Rome in 1953, where his lecture was loudly interrupted by heckling. As a result, Milanković returned visibly disappointed from the congress and did not see how his theory found an increasingly broader reception in paleoclimatology from the mid-1970s.

The breakthrough came with the development and application of marine oxygen isotope stratigraphy . James D. Hays , Nicholas Shackleton and John Imbrie were able to show in a fundamental Science article in 1976 (the so-called "Pacemaker" study) that the ratio of the stable oxygen isotopes 16 O and 18 O in seawater depends on the increase or decrease of the great ice sheets and that these fluctuations correspond to the Milanković cycles of the last 500,000 years. This changed the hypothetical character of the orbital theory to a proven statement that changes in the orbit parameters are responsible for the occurrence of the Pleistocene glacial cycles. The orbital theory had thus passed its first geological “proof test”. This also led to a change in the focus of research: While geological findings initially formed the guideline for the representation of climatic periodicities of the geological past, the "Milanković tape" has now developed into the binding standard for classifying cyclical climate changes during the Quaternary and beyond.

The European Geosciences Union (EGU) has been awarding the Milutin-Milanković Medal for work in long-term climatological research since 1993. In addition to Sir Nicholas J. Shackleton (1999), John Imbrie (2003) and James Hays (2010), who had received them for confirming the Milanković cycles (1976), the 2019 award was given to Jacques Laskar , who has been working on the theory since The 1990s expanded and applied to large parts of the Cenozoic era .

Applications

The Milanković cycles developed into an indispensable tool for dating Pleistocene deep-sea sediments or determining the time of sedimentation rates. The role of external cosmic factors on the earthly climatic course also exerted a lasting influence on natural philosophy , since until then geological processes were not understood as a reaction to astronomical forcings . It was only because of Milanković's work that astronomical quantities came into consideration as starting points and initiators for climate change. The fact that the theory was accepted with a considerable delay was largely due to the lack of precise verification procedures. Only with advances in oceanography and isotope analysis did the Milanković theory - in its temporal development roughly comparable to Alfred Wegener's idea of continental drift - find its full confirmation. Even before the Second World War, the dating of the ice ages was based exclusively on terrestrial deposits, as documented in particular by Albrecht Penck and Eduard Brückner (in their three-volume standard work Die Alpen im Eiszeitalter , 1901 to 1909) through the stratigraphic exploration of the Alpine foothills in the form of the glacial series . Only when a reliable method for the chronology of the Quaternary on a global scale became available with the evaluation of deep-sea sediments, the regional alpine dates could be adjusted accordingly.

The outstanding position of the Milanković theory in geology can be found in the standard time scale " SPECMAP " (SPECtral MApping Project), which enables reliable information on the basis of oxygen isotope data sets for the last 650,000 years (including the marine oxygen isotope stage 16 = MIS 16). The Milanković theory was the first conclusive explanation for the existence of cold-age cycles, clearly emphasizing the central importance of the (sub) polar regions of the northern hemisphere for cyclical climate change events.

Impact on ice sheets and glaciers

While the effect of orbital steering on global climate and cold ages is widely recognized, this influence on the dynamics and expansion of ice sheets and glaciers is less clear. Within glacial periods, precession cycles and seasonal patterns had a major impact on global ice volume. Melting ( ablation ) of glaciers was triggered by every fourth or fifth precession cycle and was largely dependent on summer temperatures. This had a direct impact not only on melting, but also on glacier growth at the beginning of a glacial. Many glacial cycles with the largest ice sheets in the northern hemisphere matched the lowest solar irradiation at the beginning of the glacials in the interglacials and agreed for the isotope stages MIS 5d – 2, 6, 8, 12, 14, 16, 20 and 24–22. MIS 18 and 10 were prepared by solar valleys that were surpassed to the cold cycles. The fact that many of the main cold periods were associated with a northern hemisphere solar radiation minimum indicates a cold phase with pronounced glacial growth in the northern hemisphere. Although the orbital control is an essential variable for the start of glacial periods, it was not decisive for the glacier dynamics during a glacial. In contrast, the amplitude fluctuation of peak and valley at the beginning of the northern hemisphere glaciations played a primary role for the duration of the glacial periods. At the beginning of MIS 5d – 2 and MIS 24–22, the absolute peak changed to a valley in solar radiation. The ice growth showed that a feedback process was set in motion between the Euro-Asian inland glaciers and the Atlantic coastal regions, resulting in increasingly arid conditions. Thus, after initial growth, the ice masses receded under the influence of the arid climate that they themselves produced, as is assumed for the glacials MIS 24–22, MIS 5d – 2, MIS 23 and MIS 3.

overview

On earth, the cycles are expressed as long-period changes in the solar constant and the characteristics of the seasons (more extreme or mild) in the higher latitudes of the northern and southern hemispheres. As the celestial mechanical cause of these fluctuations, three overlapping secular changes in the parameters of the earth's orbit and the earth's axis are distinguished:

  • The precession , the periodicity of which varies approximately between 19,000 and 24,000 years, and in which two different cycles overlap:
  • The variation of the ecliptic slope (angle of inclination of the earth's axis) with a cycle of 41,000 years
  • The change in eccentricity (variation of the length of the semiaxes of the earth's orbit) with a simple cycle of around 100,000 years, with an eccentricity maximum occurring approximately every 405,000 years.

As a result of the eccentricity fluctuations, there are slight changes in the amount of energy that the entire earth receives annually from the sun (magnitude of the fluctuation by 0.2%) and as a result of the precession and the change in the axis inclination, there are considerable changes in the amount of energy that the two hemispheres and in particular, their higher geographic latitudes are preserved annually (magnitude of the fluctuation to 65 ° N at the summer solstice by an average of 28%). In interaction with, for example, the distribution of land masses over the earth's surface or the global sea level, which have an impact on the reflectivity (albedo) of the earth's surface and which also vary greatly in geological time periods (see →  continental drift , →  eustasia ), this can lead to considerable fluctuations in the Radiation balance of the earth or at least one of the two hemispheres, with corresponding consequences for the global climate.

From this, the following sequence can be postulated as the basic idea of ​​the hypothesis in the interaction of orbital forces and glacial and interglacial cycles ( André Berger , 1993): a way to prevent a positive amount from remaining in the annual budget of snow and ice, and subsequently a positive feedback in the cooling from the expansion of the snow cover and the increase in the surface albedo follows ”.

The Milanković cycles in detail

Precession

Schematic representation of the earth's axis precession
Precessing Kepler orbit 280frames e0.6 smaller.gif
Schematic animation of the apsid rotation of the earth's orbit (not to scale, eccentricity of the earth's orbit and amount of rotation per revolution shown greatly exaggerated)

The earth's axis is only really fixed in the center of the earth . Outside the center of the earth, it describes a circular movement around the imaginary perpendicular position to the ecliptic plane with a period of 26,000 years (with increasing radius of the circle with increasing distance to the center of the earth). Such a "tumbling motion" is called precession . The cause of the earth's axis precision are the forces of the sun and moon on the equatorial bulge of the rotating earth ellipsoid , the so-called tidal forces . The axis precession means that the changes of the seasons do not always occur in the same orbit points of the earth's orbit ellipse. This also means, among other things, that the earth passes its closest point to the sun ( perihelion ) for a quarter cycle in northern summer and for a quarter cycle, as is currently the case, in northern winter. Accordingly, the summers and winters in the northern hemisphere are more extreme and moderate in these two sections of the cycle.

The cycle of axial precession is superimposed on the cycle of apsidal precession of the earth's orbit, the period of which is 112,000 years. During the apsidal precession of the earth's orbit, also called perihelion, the semiaxes rotate in the orbit plane in the direction of rotation around the sun. This also influences the times of the seasonal changes relative to the movement of the earth on its orbit and thus relative to the point closest to and furthest from the sun.

The superposition of the two precession movements results in the so-called tropical rotation of the apse , the cyclical change in the position of the vernal equinox relative to the perihelion. The tropical turn of the apse corresponds to a Milanković cycle of around 21,000 years on average. So the earth currently passes its perihelion around January 3rd, i.e. in the middle of northern winter, its aphelion (point furthest from the sun) around July 5th. In 11,000 years, the perihelion will be passed through in northern summer, so that the seasons in the northern hemisphere will be more extreme than today.

In addition, “precession” in connection with the Milanković cycles also means the so-called precession index. This is the mathematical product of the apsidal precession and the fluctuations in the eccentricity of the earth's orbit (see below ). The eccentricity cycles can be read from its consequently non-constant amplitude .

Change of axis inclination

Schematic representation of the variability of the inclination of the earth's axis (ecliptic inclination). The white line is the perpendicular to the plane of the earth's orbit.

The inclination of the earth's axis (obliquity, ecliptic inclination ) from the normal to the plane of the earth's orbit changes periodically between 22.1 ° and 24.5 °, with a period of approximately 41,000 years. This effect leads, among other things, to a change in the maximum and minimum angle of incidence of the sun's rays and thus to greater fluctuations in the radiation intensity in higher geographical latitudes over the course of the year. With a greater axis inclination, the winters in the higher latitudes are consequently colder and the summers warmer than with a lower axis inclination. The ecliptic is currently 23.43 ° and is roughly in the middle between the extreme values. It is slowly decreasing and is expected to reach its minimum in 8,000 years.

With a low inclination of the axis, the winters in the higher latitudes are less severe, but glaciers can accumulate larger masses of snow, as evaporation over the sea is higher and therefore more snow falls widely where temperatures are below freezing point. In the summers, on the other hand, the ablation is reduced due to the lower solar radiation and the lower average temperatures. If one ignores non-astronomical climatic factors (see below ), the prerequisites for the formation of continental ice sheets are consequently more favorable with a low axis inclination than with a high axis inclination. In fact, the cold periods of the Pleistocene are often in phases for which a low axis inclination has been calculated, while the warm periods correlate with phases of high axis inclination.

Change in eccentricity

Circular orbit with an eccentricity of 0
Orbit with an eccentricity of 0.5

The earth's orbit around the sun is an ellipse . The eccentricity indicates how much the elliptical orbit deviates from a circular path. The shape of the earth's orbit varies from nearly circular (low eccentricity of 0.0006) to slightly elliptical (high eccentricity of 0.058). The mean eccentricity is 0.028. The main component of this deviation occurs over a period of 405,000 years (variation in eccentricity by ± 0.012). Several other parameters of the earth's orbit change in cycles between 95,000 and 136,000 years and unite loosely with the main component in a cycle of 100,000 years (variation between −0.03 and +0.02).

The current eccentricity is 0.0167 (with a decreasing trend), so that the solar distance varies by 3.4% over the course of the year. This corresponds to a variation in radiation of 6.9%. If the earth's orbit is minimally eccentric, the change in radiation is only about 2%, but at a maximum it is more than 23%. These variations are caused by disturbances in the earth's orbit by the other planets in the solar system, primarily by Jupiter and Saturn .

Due to Kepler's 2nd law, a revolution through the "more distant" part of the earth's orbit around the sun (aphelion speed) takes longer than through the closer part, so that the earth is less brightly illuminated for longer than an almost circular orbit in an elliptical orbit becomes. However, the reduced irradiation is more than compensated for in the course of the year by the quadratic increase in irradiance close to the sun.

Since the earth is currently closer to the sun during winter in the northern hemisphere, the autumn-winter half-year is about 7 days shorter than the spring-summer half-year.

Further effects and critical consideration

Climate parameters of the last 420,000 years, determined from ice core analyzes of the Vostok station in Antarctica

One effect that Milanković did not take into account in his calculations is the periodic tilting of the Earth's orbit plane compared to the Sun-Jupiter plane, which, like the other disturbances, is essentially caused by Jupiter and Saturn. The cycle of around 100,000 years corresponds well with the periodicity of the cold ages during the last 700,000 years of the Pleistocene .

Variation in the duration of the Pleistocene glaciation cycles, determined from ocean sediments. Note the “jump” of the cyclicality from 41,000 years to 100,000 years at around 1.2 Ma.

Different climatic periods are known from paleoclimatological studies, which do not necessarily coincide with the astronomical cycles. Correlations between climatic and astronomical cycles can also be demonstrated for some time periods, but not with all three Milanković cycles, but only with a single one, whereby the climatic cycles can also “switch” from one Milanković cycle to another, so that in In these cases it is difficult to establish a causal relationship between the two. A study published in 2019 postulates a significant weakening of the deep water circulation in the subpolar regions of the southern ocean as the main cause of the cycle change in the Middle Pleistocene , with the result of lower carbon dioxide transport from the deep sea to the surface.

The reasons for such irregularities are that non-astronomical factors also influence the global climate, for example changes in the earth's atmosphere with regard to its aerosol and greenhouse gas content (both influenced by volcanism ) or changes in ocean and air currents in the course of continental drift (opening of Straits of the sea, mountain formations). Such factors can interact in complex ways with each other and with astronomical factors, with positive and negative feedback on the climate. These complex interrelationships can ensure that a Milanković signal is sometimes only indistinctly or not at all in the data sets. This applies above all to the components of precession and axis inclination , but less so to the long-period eccentricity cycles, which, according to recent paleoclimatological studies, can be demonstrated as a stable influencing variable over large parts of the Phanerozoic . In this way, the major cycle from 405,000 years to the Upper Triassic around 215 million years ago could be traced back and arranged chronologically. The Milanković cycles are also ascribed a significant influence on the climatic fluctuations occurring during the Permocarbon Ice Age in the late Carboniferous (about 315 to 299 mya) . The same applies to the times of crisis associated with abrupt climate changes and two mass extinctions in the Upper Devonian . In addition, according to more recent findings, the periodic changes in eccentricity could also have an impact on the carbon cycle within the various earth spheres .

outlook

Sediment cores from the deep sea show a climatic optimum in the Holocene around 8000 to 6000 years ago, the temperature values ​​of which were only reached again on a global level towards the end of the 20th century. Due to the decrease in solar radiation in northern latitudes during the summer maximum, coupled with the periodicity of the Milanković cycles, there has since been a slight decrease in temperature averaging ≈ 0.12 ° C per millennium. This cooling trend would normally mean that the interglacial of the Holocene would not be followed by a new glacial for some 10,000 years. Whether this event will occur as calculated or whether the current warm phase will be of a longer duration depends largely on the extent to which anthropogenic and natural greenhouse gases will enter the atmosphere in the future.

literature

  • Milanković, M. 1941. “Canon of Earth Radiation and its Application to the Ice Age Problem”. Royal Serbian Academy. (PDF)
  • Hays, JD, Imbrie, J., & Shackleton, NJ 1976. "Variations in the earth's orbit: pacemaker of the ice ages". Science, 194, 11 (Researchgate: PDF)
  • VM Fedorov: Earth's insolation variation and its incorporation into physical and mathematical climate models . In: Physics-Uspekhi . 2019, doi : 10.3367 / UFNe.2017.12.038267 .

Web link

Commons : Milanković cycles  - album with pictures, videos and audio files

swell

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