What are the three periodicities of Milankovitch
Both Milanković cycles (after the Serbian mathematicianMilutin Milanković, in the German-speaking area too Milankovic or Milankovich, in the English-speaking area Milankovich or Milankovitch written) are time-variant patterns in which the solar radiation hitting the earth varies beyond the annual fluctuation, i.e. long-period variations of the solar constant and the characteristics of the seasons. They partly explain the earth's natural climatic fluctuations and are therefore of great importance in the context of climatology and paleoclimatology.
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 northern and southern hemispheres. As a celestial mechanical cause for 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 fluctuates approximately between 19,000 and 24,000 years, and in which two different cycles overlap:
- The variation of the ecliptic inclination (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 around every 400,000 years.
As a result of the eccentricity fluctuations, there are slight changes in the amount of energy that the entire earth receives from the sun annually (magnitude of the fluctuation by 0.2%) and as a result of the precession and the change in the axis inclination to considerable changes in the amount of energy that the two hemispheres and especially their higher geographical latitudes receive annually (magnitude of the fluctuation to 65 ° N at the summer solstice on average by 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 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.
The axis of the earth is only really fixed in the center of the earth. Outside the center of the earth, it describes a circular movement with a period of 26,000 years around the imaginary perpendicular position to the ecliptic plane (with increasing radius of the circle with increasing distance to the center of the earth). Such a "tumbling movement" is called precession. The cause of the earth's axial precession are the forces of the sun and moon on the equatorial bulge of the rotating earth ellipsoid. The axis precision 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 the axis precession is superimposed by the cycle of the apsidal precession of the earth's orbit, the period of which is 112,000 years. In 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 change of seasons 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 turn of the apse, the cyclical change in the position of the vernal equinox relative to perihelion. The tropical turn of the apse corresponds to a Milanković cycle of around 21,000 years on average. 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, under “precession” in connection with the Milanković cycles, the so-called Precession index Roger that. 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
The inclination of the earth's axis (obliquity, ecliptic inclination) against 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 radiation intensity at higher geographical latitudes over the course of the year. If the axis inclination is greater, the winters in the higher latitudes are colder and the summers warmer than if the axis inclination is less. 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 lower inclination of the axis, the winters are less severe in the higher latitudes, but glaciers can accumulate larger amounts 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-astromic 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 413,000 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 with the main component loosely 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 distance to the sun varies by 3.4% over the course of the year. This corresponds to a variation in irradiation 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%. The cause of these variations are disturbances of the earth's orbit by the other planets of the solar system, primarily by Jupiter and Saturn.
Due to Kepler's second 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 illuminated than average for longer in an elliptical orbit compared to an almost circular 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
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 Pleistocene cold ages.
Different climatic periods are known from paleoclimatological studies, which do not necessarily coincide with the astronomical cycles. Correlations between climate 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 climate cycles can also “switch” from one to another Milanković cycle, so that in In these cases it is difficult to establish a causal relationship between the two.
The causes of 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 complexly with each other as well as with astronomical factors, with positive and negative feedback on the climate. These complex interrelationships can ensure that a Milanković signal in paleoclimatological data sets is very indistinct or not present at all.
Schönwiese points out that Milanković's idea was initially heavily questioned, even in the decades after its publication in 1920. The idea was then renewed - with considerable modifications, especially in terms of cryospheric feedback - by Nicholas Shackleton in 1976, Berger (1978 and 1984) as well as Berger and Loutre (1997a, 1997b) taken up and shared.
- ↑ 1,01,1David Parry Rubincam: The Precession Index, A Nonlinear Energy Balance Model, And Seversmith Psychroterms. NASA Technical Report, Greenbelt (MD) 2004 (PDF 2.8 MB)
- ↑Milutin Milankovitch. In: Encyclopædia Britannica. Accessed December 14, 2015 (english).
- ↑ 3,03,1based on data from the Milankovitch Orbital Data Viewer, Denning Research Group, Colorado State University
- ↑Earth's seasons. In: Naval Oceanography Portal. Archived from the original on September 1, 2012, retrieved on December 14, 2015 (english).
- ↑ Richard A. Muller, Gordon J. MacDonald: Spectrum of 100-kyr glacial cycle: Orbital inclination, not eccentricity. In: PNAS. Volume 94, No. 16, 1997, Orbital Inclination: An Alternative 100-kyr Cycle., Pp. 8329-8334 (online [accessed December 14, 2015]).
- ↑ J. D. Hays, J. Imbrie, N. J. Shackleton: Variations in the Earth's Orbit: Pacemaker of the Ice Ages. In: Science. Volume 194, No. 4270, December 10, 1976, pp. 1121-1132, doi: 10.1126 / science.194.4270.1121.
- ^ André L. Berger: Long-Term Variations of Daily Insolation and Quaternary Climatic Changes. In: Journal of the Atmospheric Sciences. 35, No. 12, December 1978, p. 2362. doi: 10.1175 / 1520-0469 (1978) 035 <2362: LTVODI> 2.0.CO; 2.
- ↑ Schönwiese, Christian-Dietrich: climatology. 4th (revised and updated) edition. Stuttgart 2013, ISBN 978-3-8252-3900-8, p. 299.
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