Milankovitch’s cycles are one of the theories with which scientists have tried to explain the existence of glaciations in Earth’s history. This hypothesis is also called orbital or astronomical. It got its name after the Yugoslav climatologist Milutin Milankovich. Despite the large number of contradictions present in this theory, it formed the basis of modern paleoclimatology.
Earth movement
As you know, the Earth revolves around the Sun in an elliptical orbit and around its axis. The latter also changes its position due to the influence of the attraction of the moon. The Earth’s axis has a certain angle of inclination, like other planets in the solar system. She describes a cone in space. This effect is called precession. A good example visualizing this feature of planetary motion is the rotation of a top.
The period of a full circle revolution is about 25,800 years. The angle of inclination of the axis also varies in the range of 22.1-24.5 ° every 40 to 100 years. This phenomenon is called nutation.
The eccentricity, or degree of compression of the Earth’s orbit during the rotation of the Sun, varies over a period of 90,800 years. When it grows, the planet moves away from the star and receives less solar radiation, and therefore, heat. There are also periods when the largest slope of the Earth coincides with the maximum eccentricity. The result is a global cooling of the climate.
Perigelium and aphelion
Since the planets of the solar system have a mutual influence on each other, the axis of the Earth’s orbit, when moving around the sun, gradually turns in the same direction as the orbital motion. As a result, the perigil is shifted - the closest point to the luminary orbit and aphelion - the most distant point. These parameters affect the intensity of exposure to solar radiation - thermal, electromagnetic, corpuscular radiation. In percentage terms, these fluctuations are small, but they affect the heating of the planet's surface.
Astronomy, geophysics and climatology are the sciences with the help of which scientists seek to establish a relationship between the activity of the Sun, secular changes in the average annual temperature and climate in general, as well as between other factors. Their tasks are not only the determination of natural laws, but also the prediction of future changes that can significantly affect human life.
What are Milankovitch cycles?
The climate of the Earth is changing under the influence of anthropogenic and non-anthropogenic factors. The second group includes tectonic movements of lithospheric plates, fluctuations in solar radiation, volcanic activity, as well as Milankovitch cycles. They describe the effect of changes in planetary movements on its climate.
In 1939, Milankovich first put forward a hypothesis about the cyclical dependence of the ice ages over the past 500 thousand years. He calculated the dynamics of changes in solar radiation, which consists of electromagnetic and particle radiation, and explained the cause of glaciation in the Pleistocene era. In his opinion, it consisted in changing the parameters of the planet’s orbit - eccentricity, the angle of the axis and the position of the perihelion. According to the postulates of his theory, glaciations caused by these factors are repeated at short intervals, and they can be predicted.
His hypothesis was built on the assumption that the planet’s atmosphere was transparent. Variants of solar radiation (insolation) were calculated by him for 65 ° north latitude. The areas corresponding to the four glaciations obtained on the insolation diagram corresponded well with the alpine glaciation scheme constructed by German scientists A. Penk and E. Bruckner.
Major factors and ice ages
According to Milankovic’s theory, the three main orbital factors listed above should normally act in different directions so that their effect does not add up. The next ice age begins when they add up and reinforce each other.
Each of them determines the influence of the Sun on the Earth, on the amount of solar radiation received by different zones of the planet. If it decreases in the Northern Hemisphere, where the bulk of the glaciers are concentrated, then more and more snow accumulates on the surface every year. The increase in snow cover leads to increased reflection of sunlight, which, in turn, contributes to further cooling of the planet.
This process is gradually increasing, global cooling begins, the next ice age begins. At the end of this cycle, the opposite phenomenon is observed. According to scientific data, the peak of cooling during the last ice age was about 18,000 years ago.
Precession effect
Scientists believe that the precession cycle is most pronounced in glaciations in the Northern Hemisphere. Now it is in the interglacial period, which will end in about 9-10 thousand years. In the coming millennia, sea level rise may continue due to melting glaciers. And first of all it concerns the Greenland ice sheet - the second largest after the Antarctic.
In the Southern Hemisphere, on the contrary, the era of “glaciation” is currently observed, but since there is much less land than in the Northern Hemisphere, this phenomenon does not look so bright.
If the day of the winter solstice falls on aphelion (that is, the inclination of the axis of rotation of the planet in the direction from the Sun is maximum), the winter will be longer and colder, and the summer will be hot and short. In the opposite hemisphere, on the contrary, there is a long, cool summer and a short warm winter. The differences in the duration of these seasons are the more noticeable, the greater the eccentricity of the orbit.
Nutation
Nutation is associated with shorter fluctuations in the position of the earth's axis. The largest amplitude is 18.6 years.
Nutation leads to a change in seasonal contrasts of solar radiation, but its annual amount remains constant. The increase in insolation in the summer (hotter and drier weather) is compensated by its decrease in the winter.
Change in orbital shape
The distance from the Earth to the Sun depends on the magnitude of the elongation of the planet’s orbit. The difference between the extreme points is 4.7 million km. In an era of small eccentricity, the planet receives more solar radiation, the upper boundaries of the atmosphere are heated more strongly, and vice versa.
Eccentricity changes the total annual solar radiation, but this difference is small. Over the past million years, it has not exceeded 0.2%. The greatest effect occurs when the maximum eccentricity coincides with the largest slope of the Earth’s own axis.
Earth Climate Change History
Modern geophysical research methods allow us to find out what the climate was like on our planet hundreds of millennia ago. Temperature is indirectly estimated by the amount of isotopes of heavy hydrogen and oxygen. The rate of global warming is currently around 1 ° per year.
Over the past 400,000 years, 4 ice ages on Earth have been noted. The sharp warming, which began about 12 thousand years ago, led to an increase in the sea level by 50-100 m. Perhaps this phenomenon was described in the Bible as a Flood.
Warming in the modern era is accompanied by differences in average annual temperature by 2-3 degrees. In the constructed dependences, jumps in the temperature of the surface of the planet are noted, the duration of which is no more than 1000 years. There are fluctuations in a smaller cycle - every 100-200 years by 1-2 °. As scientists suggest, this is due to fluctuations in the amount of methane and carbon dioxide in the atmosphere.
Theory flaws
In the 60-70s. XX century, scientists have obtained new experimental and calculated data that diverged from the concept of Milankovitch cycles. The following contradictions exist in it:
- The atmosphere of the Earth was not always as transparent as it is now. This is confirmed by studies of the ice of Greenland and Antarctica. A large amount of dust, presumably associated with active volcanic activity, reflected solar heat. As a result, the surface of the planet cooled.
- According to Milankovitch’s theory, glaciations in Greenland and Antarctica occurred at different time periods, however, this diverges from paleontological data.
- Global cooling should be repeated at approximately equal intervals, but in fact they were not in the Mesozoic and Tertiary period, and in the Quaternary they followed one after another.
The main drawback of this theory is that it is based only on astronomical factors, namely, a change in the motion of the Earth. In reality, there are many other reasons: variations in the geomagnetic field, the presence of numerous feedbacks in the climate system (the resonance response mechanism that occurs in response to orbital influences), tectonic activity (volcanism, seismic activity), and in recent centuries, the anthropogenic component, that is, the impact of human activities on nature.