Milankovitch cycles are one of the theories with which scientists tried to explain the existence of glaciations in the history of the Earth. This hypothesis is also called orbital or astronomical. It got its name from the Yugoslav climate scientist Milutin Milanković. Despite the large number of contradictions in this theory, it formed the basis of modern paleoclimatology.
As you know, the Earth revolves around the Sun in an elliptical orbit and around its own axis. The latter also changes its position due to the influence of the moon's gravity. The earth's axis has a certain angle of inclination, like other planets in the solar system. It describes a cone in space. This effect is called precession. A good example visualizing this feature of the planet's motion is the rotation of a top.
The period of a complete revolution around the circumference is about 25,800 years. The tilt angle of the axis also changes in the range of 22.1-24.5° every 40,100 years. This phenomenon is called nutation.
Eccentricity, orthe degree of compression of the Earth's orbit during the rotation of the Sun changes over a period of 90,800 years. When it increases, the planet moves away from the star and receives less solar radiation, and, accordingly, heat. There are also periods when the greatest slope of the Earth coincides with the maximum eccentricity. The result is global cooling.
Perihilion and Aphelion
Since the planets of the solar system have 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 movement. As a result, the perihelion is shifted - the point of the orbit closest to the star and aphelion - the most distant point. These parameters affect the intensity of the impact of 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 the relationship between solar activity, secular changes in the average annual temperature and climate in general, as well as between other factors. Their task is not only to determine natural patterns, but also to predict future changes that can significantly affect human life.
What are Milankovitch cycles?
The Earth's climate 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, and Milankovitch cycles. They describe the impact of changes in the planet's movements on its climate.
In 1939, Milankovitch first put forward a hypothesis about the cyclic dependence of ice ages over the past 500 thousand years. He calculated the dynamics of changes in solar radiation, which consists of electromagnetic and corpuscular 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 - the eccentricity, the angle of inclination 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 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 sections obtained on the insolation diagram, corresponding to four glaciations, correlated well with the Alpine glaciation scheme, built by the German scientists A. Penk and E. Brückner.
Main factors and ice ages
According to Milankovitch'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 comes 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 differentzones of the planet. If it decreases in the Northern Hemisphere, where the bulk of glaciers are concentrated, then more and more snow accumulates on the surface every year. The increase in snow cover increases the reflection of sunlight, which in turn contributes to further cooling of the planet.
This process is gradually increasing, global cooling begins, another ice age begins. At the end of such a cycle, the opposite phenomenon is observed. According to scientific data, the peak of cooling during the last ice age was approximately 18,000 years ago.
Influence of precession
Scientists believe that the precessional 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, the sea level may continue to rise due to the melting of glaciers. And first of all, this concerns the Greenland ice sheet - the second largest after the Antarctic one.
In the Southern Hemisphere, on the contrary, the epoch of "glaciation" is currently observed, but since there is much less land here than in the Northern, this phenomenon does not look so bright.
If the day of the winter solstice falls on aphelion (that is, the tilt of the planet's axis of rotation in the direction from the Sun is maximum), winter will be longer and colder, and summer - 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 moreorbital eccentricity.
Nutation is associated with more short-term fluctuations in the position of the earth's axis. The largest magnitude of the amplitude is 18.6 years.
Nutation leads to a change in the seasonal contrasts of solar radiation, but its annual amount remains constant. The increase in insolation in summer (hotter and drier weather) is offset by its decrease in winter.
Changing the orbital shape
The distance from the Earth to the Sun depends on the elongation of the planet's orbit. The difference between the extreme points is 4.7 million km. In the era of small eccentricity, the planet receives more solar radiation, the upper boundaries of the atmosphere heat up more, and vice versa.
Eccentricity changes the total annual solar radiation, but this difference is small. During the last million years, it has not exceeded 0.2%. The greatest effect occurs when the maximum eccentricity coincides with the greatest inclination of the Earth's own axis.
History of Earth's climate change
Modern geophysical research methods allow us to find out what the climate on our planet was like hundreds of millennia ago. The temperature is indirectly estimated by the number 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 have been recorded inEarth. A sharp warming, which began about 12 thousand years ago, led to a rise in the ocean level by 50-100 m. Perhaps this phenomenon was described in the Bible as the Flood.
Warming in the modern era is accompanied by average annual temperature fluctuations of 2-3 degrees. On the constructed dependencies, jumps in the temperature of the planet's surface 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.
Flaws of the theory
In the 60s and 70s. In the 20th century, scientists obtained new experimental and calculated data that diverged from the concept of Milankovitch cycles. It contains the following contradictions:
- Earth's atmosphere has not always been as transparent as it is now. This is confirmed by studies of ice in 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, but this is at odds with paleontological data.
- Global coolings should be repeated at approximately equal intervals, but in fact they were not in the Mesozoic and Tertiary periods, and in the Quaternary they followed one after another.
The main drawback of this theory is thatit is based only on astronomical factors, namely the 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 impacts), tectonic activity (volcanism, seismic activity), and in recent centuries, the anthropogenic component, that is, the impact of human economic activity on nature.