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Milanković Cycles

[Plumbing the Russian internet, I have come across some wondrous sites. In this post, I translate an article from the site Nature of Science.]

Milanković Cycles
Due to periodic changes in the nature of the Earth’s orbit, the planet passes through repeated glaciations

In the XIX century, geologists made a startling discovery: it turned out that huge Arctic glaciers had advanced onto land and covered almost all of Europe and North America. In particular, the glaciation of these zones indicated two geological features. Imagine that a glacier acts as a bulldozer: it pushes forward soil and rock. When the glacier achieves its greatest extent and begins to retreat, the rock debris that remains turns into a chain of hills, the so-called glacial moraine. In addition, the slow movement of the ice erodes the rock below it. If you look at the surface of a mountain valley formed by the glacier, you will find deep parallel grooves in it. The origin of these scratches is easily explained if you imagine that the compressed lower portion of the glacier acts as a scraper or sandpaper. Moraines and scratches – vivid proof that once there were glaciers.

Shortly after this discovery, it became clear that the ice age on Earth was not a singular occurrence. Glacial periods have occurred in the past, repeated at regular intervals. Why this happened, nobody could explain until the beginning of the XX century, when the riddle was taken up by a prominent scientist. In his memoirs, the Serbian researcher Milutin Milanković recounts how he began to think about the causes of glacial periods. A friend of Milanković had published a collection of his patriotic poems, and they were celebrating the event in a cafe (young teachers in the University of Belgrade could only afford coffee!), when a wealthy man, overhearing them and liking the poems, immediately bought ten copies of the book. The friends ordered wine to celebrate this event. After the first bottle, Milanković recalled his previous achievements, which now seemed narrow and limited. By the end of the third bottle, the poet had begun to write an epic poem, and Milanković decided to comprehend the entire Universe, and to bring a ray light to its darkest corners.

During the First World War, Milanković served with the Serbian General Staff Headquarters. He was captured by Austro-Hungarian troops in battle, and served out the remaining hostilities imprisoned in Budapest. Fortunately for Milanković (and for science), his colleagues at the Hungarian Academy of Sciences created conditions for him to conduct scientific research, in return for his parole that he would not attempt to escape. He agreed, and during the rest of the war, developed his theory of the frequency of glacial periods.

His explanation is connected to changes in the Earth’s orbit (now called Milanković Cycles). According to Newton’s Law of Gravitation (and the first of Kepler’s laws of planetary motion), each planet revolves around the Sun in an elliptic orbit. In addition, under the Law of Conservation of Angular Momentum, the direction of the Earth’s rotational axis in space should remain unchanged. But in the Solar System, the Earth doesn’t orbit in the Sun in lonely splendour. It is attracted by the Moon and other planets, and these attractions exert an important (albeit weak) effect on the Earth’s orbit itself, as well as on the rotation of the Earth.

This influence is threefold:

Precession: Similar to a spinning gyroscope, whose axis describes a cone while it rotates, the Earth’s axis itself describes a cone with a period of 26,000 years. At present, the Earth is inclined such that in January (when it is closest to the Sun), the northern hemisphere, where the bulk of the landmass lies, is inclined away from our star. In 13,000 years, the situation will be the opposite: in January, the northern hemisphere will incline towards the Sun, and January will the mid-summer in the north.
Nutation: in addition to the slow precession of the Earth, the planetary axis itself wobbles, changing its angle in small fluctuations called nutations. At present, the axis is inclined at 23 degrees to the orbital plane. Every 41,000 years, under the influence not only of the Moon but distant Jupiter, the tilt angles changes to 22 degrees, and then returns to 23 degrees.
Change in the orbital shape: Because of the gravitational pull of other planets over time, the Earth’s orbit changes shape. The elliptical orbit becomes stretched along its minor axis, becoming more circular, followed by a stretch along the perpendicular axis, becoming elliptical again, and so on. This cycle takes 93,000 years to complete.

Milanković concluded that each of the factors affects the amount of sunlight received by different areas of our planet. For example, precession of the axis affects the nature and the years of winter in the northern hemisphere (I draw particular attention to the north because that’s where the majority of the landmass is found, and hence the bulk of glaciers).

Milanković realised that over time, the Earth’s climate would change. If the amount of sunlight in the northern hemisphere decreases, the snow every year will persist longer on the surface. Snow reflects light well, and the increased snow will reflect sunlight more and more, leading to a further cooling of the planet. The next winter, therefore, more snow will fall, further increasing the area under snow cover, which will reflect more sunlight, and so on. Over time, accumulating much snow, the glaciers move southwards. The land enters a glacial period. At the end of the cycle, the northern hemisphere will begin to receive more solar energy and the opposite effects will obtain: the ice will melt, exposing soil (which absorbs light well), the Earth will warm up again, and the three factors of the variability of the Earth’s axis will ensure that the glaciers retreat.

Milanković thought that the climate of the Earth is influenced by these three cycles, each associated with a certain astronomic cause. When they reinforce each other, we can expect deep cold and the onset of an Ice Age. However, the periods of these cycles are different, and their effects are often not additive, so that the climate quickly returned to its norm. In effect, the glacial periods occur when the three orbital factors are all in the same direction, where their cumulative effects depressed the planetary climate into cooling. This phenomenon has been repeated several times in the history of the planet.

Over the past three million years, there have been at least four periods of large-scale glaciation, and even before them, there have been many others. I would like to point out that the last Ice Age reached its maximum intensity about 18,000 years ago, and that the time in which we live is what the scientists call interglacial: a very encouraging term.

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