Milankovitch cycles
July 7, 2010, 4:09 pm
August 1, 2012, 12:53 pm
Milankovitch cycles refer to long term variations in the orbit of the
Earth which result in changes in climate over periods hundred of
thousands of years and are related to ice age cycles.
Once Isaac Newton
described his laws of motion and of gravity, the orbit of each planet
became predictable, not only under the influence the Sun, but the much
weaker influences of all the other planets and the Moon as well.Milutin Milankovitch did not discover the cycles, nor was he the first to calculate their changes. He did, though, improve on the methods of calculating them and relating them to Earth’s climatic variations. Here is a brief description of the three cycles.
Precession (also called Precession of the Equinoxes): the gravitational pull of the Sun and Moon on Earth’s equatorial bulge causes the poles to slowly wobble. Over 25,800 years, the polar axis traces out a circle with respect to the stars.
Milankovitch cycles. Source: UCAR
Now the North Pole points to Polaris and all other stars visible in the Northern Hemisphere appear to rotate around that star in the night sky.
In one half cycle, or 12,900 years, the North Pole will point to the star Vega, which is forty-seven degrees away from Polaris. In another 12,900 years, the North Pole will be back to Polaris.
Astronomers in 4,000 BC, for example, noted that the axis pointed to the handle of the Big Dipper (part of the constellation Ursa Major), not Polaris, which is the end of the handle of the Little Dipper (part of the constellation Ursa Minor). An effect of this change is that the time of year that Earth is closest to the Sun, called perihelion, varies through the cycle.
Now perihelion is January 3, so the Northern Hemisphere has slightly milder winters and the Southern Hemisphere has slightly cooler winters. And, conversely, summers are a bit cooler in the North and warmer in the South. In 12,900 years, the North will have colder winters because Earth will be furthest from the Sun (aphelion) in January.
Another aspect of the precession is the length of winter and summer. Because the Sun is not at the center of the orbital ellipse (discussed in the next paragraph), it currently takes seven more days for Earth to travel from the vernal equinox to the autumnal equinox than from the autumnal to the vernal. In other words, the Northern Hemisphere winter now is shorter than the Southern Hemisphere winter. In 12,900 years, the North will have longer winters and shorter summers.
Eccentricity of Orbit: Earth travels around the Sun along a flat surface called the plane of the ecliptic, called that because eclipses occur when the Moon intersects this plane. The path taken along this plane is almost a circle, but not quite. It is elliptical, with the Sun just off center as one ‘foci’ of the ellipse. Gravitational pull of other planets causes the path to become slightly more or slightly less elliptical. In other words, it becomes more or less of a flattened circle. Venus, because it is close to Earth, and Jupiter, because it is so massive, have the greatest effect on the eccentricity. There are peaks in eccentricity every 95,000 years, but superimposed on those are larger peaks at 125,000 and 400,000 years. When the orbit is more elliptical, the perihelion is closer to the Sun and the aphelion is farther away than when the orbit is more circular.
Axial Tilt (also called Obliquity): The axis of rotation intersects the plane of the ecliptic at an angle and that angle changes over time. This change is caused by the fact that the Moon’s orbital path is not precisely along Earth’s plane of the ecliptic and so the gravitational attraction of the Moon varies in direction over time. The angle of axial tilt affects the difference between winter and summer in each hemisphere, especially at higher latitudes. Not only does the axial tilt vary over time, but the plane of the ecliptic varies, too. Taking the two into consideration, the obliquity of the axis varies on a 41,000 year cycle and varies from 22.1° to 24.5° from a line perpendicular to the plane of the ecliptic, with the current value at about 23.44°.
Milankovitch cycles over the past 1 000 000 years. Source: Global Warming Art
Soon after the existence of an ice age had been proposed, scientists sought an explanation of their cause. In 1842, Frenchman Joseph Alphonse Adhémar suggested that the varying lengths of winter and summer, an effect of the precession, causes ice to accumulate in the hemisphere with the longer winter. He used the massive ice sheet in Antarctica as evidence, since the Southern Hemisphere currently has longer winter and shorter summer.
Scotsman James Croll combined the eccentricity of the orbit and the precession and in the 1860s and 1870s presented his ideas on the effects of the cycles and how they might influence climate, especially the colder winters when they correspond with the aphelion. In fact, what are typically called ‘Milankovitch Cycles’ are sometimes referred to as ‘Croll-Milankovitch Cycles.’
Milankovitch gets most of the credit for relating the cycles to ice ages because he incorporated all of the pertinent cycles, dealt with them in much greater mathematical precision and showed much more thoroughly how they affect climate. He, at Wladimir Köppen and Alfred Wegener’s suggestion, investigated the role of cooler summers in instigating ice ages. Milankovitch Cycles clearly play an important role in the comings and goings of ice sheets, but the details of just how this happens are far from well understood.
Further Reading
- Milankovitch, M. 1941. Canon of Insolation and the Ice-Age Problem. Israel Program for Scientific Translations. Jerusalem (1969).
- Muller, Richard A. and MacDonald, Gordon J. 2000. Ice Ages and Astronomical Causes: Data, spectral analysis and mechanisms. Springer. London.
- Raymo, Maureen E. and Huybers, Peter. 2008. Unlocking the Mysteries of the Ice Ages. Nature 451: 284-285.
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