THE MILANKOVITCH CYCLES are recurring variations in the Earth-Sun orbital geometry. They collectively account for deviations in the amount and intensity of solar radiation received by the Earth. The cycles are named after Serbian astrophysicist Milutin Milankovitch (1879-1958), who developed the modern mathematical theory and formulas upon which these orbital perturbations are based. The central assertion of the Milankovitch theory is that the Earth's orbital relationship with the Sun is not stagnant, but dynamic, with three cyclic modes, which, when precisely combined, may significantly alter the Earth's insolation, resulting in a major global climate shift. While the Milankovitch theory was first proposed in the 1930s, the first supporting scientific evidence was not brought forth until the 1970s, when researchers of the CLIMAP project linked the timing of changes found in ocean sediment core data with associated orbital shifts of the Milankovitch cycles.
The three astronomical attributes constituting the Milankovitch cycles are: eccentricity or shape of the Earth's orbit about the Sun; obliquity or tilt of the Earth's axis from the perpendicular plane to the Earth's orbit; and precession or orientation of the Earth's rotational axis relative to the Sun. All three cycles fluctuate gradually at different rates with periodicities of thousands of years. These oscillations can operate jointly to control the strength and timing of seasons, including being a forcing mechanism for the frequency of glacial retreats and advancements.
Eccentricity is the change in the shape of the Earth's orbit around the Sun, fluctuating from a more circular (low eccentricity) to a more oval-shaped (high eccentricity) orbit. This cycle operates on several time periods depending on the Earth's juxtaposition to the gravitational pull of other planetary bodies (such as Jupiter), with a cycle of approximately 100,000 years most prominent. Currently, the Earth's orbit is in a low eccentricity phase, denoting a nearly circular orbit and a relatively small 3 percent difference in distance between the point of longest (aphelion) and shortest (perihelion) Earth-Sun distance. Because perihelion occurs on July 3 (Southern Hemisphere summer), this distance difference translates to approximately 7 percent more solar radiation received in the Southern than the Northern Hemisphere. At maximum eccentricity, the distance difference between aphelion and perihelion can be about 10 percent and account for over a 25 percent difference in solar radiation between the two hemispheres.
Obliquity is the deviation in the tilt of the Earth's axis away from the orbital plane. The current tilt of the Earth's rotational axis is 23.5 degrees, varying between 22.1 and 24.5 degrees over its 41,000-year cycle. Since the last peak obliquity occurred approximately 10,000 years ago, the current cycle is nearly at the midpoint of its range.
While the limits of the axial tilt cycle appear relatively small, the impact on seasons can be considerable, minimizing (exaggerating) seasonal differences with decreasing (increasing) obliquity, especially for the middle and high latitudes. During periods of less tilt, the severity of extratropical winters and summers is reduced, resulting in milder winters and cooler summers. This reduction in a seasonal contrast may promote the growth of ice sheets by decreasing the amount of summer melting and increasing high latitude winter snowfall accumulation, as higher winter temperatures increase the moisture capacity of the air. The present decreasing obliquity of the Earth has expanded the temperate latitudes and reduced the tropical and polar regions, effectively altering the position of the tropics and the polar circles at a rate of about a mi. (more than a km.) per century.
Precession is the change in the direction of the Earth's rotational axis, influencing the timing and intensity of the seasons. Often compared to the winding-down of a spinning top, the Earth wobbles on its axis, moving slightly forward as it travels in orbit around the Sun. During its average 26,000-year cycle, the Earth's axis of rotation pivots such, that the celestial poles are shifting, altering the current pole star of Polaris to other stars such as Vega in 14,000 c.E.
As a consequence of this wobble, the orbital locations at which the summer and winter solstices transpire periodically reverse; such as will be the case when Vega is the North Star. In 14,000 c.E., aphelion (perihelion) will coincide with the Northern Hemisphere winter (summer) solstice, resulting in the colder winters and warmer summers for the Northern Hemisphere and weaker seasonal temperature contrasts for the Southern Hemisphere. Precession is the only orbital parameter that is noticeable on a time scale of hundreds of years, influencing the timing of seasons by 1-2 days per century, on average. For the Northern Hemisphere, changes in precession during the past century have delayed the onset of winter and lengthened the summer.
sEE ALsO: CLIMAP Project; Milankovitch, Milutin; Orbital Parameters, Eccentricity; Orbital Parameters, Obliquity.
bibliography. E. Aguado and J.E. Burt, Understanding Weather and Climate (Prentice Hall, 2006); C.D. Ahrens, Meteorology Today (Thomson Brooks/Cole, 2007); E.A. Bryant, Climate Processes and Change (Cambridge University Press, 2004).
Jill S.M. Coleman Ball State University
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