Milankovitch hypothesis

Milankovitch cycles refer to various periodic changes in Earth's orbital revolution and axial rotation—cycles that cause corresponding changes in the amount of solar radiation the earth receives. The Milankovitch hypothesis seeks to correlate these cycles with long-term climate changes on Earth, including the ice ages that have occurred about every 100,000 years during the last one or two million years.

Attempts to Explain the Ice Ages

The Milankovitch hypothesis attempts to explain the periodic occurrence of ice ages on Earth through the last one million years by calculating the combined effects of various periodic Earth motions on Earth's long-term climate patterns. Milutin Milanković, a Serbian geophysicist, engineer, and professor of applied mathematics at the University of Belgrade, worked on these calculations while interned during World War I. His interest in this topic began with a 1912 scientific paper, “Contribution to the Mathematical Theory of Climate.” In 1913, he published The Schedule of Sun Radiation on the Earth's Surface, which was followed by another scientific work, “About the Issue of the Astronomical Theory of Ice Ages” (1914). After the war, Milanković published a monograph, “Mathematical Theory of Thermal Phenomena Caused by Solar Radiation” (1920), which was based on his scientific calculations and established the Milankovitch cycles.

Milanković's ideas were first suggested by the French mathematician Joseph Adhemar in 1842. Adhemar recognized that the earth's elliptical orbit brings it closer to the sun (perihelion) in winter (on January 3) and farther from the sun (aphelion) in summer (July 4) in the Northern Hemisphere. The opposite occurs in the Southern Hemisphere, with the earth farther from the sun in winter, so that Antarctic winters are colder than Arctic winters. Kepler's laws show that the earth moves more slowly at aphelion, so Antarctic winters are several days longer than Antarctic summers. Adhemar believed this would cause more cooling and would produce a larger Antarctic ice sheet; he made this prediction before the ice sheet was discovered.

Adhemar also recognized that the precession of the equinoxes slowly shifts the axis of Earth's rotation and the corresponding seasons. The axis leans toward the sun in summer and away from the sun in winter. Precession reverses this effect during half of the precession cycle, and thus might cause the ice ages in the Northern Hemisphere when the axis is leaning away from the sun at aphelion.

Adhemar's ideas were developed further by Scottish scientist James Croll, who taught himself physics and astronomy. After corresponding with British geologist Sir Charles Lyell on the ice ages and their possible connection with orbital variations, Croll devoted himself to research on this topic and published his results in the books Climate and Time, in Their Geological Relations in 1875 and Climate and Cosmology in 1885. Croll related the 26,000-year precession of the equinoxes to a series of ice ages. He followed Adhemar in suggesting that when winter occurs near aphelion it will be colder for a longer period of time. Croll was the first to suggest that the resulting ice sheet will reflect more sunlight and remain frozen longer, producing colder temperatures for a “positive feedback” that reinforces the severity of an ice age even during summers. However, his calculation that the last ice age ended some 80,000 years ago was contradicted by new evidence at the end of the nineteenth century, suggesting a more recent end to the last ice age. Croll's ideas fell out of favor until they were revived by Milanković.

Milankovitch Cycles

Milanković calculated the changes of solar radiation on Earth caused by various periodic motions of Earth, including the 26,000-year axial precession of the equinoxes, the 112,000-year orbital precession of Earth's elliptical orbit, the 41,000-year nutation (nodding) of Earth's rotational axis, and changes in the eccentricity (elongation) of Earth's orbit during a cycle of about 100,000 years.

The precession of the equinoxes was discovered about 150 BCE by Greek astronomer and mathematician Hipparchus, who determined the precession to be a slow shift of the stars, in which the equinoxes shifted along the celestial equator by about one degree every 72 years, or 360 degrees in about 26,000 years. When Polish astronomer Nicolaus Copernicus proposed in his monumental work On the Revolutions of the Heavenly Spheres (1543) that the earth rotates on its axis and revolves around the sun, he recognized that precession of the equinoxes was caused by a 26,000-year wobble of the earth's axis, which traces a 23.5-degree cone around a perpendicular to the earth's orbital plane. English scientist-mathematician Sir Isaac Newton showed in the late seventeenth century that this wobble results from gravitational forces of the sun and moon acting on the equatorial bulge of the earth. Newton attributed this bulge to centrifugal forces caused by Earth's rotation, which resulted in its flattened spherical shape (oblate spheroid).

Although the earth's axis precesses once every 26,000 years relative to the stars, its axis precesses faster relative to the perihelion of the earth's orbit. This is true because the orbit itself is precessing about once every 112,000 years due to a slight inclination of Earth's orbit relative to the orbits of Jupiter and Saturn and due to gravitational forces acting to reduce this inclination. The result of this rotation of Earth's orbit to the ecliptic plane is that the 26,000-year axial precession relative to the stars is only about 21,000 years relative to the earth's orbit and its perihelion and aphelion points.

Because the greatest seasonal differences between summer and winter occur when the earth is closest to the sun (perihelion), at the summer solstice, and farthest from the sun (aphelion), at the winter solstice, it is this 21,000-year cycle that should have the greatest climate effect. The coldest and longest winters should occur when the earth is farther from the sun, near aphelion every 21,000 years. The increasing ice sheet on Earth would then reflect the sun's radiation during the following summers, slowing any melting of ice and helping to offset the larger amount of radiation that Earth receives when it is closer to the sun at perihelion.

The tilt of Earth's axis also varies in a 41,000-year nutation period, bobbing between 22.1 and 24.5 degrees of obliquity. The nutation of Earth's axis was discovered by English astronomer James Bradley in 1728 and was later shown to be caused, primarily, by the changing gravitational forces of the sun and moon on the earth's equatorial bulge. Increasing obliquity increases the amount of solar radiation in summers, when Earth's axis leans farther toward the sun and decreases it in winters, when the axis leans farther from the sun. This effect is amplified at the higher latitudes, where there are greater amounts of land that experience faster temperature changes than the oceans, in which mixing and cooling can occur. In a similar fashion, decreasing obliquity decreases solar radiation intensity in summers and increases it in winters. Even though winters are warmer, it is thought that the cooler summers melt less of the previous winters' ice buildup, contributing to the onset of an ice age.

Studies have shown that tilts of more than 24.5 degrees could raise temperatures enough to prevent the emergence of higher forms of animal life. Studies also have shown that the earth's unusually large moon stabilizes the tilt of the earth in a narrow range in contrast to the tilt of Mars, which varies widely and prevents the long-term stability of seasons needed for the development of living organisms. The current 23.5-degree obliquity of Earth is near the middle of the nutation range and is moving toward decreasing obliquity and its associated cooling trend; however, this may be offset by global warming, which is caused by human contributions to greenhouse gases in the atmosphere.

The eccentricity (elongation) of the earth's elliptical orbit has several periodic variations, mainly due to the gravitational forces of Jupiter and Saturn on the earth. The longest of these variations is 413,000 years, but several shorter variations combine to give a stronger periodic variation of about 100,000 years. The major axis of Earth's elliptical orbit remains nearly constant, which stabilizes the length of the year; but the minor axis varies, with an associated change in the distance between the focal points on the major axis, and thus the distance between the earth and the sun located at one focus.

The elongation of the earth's orbit varies from near circularity (lowest eccentricity of 0.005) to mildly elongated (highest eccentricity of 0.058) and has an average eccentricity of 0.028. Earth's orbit is currently in the lower end of the range, with an eccentricity of 0.017. Solar radiation received by the earth at perihelion is 6.8 percent more than radiation received at aphelion. The maximum variation in radiation received by the earth is 23 percent, when its orbit is at maximum elongation.

Confirmation of the Milankovitch Hypothesis

Milanković combined the various Milankovitch cycles to predict that temperatures on Earth would oscillate with a variety of different periods. The shorter term periods, caused by the combined effect of axial and orbital precessions (21,000 years), and the changing axial tilt (obliquity) caused by nutation (41,000 years), would be superposed on the longer variations, caused by changes in the eccentricity of the earth's orbit (about 100,000 years). Milanković believed that decreasing obliquity, with its cooler summers at aphelion, would have the strongest influence on increasing glaciation, especially at higher northern latitudes (which have larger landmasses), and thus would cause ice ages approximately every 41,000 years. This led Milanković to recognize that ice ages would not alternate between the Northern and Southern Hemispheres but rather would be dominated by the higher-latitude landmasses and occur simultaneously in both hemispheres. Information about previous ice ages was limited at the time, so no clear confirmation of this hypothesis was possible.

The development of radiocarbon dating in the 1950s appeared to conflict with the Milankovitch hypothesis, and for a time the hypothesis was abandoned. More recent research has shown that the longer 100,000-year variations in the elliptical shape of the earth's orbit have a stronger effect than nutation, and that ice ages have occurred about every 100,000 years during the last couple of million years.

A more complete and accurate record of ice ages began to emerge in the 1970s from deep-ocean sediment cores and Antarctic ice cores, which together revealed past global sea levels and temperatures. These data were the basis for an influential paper by J. D. Hays, J. Imbrie, and N. J. Shackleton (“Variations in the Earth's Orbit: Pacemaker of the Ice Ages,” 1976) that clearly revealed the three Milankovitch cycles. This revival of the Milankovitch hypothesis differed in the relative strengths of the cycles, with the measured peaks separated by about 23,000 years, 42,000 years, and 100,000 years, corresponding to approximately 10, 25, and 50 percent of the climatic variance caused by precession, obliquity, and eccentricity.

Problems, Proposals, and Predictions

The revival of the Milankovitch hypothesis in the 1970s had strong support in the observed climatic cycles, with their periodicities closely matching each of the Milankovitch cycles. All the cycles were evident in temperature variations, although the 100,000-year eccentricity cycle dominated and gave the best correlation with ice-age glaciations of the last 1 million years. This success, however, was not without problems, although these problems did result in new research and theories.

One problem immediately evident was that the 100,000-year eccentricity cycle had the weakest theoretical influence on solar radiation on Earth, yet it appeared to have the strongest climatic influence, as seen in the data from core samples of ocean sediments and Antarctic ice. Another problem was a clear shift from the dominant influence of the 41,000-year obliquity cycle to the recent dominance of the 100,000-year cycle, beginning about 1 million years ago.

In 1994, Richard A. Muller, at the University of California, Berkeley, proposed a new theory to resolve these problems. He focused on another orbital motion not considered in the original Milankovitch hypothesis: the changing inclination of Earth's orbit relative to the orbit of Jupiter. This orbital inclination varies over a few degrees with a 100,000-year period similar to that of the earth's orbital eccentricity, both of which result mainly from the gravitational influence of Jupiter and Saturn.

In 1983, the Infrared Astronomical Satellite detected a thin ring of dust and debris around the sun approximately in the plane of Jupiter's orbit. This ring apparently resulted from collisions between asteroids of the Themis and Koronis families. The earth still passes through it, on January 9 and July 9 of each year, an event accompanied by an increase in radar-detected meteors. Muller postulated that about every 100,000 years, when Earth's orbit aligns with this ring of debris, the ring itself gathers enough dust by accretion in the atmosphere to cool Earth's climate and reinforce the weak eccentricity cycle. Muller also proposed that if asteroid collisions occurred about every 1 million years, the resulting increase in dust would explain the shift from the 41,000-year cycle to the 100,000-year cycle.

Muller's proposal has not been widely accepted, and other problems remain unresolved. The Milankovitch cycles continue to have wide support as the best explanation for the ice ages. Summer radiation received in northern latitudes (calculated at 65 degrees north) appears to be the strongest driver of the 100,000-year ice-age cycle. Extrapolating the Milankovitch cycles into the future has led to various predictions of long-term temperature trends and ice-age glaciations. A widely quoted 1980 study by John Imbrie and John Z. Imbrie concluded that orbital forces upon Earth's climate will continue the long-term cooling trend that began 6,000 years ago for another 23,000 years unless the trend is reversed by greenhouse gases. A 2009 study, led by Darrell Kaufman of Northern Arizona University, indicated an Arctic cooling trend during the last 2,000 years that should continue for another 4,000 years, but this cooling trend appears to have been reversed by global warming.

Principal Terms

aphelion: Earth's greatest distance from the sun in its elliptical orbit

celestial equator: intersection of Earth's equatorial plane with the celestial sphere

eccentricity: the degree of elongation of an elliptical orbit from a circular orbit with zero eccentricity

ecliptic plane: the plane of Earth's orbit around the sun

equinox: the points on the celestial sphere where the sun appears to cross the celestial equator, moving northward at the vernal equinox and southward at the autumnal equinox; corresponds to equal hours of night and day

greenhouse gases: atmospheric gases such as water vapor, carbon dioxide, and methane that trap heat by absorption of solar radiation and reemission of longer wavelengths that cannot escape from the atmosphere

insolation: the amount of incident solar radiation on a unit area of the earth's surface at any given latitude

nutation: the periodic change in the angle of tilt of Earth's rotational axis that results in a bobbing of Earth's axis during precession

obliquity: the angle of tilt of Earth's rotational axis (about 23.5 degrees) from a perpendicular to its orbital plane (its ecliptic)

perihelion: the closest point of Earth from the sun on its elliptical orbit of the sun (located at one focal point of the ellipse)

precession of the equinoxes: the 26,000-year wobble of the earth's axis that causes the axis to slowly shift along a conical path away from its current direction toward the North Star Polaris

solstice: the farthest points on the celestial sphere of the sun's apparent path above and below the celestial equator, where the sun stands at midsummer and midwinter

Bibliography

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Ganopolski, Andrey. "Toward Generalized Milankovitch Theory (GMT)." Climate of the Past, vol. 20, no. 1, 18 Jan. 2024, doi.org/10.5194/cp-20-151-2024. Accessed 10 Feb. 2025.

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Imbrie, John, and Katherine Palmer Imbrie. Ice Ages: Solving the Mystery. 2d ed. Cambridge, Mass.: Harvard University Press, 1986.

Macdougall, Doug. Frozen Earth: The Once and Future Story of Ice Ages. Berkeley: University of California Press, 2006.

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Raymo, Maureen E., and Peter Huybers. “Unlocking the Mysteries of the Ice Ages.” Nature 451 (January 2008): 284-285.

Roe, Gerard. “In Defense of Milankovitch.” Geophysical Research Letters 33, no. 24 (December 2006).