Milankovitch Cycles

Introduction

Milankovitch cycles are regular changes in Earth's climate over tens and hundreds of thousands of years that are caused by shifts in Earth's orbit. They are named after Serbian scientist Milutin Milankovitch (1879– 1958), who proposed such cycles in the early twentieth century. Since the 1970s, when the theory was revived on a firmer scientific footing, much research effort has gone into investigating various indicators of ancient climate (paleoclimate) in the geological record to see if Milankovitch cycles are real and, if so, what aspects of Earth's long-term climate they control. Although there is agreement among the majority of geophysicists and paleoclimatologists that Milankovitch cycles account for many changes in the climate record, the theory continues to be challenged.

Historical Background and Scientific Foundations

Development of the Theory

The idea that cyclic or repeating changes in Earth's orbit cause cyclic climate changes actually predates by almost a century detailed evidence of such cyclic climate changes. It also predates Milankovitch, the man whose name is most often associated with such cycles. In 1876, amateur scientist James Croll (1821–1890), who taught himself science from books while working as a janitor at the library of the Andersonian College and Museum in Glasgow, Scotland, published a treatise in which he proposed that changes in Earth's orbit should change the amount of sunlight striking Earth, or the different hemispheres of Earth, and so alter Earth's climate cyclically over thousands of years. Croll's theory received little attention for several decades but was revived in the early 1900s by Milutin Milankovitch.

Milankovitch realized that because two thirds of Earth's land is located in the Northern Hemisphere, insolation (amount of sunlight) in that hemisphere should dominate the occurrence of warm and cold climate periods. After years of hand calculations, Milankovitch concluded that changes in the tilt of Earth's axis and the shape of its orbit around the sun should produce ice ages on 23,000-year, 41,000-year, and 100,000-year cycles.

Data were not yet available, however, on when Earth's ice ages had actually occurred. When such data began to become available from radiocarbon dating after 1949, they did not seem to match Milankovitch's

prediction and the idea of orbital forcing of climate cycles was once again discredited for a time.

The idea revived again after W. Broecker and J. van Donk showed in 1970 that the dominant ice-age cycle has a 100,000-year interval, which is in agreement with Milankovitch cycling. In 1976, J. D. Hays and colleagues published what has been called “one of the great papers in paleoclimatology,” a paper titled “Variations in the Earth's Orbit: Pacemaker of the Ice Ages.” The paper showed in convincing detail that 23,000-year and 41,000-year orbital climate cycles could be detected in sea-floor sediment layers.

Physical Basis for Milankovitch Cycles

There are three major factors that contribute to Milan-kovitch cycles, namely: 1) precession of Earth's axis; 2) obliquity or tilt of Earth's axis; and 3) changing eccentricity of Earth's orbit. To understand what these are and how they can influence climate, it is necessary to know the basic motions of Earth.

Earth circles or orbits the sun once a year. Also, Earth spins like a top around an imaginary line called its axis. The axis passes through the North and South geographic poles. If Earth's orbit is thought of as lying in a plane like a vast sheet of glass, the sun also lies in that plane and Earth's axis is tilted with respect to it. That is, Earth's axis does not stick straight up out of the plane (which astronomers term the ecliptic), but is tipped at an angle of about 23°.

The rotation of Earth on its axis gives us day and night; the tilt of Earth's axis gives us seasons. As Earth orbits the sun, each pole is rotated toward the sun for half the year. The hemisphere centered on that sunnier pole receives more insolation and experiences summer while the other, shadier hemisphere experiences winter.

Earth's orbital motion does not inscribe a precise geometric circle—a circular path would keep Earth at exactly the same distance from the sun at all times— but instead traces a geometric figure called an ellipse, which resembles a slightly stretched circle. The sun is near the center of this ellipse, but slightly nearer to one end. This off-centeredness is called eccentricity. Because of eccentricity, Earth is closer to the sun at some points in its orbit (i.e., some times of the year) than at others, and so receives more sunlight at those times. The shape of Earth's orbit is always changing, so that sometimes it more closely resembles a circle (low eccentricity) and sometimes is more elliptical or squashed (high eccentricity). The change from lower to higher eccentricity happens every 413,000 years.

Also, the tilt of Earth's axis is not constant, but bobs up and down slightly every 41,000 years. And the direction in which Earth's axis points twirls slowly around, tracing out a cone in space. This twirling process, called precession, completes one cycle every 25,800 years.

WORDS TO KNOW

ECCENTRICITY: In the Keplerian orbit model, the satellite orbit is an ellipse, with eccentricity defining the ´shape´ of the ellipse. When e=0, the ellipse is a circle. When e is very near 1, the ellipse is very long and skinny.

ICE AGE: Period of glacial advance.

ORBITAL FORCING: Increases or decreases in the amount of thermal energy being absorbed by Earth's climate system governed by Milankovitch cycles, that is, regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun

PALEOCLIMATE: The climate of a given period of time in the geologic past.

PRECESSION: The comparatively slow torquing of the orbital planes of all satellites with respect to Earth's axis, due to the bulge of Earth at the equator, which distorts Earth's gravitational field. Precession is manifest by the slow rotation of the line of nodes of the orbit (westward for inclinations less than 90 degrees and eastward for inclinations greater than 90 degrees).

RADIOCARBON DATING: Method for estimating the age of a carbon-containing substance. Small amounts of an unstable isotope of carbon, carbon–14, are formed continuously by cosmic rays striking nitrogen atoms in Earth's atmosphere. Plants incorporate a small amount of this carbon when they ingest carbon dioxide from the atmosphere; animals ingest the radioactive carbon when they eat plants or animals that have eaten plants. Over time, carbon–14 breaks slowly down into stable carbon–12. By measuring the ratio of carbon–14 to carbon–12 in a sample of ancient wood or other organic material, scientists can estimate how long ago the plants lived that took the carbon from the air. The method is valid for samples up to about 60,000 years of age.

The effects of changing eccentricity, changing axial tilt, and precession overlap to produce the cycles of Milankovitch forcing, which can be as short as a few tens of thousands of years to over a million years. As Earth's axis precesses, the Northern and Southern Hemispheres will be turned to face the sun at different times of year, that is, when Earth is at different distances from the sun. Ice ages will tend to occur when the Northern Hemisphere, where most of Earth's land area is located, gets the least sunlight in summer.

Impacts and Issues

Since the revival of the Milankovitch theory in the 1970s and its wide acceptance in following decades, many associations between orbital cycles and geological records of climate change have been found. At the same time, several nagging puzzles have persisted. For example, in 1992, what paleoclimatologists term the “causality problem” appeared—namely, the observation that at least some of the effects that were attributed to changes in Earth's orbital condition seem to have occurred before the orbital changes. The 1992 data came from sediments in a water-filled cave known as Devil's Hole in Nevada in the United States, where shifts in oxygen isotopes seemed to show that the beginning of an inter-glacial (warmer) period about 135,000 years ago began significantly before the orbitally forced increase in insolation that was supposed to have caused the warming.

As climatologists D. B. Karner and R. A. Muller stated in 2000, “The termination event appeared to precede its own cause.” However, they were not the only data indicating a causality problem, though they were the strongest at the time. Sea-level records from around the world collected in the 1990s reinforced the problem, agreeing with the Devil's Hole data and ruling out the possibility that the researchers handling that data had simply misdated it. Karner and Muller urged that the causality problem was indeed real, and that researchers had to “look at the data again, as if for the first time, regard climate to be multidimensional, and be open to new ideas unbiased by our prior theoretical prejudices.”

It is unlikely that orbital forcing does not have any influence on Earth's climate—far too many agreements between orbital forcings and various geological records have been found—but it does seem likely that other factors are also involved. For example, one well-known problem with Milankovitch theory is that the climate changes observed on the 100,000-year cycle are too strong to be explained by orbitally forced changes in insolation; that is, the changes in how much energy Earth receives from the sun are not large enough, by themselves, to explain the observed cooling and warming.

In 1997, R. A. Muller and G. J. MacDonald proposed the inclination hypothesis. On this theory, 100,000-year changes in Earth's inclination—a cyclic tipping back and forth of Earth's orbital plane—cause Earth to pass through a cloud of interplanetary dust particles. These particles, according to the inclination theory, block sunlight, cooling Earth. The theory is controversial. In 2004, G. Winckler and colleagues announced that they did not find increased amounts of interplanetary dust particles in ocean sediments on a 100,000-year cycle. Increased interplanetary dust was found on a 41,000-year cycle, but this cannot apparently be due to orbital changes, so the scientists theorized that it was probably due to changes in how sediments are deposited. They concluded that the inclination hypothesis is probably incorrect. Milankovitch theory and its various problems remain an active area of research in paleoclimatology.

The changes in Earth's climate caused by Milankovitch cycles occur over many thousands of years. The rapid global warming seen in recent decades is not caused by Milankovitch forcing, but by human releases of greenhouse gases.

See Also Ice Ages; Millennial Climate Oscillations.

BIBLIOGRAPHY

Periodicals

Crowley, Thomas J. “Cycles, Cycles Everywhere.” Science 295 (2002): 1473–1474.

Hays, J. D., et al. “Variations in the Earth's Orbit: Pacemaker of the Ice Ages.” Science 194 (1976): 1121–1132.

Karner, Daniel B., and Richard A. Muller.“Paleoclimate: A Causality Problem for Milankovitch.” Science 288 (2000): 2143–2144.

Muller, Richard A., and Gordon J. MacDonald. “Glacial Cycles and Astronomical Forcing.” Science 277 (1997): 215–218.

Winckler, Gisela, et al. “Does Interplanetary Dust Control 100 kyr Glacial Cycles?” Quaternary Science Review 23 (2004): 1873–1878.

Wunsch, Carl. “Quantitative Estimate of the Milankovitch-Forced Contribution to Observed Quaternary Climate Change.” Quaternary Science Review 23 (2004): 1001–1012.

Larry Gilman