Milankovitch weather cycles

According to the theory of the Milankovitch weather cycles, ice ages are cyclical, caused by changes in the earth's orbit. The theory was developed by Serbian geophysicist Milutin Milankovitch (1879–1958) in the 1930s, and it postulates that the amount of available sunlight in the northern hemisphere is affected by the earth's orientation in space. Because the earth is a globe in motion, sunlight strikes the earth differently depending on the following factors: The eccentricity of its orbit, which returns to the same point every 100,000 years; the tilt of the axis of its rotation, a 41,000-year cycle; and the precession of the equinoxes, a 23,000-year cycle. Milankovitch proposed that decreased sunlight prevents ice and snow from melting in the summer in the northern latitudes. This, in turn, cools the atmosphere , because ice reflects 90 percent of solar radiation back into space, and over a long period of time, the ice accumulates and moves south.

The theory has gone through its own cycles of acceptance and rejection. The first problem for proponents of the theory was to prove the occurrence of glacial-interglacial patterns during the Pleistocene era, the epoch preceding ours. It was also necessary to date the onset of each ice age in order to calculate the date of the next one. During the last fifty years, data collection improvements in astronomy and geology, especially in satellites and deep-sea sediment coring, have challenged and refined the theory. Supported first by astronomical and geological data, the theory encountered opposition in the 1950s when scientists used the newlydeveloped Carbon-14 method to date warm-era fossils in supposed ice-age deposits. But examination of fossilized microscopic sea creatures on the ocean bed, known as foraminifera, revealed two types, the larger of which flourished when the ocean was warm. In 1955, geologist Cesare Emiliani refined dating techniques of the foraminifera retrieved by deep-sea coring. He isolated oxygen isotopes. Oxygen 16, a lighter isotope , evaporates more quickly and becomes trapped in ice, while oxygen 18, which thrives in warm waters, was absorbed by the shells of the foraminifera. From the evidence of theses oxygen isotopes, Emiliani was able to reconstruct glacial-interglacial periods for 300,000 years. His date agreed with Milankovitch's orbital dates.

In 1971 John Imbrie of Brown University created Project CLIMAP, which coordinated all of the data relevant to Milankovitch's cycles. From this data, scientists were able to develop a "rosetta stone" for Pleistocene glacial-interglacial dates. In the 1980s, Project SPECMAP brought together all of the deep-sea core data. It was from this data that scientists predicted the next ice age was imminent.

But critics have argued that the Milankovitch orbital theory does not take into account the concept of chaos as well as the climatic influence of the heat transfer between the atmosphere and the ocean. In 1992, United States Geological Survey hydrogeologist Isaac Winograd challenged the Milankovitch theory when a scuba team with a submersible drill retrieved a 14-in (36-cm) core of calcite in a water-filled fault called Devils Hole in Nevada. The ice age dates for this core of calcite differed from the Milankovitch dates, and this suggested to Winograd that ice ages were tied less to orbital cycles than to an interaction of heat and moisture between the atmosphere, the ocean, and ice sheets. Imbrie and many others defended Milankovitch against these findings. They stood on their mountain of ocean-sediment core data, and they argued that the calcite core from Devils Hole reflected local rather than global changes.

Another problem for the Milankovitch theory is identifying the effects of human activity on the environment . The current interglacial period began 10,000 years ago, and according to Imbrie, it might become a "super interglacial" period due to the burning of fossil fuels and the subsequent greenhouse effect . Imbrie has argued that the natural cooling cycle which began 7,000 years ago will be postponed until the excess carbon dioxide is exhausted in 2,000 years. Then, after another 1,000 years, serious cooling will set in, expected to last for 23,000 years.

See also Climate; Glaciation; Ice age refugia; Ozone layer depletion; Radiocarbon dating

[Stephanie Ocko ]

RESOURCES

BOOKS

Imbrie, J., and K. P. Imbrie. Ice Ages: Solving the Mystery. Short Hills, NJ: Enslow, 1979.

PERIODICALS

Kerr, R. A. "Milankovitch Climate Cycles Through the Ages." Science 235 (February 27, 1987): 973–4.

Winograd, I. J., et al. "Continuous 500,000-Year Climate Record from Vein Calcite in Devils Hole, Nevada." Science 258 (October 9, 1992): 255–260.

OTHER

Berger, A., et al. Milankovitch and Climate. Proceedings of the NATO Advanced Research Workshop on Milankovitch and Climate. Dordrecht: D. Reidel Publishing, 1982.