Milankovic produced a curve demonstrating the variation in intensity of summer sunlight over the past 600,000 years. Once established, the curve enabled scientific exploration of past and future relationships between solar radiation, eccentricity, and climate.

Milutin Milankovi6 is credited with developing the concept of astronomical cycles that affect global climate and the timing of ice ages. In December, 1904, he received a doctorate of technical science from the Vienna University of Technology. Late in 1909, he was offered a professorship at the University of Belgrade, where for almost a half century, he would lecture on rational mechanics, theoretical physics, and celestial mechanics. Around 1915, he began to study the astronomical explanation for the Pleistocene epoch ice age. This work examined long-term solar radiation at various latitudes and seasons to determine the influence of the astronomical input to Earth's climate. The research was completed in the 1930's, culminating in the 1941 publication of Kanon der Erdbestrahlung (Canon of Insolation and the Ice Age Problem).

Milankovi6 proposed that if incoming solar radiation cyclically varies, then those variations of solar energy may be correlative with glacial and interglacial ages, which may arise from the orbital geometries between the Sun and Earth. About every 100,000 years, Earth's orbit around the Sun changes from a near-circular orbit to a slightly elliptical orbit, a variable known as eccentricity (Eyles, 2007). Circular orbits are said to exhibit low eccentricity (around 0); more elliptical orbits have a high eccentricity (around 0.07).

An additional orbital cycle is the tilt (obliquity) of Earth's rotational axis relative to the ecliptic. The tilt varies from 22œ to 25œ over a period of forty one thousand years and increases seasonal cycles at high latitudes, whereas lower latitudes undergo a reduced seasonal effect. The greater the obliquity, the more warmth high latitudes receive in summer, and the less they receive in winter. Only the eccentricity cycle affects the amount of solar radiation reaching Earth, so when Earth's eccentricity is at a maxim (causing less solar input) and its obliquity is at a minimum (resulting in less warmth in high latitudes), Milankovi6 proposed that the onset of glacial conditions would occur.

An additional cyclic variation is the precession of the equinoxes, which has two components that regulate the seasonal change of the distance between Earth and the Sun. One component is elliptical precession, which relates to Earth's rotation around one of the foci of the orbit, having a periodicity of twenty-six thousand years; the other component is the "wobble" of the Earth on its rotational axis. Recall the slowing motion of a stationary top: It spins around its rotational axis, but the top of the top also wobbles (orbits). The effects of combining these two components of precession are present at the low latitudes (with periodicities of nineteen thousand and twenty-three thousand years).

With these three major variations in operation, the essence of Milankovi6's theory is demonstrated: Cooler summers would retard the melting of winter snow and ice; in turn, the relative mildness of winter at low latitudes would lead to sizable evaporation, which would produce abundant snowfall at middle and high latitudes. As snow and ice accumulate on land and remains, the icy surface reflects more energy back into space; over time, this snow and glacial ice can develop into continental ice sheets and mountain glaciers. Also, due to reduced insolation in the cooler, middle- and higher-latitude oceans, less greenhouse gas (GHGs) is available. Less GHG allows more long-wave radiation to leave the Earth, further cooling the planet. Until the 1960's, Milankovi6's astronomical explanation for the presence or absence of glaciers was disputed because of a lack of geologic evidence.

However, a complex study using climate-sensitive microorganisms in deep-sea sediments was undertaken to establish a chronology of temperature changes over the past one-half million years. The microorganisms used were foraminifera ("forams"), a protozoa that secretes a shell and whose shell, upon death, is deposited on the ocean floor, mixing with other sediments. The foram shell contains specific percentages of oxygen 18 (O18) and oxygen 16 (O16). Since ordinary oxygen (O16) is atomically lighter than heavyO18, during evaporation of ocean water O16 would go into ice formation in ice sheets and O18 would remain in ocean waters. Thus, when forams with shells rich in O18 were plentiful in ocean sediments, Earth was in a phase of glaciation.

Using isotope analysis, temperature variations over time can be compared (Berger, 2002). The established climate time scale was then compared to the astronomical calculations of eccentricity, obliquity, and precession to determine whether there was a correlation.

The conclusion of Milankovi6's mathematically complex research is that major variations in climate are closely linked to periods of obliquity, precession, and orbital eccentricity. Research into future climate regimes investigates the relationship between solar radiation and eccentricity. Studies have demonstrated that for the next twenty-five thousand years, insolation will increase only 25 watts per square meter as received at 65œ north latitude in June. Eccentricity will approach 0 for the same period, which will subdue the variations of precession. With these values (which do not consider anthropogenic changes to climate), the present interglacial cycle may continue into the future for twenty-five thousand years. Climate modeling that considers CO2 input from anthropogenic sources and insolation variation over the next 100,000 years suggests that CO2 concentrations over 220 parts per million volume will lead to protracted interglacial periods--about fifty thousand years into the future. An overview of most climate models confirms that future global climates will be similar to the warmest portions of the last few tens of millions of years. If the models are correct in predicting long, warm interglacial periods, the additional heat of global warming may strongly modify Earth processes and severely stress living organisms.

References

1. Berger, A., and M. F. Loutre. "An Exceptionally Long Interglacial Ahead?" Science 297 (August 23, 2002): 1287-1288.

2. Eyles, Nick, and Andrew Miall. Canada Rocks: The Geologic Journey. Markham, Ont.: Fitzhenry & Whiteside, 2007.

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