The current and historical climatic records of planet earth are engraved in ice cores of glaciers and polar ice sheets, sea sediments, coral tree rings, cave rocks, landscapes and rocks of earth. Based on the study of these clues of paleoclimate, landscape modified by retreating/advancing glaciers and different ice ages, scientists found that these ice ages are connected to variations in the earth’s orbit (box 1). The effects of orbital fluctuation on earth’s climate are summarised by Serbian mathematician ‘Milutin Milankovitch’ in 1930 in his publication of “Mathematical Climatology and the Astronomical Theory of Climate Change”.

Milankovitch (1941) considered the changing seasonal (precession) and latitudinal (obliquity) patterns of incoming radiation to be critical factors in the growth of continental ice sheets and in the initiation of ice ages. He hypothesised that when axial tilt was small (large latitudinal temperature gradient), eccentricity was large and perihelion occurred during the northern hemisphere winter (warmer winters and colder summers), such a configuration would allow the persistence of accumulated snow throughout the summer months in the northern hemisphere. Additionally, the warmer winters and stronger atmospheric general circulation due to the increased temperature gradient would increase the amount of water vapour at the high latitudes available for snowfall. Based on the orbital variations, Milankovitch predicted that the ice ages would peak every 100,000 and 41,000 years, with additional “blips” every 19,000 to 23,000 years (Fig. 1)

The paleoclimate record shows peaks at exactly those intervals. Ocean cores showed that the earth passed through regular ice ages—not just the 3 or 4 recorded on land by misplaced boulders and glacial loess deposits—but 10 in the last million years, and around 100 in the last 2.5 million years. Evidence supporting Milankovitch’s theory of the precise timing of the ice ages first came from a series of fossil coral reefs that formed on a shallow ocean bench in the South Pacific during warm interglacial periods. As the ice ages came, more and more water froze into polar ice caps and the ocean levels dropped, leaving the reef exposed. When the ice melted, the ocean rose and warmed, and another reef formed. At the same time, the peninsula on which the reefs formed was steadily being pushed up by the motion of earth’s shifting tectonic plates. Today, the reefs form a visible series of steps along the shore of Papua New Guinea. The reefs, the age of which was well-defined because of the decaying uranium in the coral, measured out the millennia between ice ages. They also defined the maximum length of each ice age. The intervals fell exactly where Milankovitch said they would.

Ice cores and orbital fluctuation
When scientists started to analyse the paleoclimate evidence in the Greenland and Antarctic ice cores, they found that the record also supported Milankovitch’s theory of when ice ages should occur. But they also found something that required additional explanation: some climate changes appeared to have occurred very rapidly. Figure. 2 Interglacial and ice ages in Earth’s history (Robert and Alley 2004)

Because Milankovitch’s theory tied climate change to the slow and regular variations in earth’s orbit, the scientific community expected that climate change would also be slow and gradual. But the ice cores showed that while it took nearly 10,000 years for the earth to totally emerge from the last ice age and warm to today’s balmy climate, one-third to one-half of the warming—about 15 degrees Fahrenheit—occurred in about 10 years, at least in Greenland. A closer look at marine sediments confirmed this finding. Although the overall timing of the ice ages was clearly tied to variations in the earth’s orbit, other factors must have contributed to climate change as well. Something else made temperatures change very quickly, but what? Another theory for explaining such sudden climate changes is known as global ocean conveyer belt or global oceans circulation theory. The atmosphere and ocean work together to absorb heat and redistribute it from one part of the globe to another. Otherwise, the tropics would get hotter and hotter, and the polar regions would get colder and colder. The ocean circulates by surface currents that are driven mainly by the wind and by deep currents that are driven mainly by density contrasts in the water produced by temperature and salinity variations (Karen Grove 2001).