Milankovitch Cycles vs. Greenhouse Gases: Global Warming Causes

Introduction: A Planet in Flux

Global warming is one of today's most pressing issues. Though some dispute its existence, the great majority of individuals — scientists and lay people alike — believe that the Earth is undergoing an unnatural warming. Hot summers, freak storms, melting glaciers, and other signs all seem to point toward a shift in the kind of weather enjoyed by many regions of the globe. Much of this change has occurred suddenly and rapidly, over the course of the last generation or so.

Scientists know that there have been periods of climatic change in the Earth's past. The fossil and geological records bear witness to these fluctuations. Areas of the world that are today desert were once lush grasslands. Regions now frozen in the grip of perpetual winter were at one time home to tropical rainforests. Seas were to be found where at the present time there is only dry land. Various animal species came and went as the environments to which they had been adapted disappeared.

Perhaps most graphic of all the great climatic changes to which the Earth has been subjected during its long history were the series of ice ages that covered much of the globe in glaciers. Vast sections of Canada, the United States, Europe, and Asia were buried beneath miles of ice and snow, home to specially adapted animals such as the woolly mammoth and rhinoceros. These great ice sheets retreated only in the geologically recent past. The planet warmed again only a few thousand years before human beings first began to plant crops and build cities. There have also been minor fluctuations in temperature over the past several millennia — it was a few degrees warmer during much of the Middle Ages, and a few degrees cooler than it is today for the ensuing five centuries or so. But what most disturbs today's scientists, environmentalists, and politicians are the sudden changes of the past third of a century.

Are we humans at fault? Are the greenhouse gases emitted by our cars and factories rapidly smothering the Earth? Or is it the fault of the Earth itself — the result of fluctuations in the planet's motions and orbit, as hypothesized by the great Serbian scientist Milutin Milankovitch?

Milankovitch's Theory of Orbital and Axial Variation

Educated at the Vienna Institute of Technology, and for most of his life a professor at the University of Belgrade, Milutin Milankovitch proposed a theory of planetary motion that attempted to explain the Earth's periods of warming and cooling. In particular, his theories offered an explanation for the ice ages. His studies, published in 1930 and 1938, maintained that the ice ages were caused by variations in solar energy reaching the Earth — variations that resulted from changes in the tilt of the Earth's axis, the precession of the Earth's axis, and the eccentricity of the Earth's orbit around the sun over time.

In other words, these variations in the Earth's orbit, combined with changes in the angle of the Earth on its axis, altered the amount of solar radiation that reached the globe's surface. It was as if one had lit a fire in the hearth of a large room: sit close to the fire and you are warmed; move back only a few feet and the air is significantly cooler. As explained by Professor Melvin Benard in Reasons for Seasons:

"As the earth rotates about its axis, the axis is tilted with respect to the sun… And this is the cause for the seasonal changes from winter to summer. But the axis also rotates, which occurs over a very long time cycle of 20,000 years; this axis rotation is called precession… There are wobbles or small variations in the axis precession, as the tilt slowly changes from 21.5° to 24.25° with a cycle of about 40,000 years."

As Benard's description illustrates, Milankovitch's theory explains not only severe climatic changes such as those represented by the ice ages, but also the seasonal variations in temperature experienced across much of the Earth's surface. After all, in simplest terms, an ice age is nothing but an extremely prolonged period of winter temperatures. If small changes in the Earth's precession occurring during each year can significantly affect temperatures for a brief period — a few months — then a more permanent or long-lasting change in precession should produce a climatic change of considerable duration, on the order of thousands of years or more.

Astronomical observations can help us chart these changes in the Earth's position. Indeed, as far back as the second century B.C., the ancient Greek thinker Hipparchus had noted alterations in the positions of the stars relative to the Earth. It was the observations of the French mathematician Jean le Rond d'Alembert that first suggested the idea that ice ages might be caused by the Earth's wobble as it circled the sun. D'Alembert reasoned that:

"The two components causing the precession of the equinoxes — the 26-ka wobble and the 19-ka [ka = 'one thousand years'] rotation of the elliptical orbit itself — shifted the positions of the equinoxes along Earth's orbit with a 22-ka cyclicity that would cause an ice age in whichever hemisphere experienced winter. Ice ages would occur every half cycle — that is, every 11 ka — and would grow in the Northern Hemisphere while decaying in the Southern Hemisphere, and vice versa."

Ice Ages, Ocean Cores, and the Evidence of Climate Cycles

Traditionally, it was believed that there had been four major ice ages during the Pleistocene Epoch, each followed by an interglacial period during which climatic conditions approximated those of present-day Earth. However, more advanced methods of measuring climatic change have resulted in a revision of this picture. It is now believed that there were at least seventeen distinct ice ages during the course of the past 1.6 million years, based on evidence from ocean cores.

By studying the remains of life forms found in both ocean cores and terrestrial cores, scientists can postulate what kinds of organisms existed at a given time, and from this data project what kind of climate prevailed. Even more revealing, oceanic core samples disclose the presence of oxygen isotopes in the microscopic organism Foraminifera. The quantity of two isotopes, O18 and O16, in Foraminifera is directly related to temperature change:

"During the glacial episodes, immense ice sheets of isotopically light ice (depleted in O18) accumulated in northern America and Europe. When this occurred, the oceans diminished in volume, became slightly more saline, and became isotopically more positive (i.e., enriched in O18). This enrichment is recorded in the isotopic composition of calcareous Foraminifera preserved in deep-sea sediments."

Most interesting of all — because it bears most strongly on arguments over global warming — is the astonishing discovery, made through isotopic and other advanced methods of analysis, that interglacial periods have actually been much shorter than glacial periods themselves:

"During the last 0.9 million years there have been nine episodes with global climate comparable to today's. In other words, there have been nine interglacials. The interglacials only constitute about 10 per cent of the time, and seem to have had a duration on the order of 10⁴ years, while a full glacial cycle seems to have lasted on the order of 10⁵ years. Conditions such as those we experience today have thus been relatively short-lived and atypical of the Pleistocene as a whole."

Clearly, this new information raises a very important question: is our current, increasingly warm climate the result of natural cycles, or is it the result of something else — namely, human interference? The belief that the Earth's climate is getting progressively and unnaturally warmer is derived from numerous studies based on generally accepted methods of measuring Earth surface and atmospheric temperatures. And though the methods of temperature measurement are not in dispute, the data they generate are open to interpretation. Consider the following study:

In the spring of 1998, a research group (Group A) analyzing data from weather satellites concluded that over a twenty-year period there had been a slight cooling of the upper atmosphere, rather than the slight warming inferred from surface measurements. However, a second group (Group B) reexamined the data and pointed out that the analysis failed to take atmospheric drag into account. Correcting for drag would place the satellite trajectory fifteen kilometers closer to Earth, effectively turning slight cooling into slight warming. Group A thanked Group B for pointing out the correction but was led thereby to reexamine the data themselves. They found that two further corrections — for orbital precession of the satellites and calibration drift in the radiometer — largely offset the effect of atmospheric drag. Group B appreciated this refinement but felt these effects were too small to change the conclusion that the troposphere is warming.

This episode perfectly illustrates the difficulties involved in any scientific experiment. Such discrepancies, however, are the mainstay of the anti-global-warming group. If the world's scientists cannot even agree on whether the climate is warming, the argument goes, then there cannot be much of a problem — or, conversely, if the Earth's climate has actually cooled in recent years, then greenhouse gases must not be having the adverse effect that so many claim.

Conclusion: Toward a Fuller Understanding

Milutin Milankovitch advanced an intriguing theory in regard to climate and climatic change. Supporters of the greenhouse gas and human pollution theory also possess a compelling argument to explain worldwide warming and cooling. In the end, it seems that we must first have a better understanding of weather itself, and a more complete picture of the Earth's climatic history, if we are ever to resolve the debate over global warming. Neither theory alone fully accounts for the rapid pace of contemporary climate change, and the scientific community's ongoing efforts to refine temperature measurements and paleoclimatic records will be essential to reaching any definitive conclusion.

Works Cited

Cronin, Thomas M. Principles of Paleoclimatology. New York: Columbia University Press, 1999.

Gelbspan, Ross. The Climate Crisis, the Cover-Up, the Prescription. Cambridge, MA: Perseus Books, 1998.

Goudie, Andrew. Environmental Change. 3rd ed. Oxford: Oxford University Press, 1992.

Halpern, Paul. A Scientific Exploration of the End of the World. Cambridge, MA: Perseus Publishing, 2000.

Kaufman, Yoram. "Milutin Milankovitch." Earth Observatory: On the Shoulders of Giants. National Aeronautics and Space Administration (NASA). URL: http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/.

Park, Robert L. Voodoo Science: The Road from Foolishness to Fraud. New York: Oxford University Press, 2000.

Parsons, Michael L. Global Warming: The Truth behind the Myth. New York: Insight Books, 1995.