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From Milutin Milankovitch to Greenhouse Gases
Global warming is one of today's most pressing issues. Though some dispute its existence, the great majority of individuals - scientists and lay people - do 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, the home of specially-adapted animals such as the wooly mammoth and rhinoceros. These great ice sheets retreated only in the geologically recent past. The planet warmed up again only a few thousand years before human beings first began to plant crops and to build cities. As well, there have 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 are 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?
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 the variation in solar energy reaching the earth that was a result of changes in the tilt of the earth's axis, precession of the earth's axis, and eccentricity of the earth's orbit about the sun over time.
In other words, these variations in the Earth's orbit around the sun, 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's, 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.
So as described by Benard, Milankovitch's theory explains not only severe climactic changes such as those represented by the ice ages, but also even the seasonal variations in temperature that are 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 the course of each year can significantly affect temperatures for a brief period of times - a few months - then a more permanent, or long lasting change in precession should have the effect of producing a climatic change of considerable duration, say, of thousands of years or more. Astronomical observations can help us to 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 a French mathematician, Jean le Rond d'Alembert, that resulted in the first suggestion of the idea that ice ages might be caused by the Earth's wobble as it circled the sun. D'Alembert reason 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.
Traditionally, it was believed that there had been four major ice ages during the Pleistocene Epoch. Each of these had been 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 the story. It is now believed that there were at least seventeen distinct "ice ages" during the course of the past 1.6 million years. This is based on the evidence of ocean cores. By studying the remains of life forms found in both ocean cores and terrestrial cores, scientists can postulate the kind of organisms that existed at a given time, and from this data project what kind of climactic type prevailed. Even better, the oceanic core samples reveal the presence of oxygen isotopes in the microscopic organism Foraminifera. The quantity of these 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 is bears most strongly on the arguments over global warming, is the astonishing discovery - through the means of the isotopic and other advanced methods of analysis - that the interglacial periods have actually been much shorter than the 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 of the order of 104 years, while a full glacial cycle seems to have lasted of 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 i.e. human interference? The belief that the climate of the Earth is getting progressively warmer - unnaturally warmer - is derived from numerous studies that are themselves 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 that they generate are open to interpretation. Witness 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 has 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. That would put the satellite trajectory fifteen kilometers closer to Earth, which had the effect of turning slight cooling into slight warming. Group A thanked group B. For pointing out the correction but were 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 latest refinement but felt these effects were too small to change the conclusion that the troposphere is warming.…[continue]
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