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U.S. Geological Survey
The Margerie Glacier in Glacier Bay National Monument, Alaska, is a typical fast-moving mountain glacier.
The rivers that emerged after the retreat were not the same as those that had been there 100,000 years earlier, before the glaciation. The northern Missouri River, for example, drained northward into Hudson Bay, and what is now the upper Ohio flowed northeast into the Gulf of St. Lawrence. The lower Ohio drained into a now nonexistent river, which geologists have named the Teays.
In (a), the obliquity of the ecliptic (solid line), and the eccentricity of the orbit (dashed line) are shown. The dash-dot line gives the variation of the angle between perihelion and the position at vernal equinox, now about 90°, and going from 0 to 360° in about 20,000 years.
The variation of the average daily insolation from the values of the year 1950 is shown in (b), with 1 unit of the vertical scale corresponding to 25 watts per square meter.
Source: Adapted from A. Berger, 1977, Celestial Mechanics, Vol. 15, p. 53, and 1978, Quatenary Research, Vol. 9, p. 139. Reprinted with the permission of Macmillan Publishing Company, a Division of Macmillan, Inc. from Earth and Cosmos by Robert S. Kandel, Copyright 1980.
The primary purpose behind the global warming scare, is to convince you to accept the necessity of population reduction. There is no scientific evidence behind global warming, as the following article by Laurence Hecht, the first in an occasional series in the New Federalist, will show. This report, which first appeared in 21st Century Science & Technology magazine, Winter 1993-1994, was written shortly before Hecht, an associate editor of 21st Century magazine, began serving a 33-year sentence as a political prisoner in the state of Virginia, along with five other associates of Lyndon LaRouche.
Until the early 1970s, when climate science became ideology-driven, it was generally assumed that long-term astronomical cycles--those measured in tens, or hundreds of thousands of years, and described in this article--were the proper way to situate climate. No scientist who knew these astronomical cycles could be trapped into worrying about the ups and downs of local or global temperatures in time spans of years, or even decades, or be seriously concerned with short-term computer modelling and associated scare stories about global warming.
How then, have we come to a situation where an international climate treaty is on the table, buttressed by a ``consensus'' that contradicts the reality that, based on the last several million years of history, the world is inexorably moving into another Ice Age? We can look to the efforts of a leading Malthusian activist, Dame Margaret Mead, for the answer. Mead chaired a conference in November 1975 on ``The Atmosphere: Endangered or Endangering.'' Scientists who attended that conference warning about a coming Ice Age, such as Stephen Schneider, left the conference promoting global warming.
Mead told the assembled scientists:
``The unparalleled increase in the human population and its demands for food, energy, and resources is clearly the most important destabilizing influence in the biosphere. We are facing a period when society must make decisions on an planetary scale. Unless the peoples of the world can begin to understand the immense and long-term consequences of what appear to be small immediate choices: to drill a well, open a road, build a large airplane, make a nuclear test, install a liquid fast breeder reactor, release chemicals which diffuse throughout the atmosphere, or discharge waste in concentrated amounts into the sea, the whole planet may become endangered. What we need from scientists are estimates, presented with sufficient conservatism and plausibility, that will allow us to start building a system of artificial, but effective warnings, warnings which will parallel the instincts of animals which flee the hurricane....''Mead went on to say, that the point is to draw from people their capacity for ``sacrifice.'' In other words, scientists should create artificial but plausible stories that will sufficiently scare people, that they will give up their standard of living and modern industrial society--and in the process kill off half or more of the world's population. We are now in an ice age and have been for about the past 2 million years. Over the past 800,000 or so years, the Earth's climate has gone through eight distinct cycles of roughly 100,000-year durations. These cycles are driven by regular periodicities in the eccentricity, tilt, and precession of the Earth's orbit. In each of the past eight cycles, a period of glacial buildup lasting about 80,000 years has ended with a melt, followed by a roughly 10,000-year period--known as an interglacial--in which relatively warm climates prevail over previously ice-covered northern latitudes.
The present interglacial has already lasted beyond the 10,000-year average. One may thus suspect that a new period of glacial advance, a new ``ice age,'' is in the making. Whether it will take a few thousand years or a few hundred, or whether the process of glacial advance is already under way is difficult to say. Of one thing we are sure: The present hysteria over global warming--with its apocalyptic forecast of melting of the polar ice caps, flooding of the coastal cities, and desertification of the world's breadbaskets--is not helping citizens to understand the real and complex forces that shape the Earth's climate.
We do not wish to counter the global-warming hysterics with a new scare tactic of our own. Those interested in a scare story will have to look elsewhere. Nor will we concern ourselves here with refuting every wild conjecture put forward by the proponents of a global warming. Enough holes have already been poked in this ``theory'' (really only a conjecture) to cause honest scientists to exercise caution. [fn1] Rather, let us take a sober look at the long-term picture of Earth's climate that has been put together over centuries of careful work in the fascinating and challenging multidiscipline science known as paleoclimatology.
The idea of large-scale glacial motion was brought to the attention of modern science by a Swiss chamois hunter in the early 19th Century, who hypothesized that unusual striations in large exposed rocks had been caused by the pressure of a glacier that had since retreated up the mountain. Louis Agassiz, the Swiss paleontologist and associate of the famous Humboldt brothers, waged the fight to convince the scientific community of the truth of this hypothesis, beginning at a conference of the Swiss Society of Natural Sciences at Neuchatel in 1837.
Northern Hemisphere glaciers have been with us for approximately the past 2 million years, a short stretch on the roughly 4.6 billion-year scale of geologic time, in which our present era, the Cenozoic, occupies the most recent 50 million years. The Cenozoic era is divided into two periods, the Tertiary and Quaternary, the latter of which began about 2 million years ago with the onset of the glacial buildup. Within our present Quaternary period, there are two further subdivisions known as epochs. These are the Pleistocene, which began about 2 million years ago, and the Holocene (or, Recent) epoch, which is roughly 10,000 to 12,000 years old. (Some paleontologists argue quite cogently, that we are still in the Pleistocene and dispense with the designation of a Recent epoch.)
Currently, the greatest area of glaciation is the continental ice sheet of Antarctica (about 5.0 million square miles), which began its expansion about 5 million years ago. The largest Northern Hemisphere glacier is the Greenland ice sheet (about 0.8 million square miles). As the glaciation expands, most of the additional growth takes place in the Northern Hemisphere.
The whole of the last 2 million years, the Quaternary period, is considered an ice age, a relatively rare state of affairs in geologic history. But this long-term ice age has been marked by ebbs and flows in glacial extent. The work of the past two centuries in climatology, paleobiology, meteorology, astronomy, geology, geophysics, and many other fields has confirmed the existence of an astronomically-determined, cyclical pattern within the Quaternary ice age. Driven by well-defined cycles in the Earth's orbital orientation to the Sun, periods of roughly 100,000 years of generally advancing glaciation have been followed by short periods, of roughly 10,000 years' duration, in which the glaciers retreat. These two periods or subdivisions of the ice age are known as glacials and interglacials.
The 100,000-year periods are not one continuous downward slope of temperature and glaciation, but are modulated by roughly 20,000-year cycles, consisting of 10,000 years of cooling and glacial advance followed by 10,000 years of warming and retreat. But these shorter-term ups and downs of the glaciation curve tend to get cooler and cooler as the 100,000-year cycle advances (Figure 1).
The maximum extent of glaciation, the glacial climax of the last 100,000-year ice age, occurred just 18,000 years ago, at a time when human societies were already well established on the Earth. At that time, a huge continental glacier covered North America down through the northeastern states of the United States, reaching across the midwestern plains and up into Canada (Figure 2)). This most recent of the great continental glaciations is known in North America as the Wisconsin (in Europe as the Weichselian). Its southernmost limit extended across the middle of Long Island, through northern New Jersey, lower New York State, western Pennsylvania, Ohio, Indiana, Illinois, Iowa, then up diagonally through the northeast corner of Nebraska, into the Dakotas, and across the southern tier of the Canadian plains.
In more southerly regions, mountain glaciers also spread downward from heights in the Colorado Rockies, the Sierra Nevadas, and the Cascade Range. In western Europe, the glacier reached down from Scandinavia over northern Germany, Poland, and the Baltic nations. It reached deep into Russia and Ukraine south of Kiev, and eastward as far as the central Siberian Plateau. It stretched southwestward over the Netherlands and covered Ireland and most of the British Isles. A separate portion extended outward from the Alps and another one from the Caucasus Mountains in Asia Minor.
An Arctic climate thus prevailed over much of what are now the major population centers of western and central Europe and the United States. The weather over most of the remaining portions of the three northern continents was quite a bit colder than today's. But hunting was apparently good along the fringes of the continental glaciers, and man survived in these regions in a fairly primitive state, wearing animal furs for warmth and seeking shelter in caves.
The melting of the glaciers that had formed during the last 100,000-year ice age cycle took a long time, and the rate of melting was varied, with the North American Laurentide ice sheet being the last to retreat. If we date the beginning of postglacial (interglacial) times to a point roughly 10,000 years ago (c. 8000 B.C.), it is then useful to look at the climate, especially temperature trends, over this recent 10,000 years. Following a number of short-term oscillations beginning about 12,000 B.C., a rise in temperature that set in about 8300 B.C. led to sustained warm climates in the northern European lands formerly covered by ice. The maximum summer temperatures experienced in Europe, over the last 10,000 years, occurred in about 6000 B.C. Over North America, where the process of glacial retreat lagged somewhat, the maximum was reached by about 4000 B.C. That period is known as the Postglacial Climatic Optimum (or the altithermal period), when mean temperatures were about 1 degree Fahrenheit warmer than today.
Beginning about 3500 B.C., a sharp reversal known as the Piora oscillation set in, marked by advance of the glaciers in Europe and large-scale migration of agricultural peoples. From 3000 B.C. to 1000 B.C., the climate regained some of its former warmth but was apparently subject to recurrent fluctuations, particularly in rainfall. From 1000 to 500 B.C., the glaciers advanced again. In Europe the most marked change appears from 1200 B.C. to 700 B.C., coinciding with the Dark Age period that Homeric scholarship suggests occurred in Greek-speaking lands. In some places (Alaska, Chile, China) there is evidence that the cooling and re-advance of the glaciers began as early as 1500 B.C. [fn2]
A period of warmth and higher sea level came to Europe around the year 400 A.D., followed by another reversion to colder and wetter climates. This was again reversed, and there was a very warm period that culminated in Greenland about 900 to 1200 and in Europe 1100 to 1300. Known as the Medieval (or Little) Climatic Optimum, temperatures in this period became, briefly, nearly as warm as in the postglacial climatic optimum. As Lamb describes it: Oats and barley grew in Iceland. The limit of tillage in northern England, Wales, the Scottish highlands, in central Norway, and in high regions of central Europe was extended hundreds of meters up the hills and mountainsides. Mining operations were begun high in the Alps. Norse colonists were catching cod in the sea off western Greenland, and a regular northern sea route developed to North America.
This warming period, which ended as early as 1100 in parts of North America and later in Europe, was followed by a roughly 500-year period of severe cooling known as the Little Ice Age--the Klima-Verschleterung, or climate-worsening in the German literature. The low point of the cooling occurred from about 1550 to 1750, but extreme cold weather began earlier and ended considerably later in many parts. The Greenland colony, for example, died out not long after the year 1400. And in England, tent cities were set up, and Frost Fairs celebrated on the frozen river Thames as late as the winter of 1813-14. Some of the symptoms of the cooling as described by Lamb were:
There are two basic requirements for an ice age:
Although the causes that give rise to these two conditions are complex and far from perfectly understood, the recognition of their importance and of some of the basic mechanisms governing their genesis, dates to no later than the early part of this century. Subsequent advances in nearly all the physical sciences and the work of thousands of researchers in the many fields related to historical climatology have greatly enhanced our understanding and documentation of the climate record. But the big challenge, to understand climate well enough to be able to predict its future course, is still out of reach.
If the name of a single person were to be identified with the birth of the modern science of paleoclimatology, it would be one that is little known, even to many specialists in the field: Vladimir Köppen (1846-1940). The St. Petersburg-born meteorologist came from a German family that had settled in Russia during the reign of Catherine II. He began his study of natural sciences in Heidelberg in 1866 and received his doctorate in 1870 with a paper, published in Moscow, on the effects of heat on plant growth. After a brief period of work at the Central Observatory in St. Petersburg, Köppen came to the German Marine Observatory in Hamburg where he stayed for 44 years, becoming first the head of the weather service and then meteorologist of the observatory.
Köppen's list of publications numbers 526 items. Of these, probably the most important for today is the one he coauthored with his son-in-law, Alfred Wegener, in 1924, Die Klimate der geologischen Vorzeit (The Climates of the Geological Past). Alfred Wegener (1880-1930) is known to students of the earth sciences today as the father of the modern theory of continental drift. Wegener's now-famous theory was initially rejected by the science establishment, and became widely accepted only in the 1960s and 1970s, well after his tragic death on the Greenland glacier in 1930.
The two theories--continental drift and the determination of the ice ages by the cycle of solar insolation (the radiation experienced at the edge of the Earth's atmosphere)--had a common thread. In the minds of Wegener and Köppen they were really one grand conception. The first notion began with Wegener no later than 1910. It is recorded in a charming letter to his wife:
``Doesn't the east coast of South America fit exactly against the west coast of Africa, as if they had once been joined? The fit is even better if you look at a map of the floor of the Atlantic and compare the edges of the dropoff into the ocean basin rather than the current edges of the continents. This is an idea I'll have to pursue.'' [fn3]The idea itself was not new; it had been noted in Alexander von Humboldt's famous Cosmos, among other locations.
But Wegener had at his command the extensive researches of the previous century, which included data of both a geologic and paleobiologic sort, suggesting the possiblity that the continents had once been linked. The similarity of South American and African fossils and the close relationship of flora and fauna of many regions separated by oceans had already been noticed by investigators. One prominent attempt at an explanation was the hypothesis that land bridges had once existed, for example, connecting South Africa with southern South America, North Africa with Florida and the Caribbean, and so forth. Twenty years before Wegener, the great Viennese geologist Eduard Suess had proposed that the continents may have been linked together in one supercontinent, which he called Gondwanaland. The similarity in geological development of the continents of the Southern Hemisphere (including the Indian subcontinent), and their marked difference from those of the north, had already suggested some such link. But Suess was not sufficiently versed in these fields to recognize the paleobiological and climatological significance of his hypothesis.
Wegener drew on Suess's differentiation of the two major types of rocks sial (for silicon-alumina) and sima (for silicon-magnesia) that make up, respectively, the bulk of the crustal material of the continents and the ocean floors. The sial, which corresponds most closely to granite, has a specific gravity (a measure of its weight in comparison to an equivalent volume of water) of 2.7, while the sima, which is like basalt, is somewhat heavier at 3.0. Thus the lighter rock making up the continental crust could be thought of as giant blocks, floating somewhat like icebergs above the denser sima.
Wegener's hypothesis was first presented in Frankfurt-am-Main on Jan. 6, 1912, at the annual meeting of the Geological Association. The first book-length account, Die Enstehung der Kontinente und Ozeane (The Origin of the Continents and Oceans), appeared in 1915. Here and in his other early papers, Wegener was somewhat at a loss to explain by what mechanism the drifting apart of these blocks would occur. In 1929, he tentatively proposed the means by which the drift is today understood to occur, referring to the possibility of convection currents in the magma--the layer of molten rock on which the Earth's crust is thought to float. The high mountain ranges found near the edge of continents--the Alps, Himalayas, and the Cordilleras, which range from Alaska to southern Chile--were seen as produced by the crumpling up of layers of rock on the leading edge of the drifting continents, produced by forces similar to that of a bow wave. [fn4]
Together, these ideas condensed in the notion that the continental blocks had once been united in a single great continent, called Pangaea, and had subsequently drifted apart, taking up various configurations before arriving at the one we know today. In its details, the Wegener hypothesis also went a long way toward explaining some of the climatic anomalies in the fossil record and other paleobiologic evidence from widely varying places on the Earth. According to the modern reconstruction of the theory of drifting continents, only in the Permian period (250 million years ago), and the present Quaternary, does the placement of the continental land masses in the higher latitudes, allow for the buildup of the great glacial ice sheets.
The idea of a correlation between long-term changes in climate and the solar-astronomical cycles goes back to a hypothesis put forth in 1830 by Sir John Herschel, the son of the great astronomer Friedrich Wilhelm Herschel and a leading figure in 19th-Century British science. Herschel thought that the 21,000-year cycle of seasonal precession of the equinox might have a determining effect on climatic history. His hypothesis was taken up and elaborated first by the French mathematician J.F. Adhémar in 1842, and then, by the self-taught Scottish climatologist James Croll, beginning in 1860, who added into his calculations the cycle of change of the eccentricity of the orbit. However, at the end of the 19th Century, the exact periodicity and extent of this cyclical variable had not been precisely calculated. Croll was also hampered by his incorrect supposition that periods of ice buildup would coincide with the harshest winters.
It has since been deduced, that mild summers, in which the glacial advance of the previous winter's snow is not erased, are more important than the harshness of winter. Nevertheless, against great opposition, Croll defended the hypothesis first advanced by Herschel into the end of the 19th century. In 1910, when Köppen and then Wegener took it up again, it was neither a popular nor a widely accepted hypothesis.
But one man, Milutin Milankovitch (1879-1958), a skilled mathematician from the University of Belgrade, had independently begun his own investigation of the astronomical theory of climate. From 1911 until his first contact with Köppen in 1920, Milankovitch carried out painstaking calculations of the long curve of the variability of solar insolation (the amount of sunlight) at northern latitudes, in hopes of demonstrating its forcing effect on the ice age cycles. He published a few small papers on his work and then, in 1920, a book in the French language, The Mathematical Theory of Heat Phenomena Produced by Solar Radiation, which came to the attention of Köppen.
In that work, Milankovitch spelled out his theory of astronomical rhythms, carefully determining the effect of three major cyclical variables:
At the encouragement of Köppen, Milankovitch calculated the effect of the three astronomical cycles on Northern Hemisphere glaciation for 650,000 years into the past and 160,000 years into the future. This came to be known as the Milankovitch-cycle theory of climatic history. In a popular book published in Leipzig in 1936, Milankovitch described his theory and his close collaboration with Köppen and Wegener in the form of letters to an imaginary girlfriend, Durch Ferne Welten und Zeiten, (Through Distant Worlds and Times: Letters from an Ambler through the Universe). [fn5]
Like Wegener's theory of continental drift, the Milankovitch theory of astronomical cycles was not widely accepted by the scientific establishment. Nevertheless, a number of paleoclimatologists in America and Europe took it up and carried out pioneering work from the 1930s onward, which tended to corroborate the Milankovitch cycles. Much of this was in the field of paleobiology, examining core samples from various marine basins under the microscope, using innovative means of dating the biota and determining sea levels and temperature levels coinciding with the time of their formation.
Although Milankovitch was still fighting an uphill battle at the time of his death in 1958, today his general theory is widely accepted. Deep-sea core samples taken in the 1970s showed the Milankovitch 20,000, 40,000, and 100,000-year periodicities going back for 1.7 million years. The new work was reported in Science magazine in 1976 in a paper written by a team of researchers at Columbia University's Lamont-Doherty Geological Laboratory. [fn6] Somewhat ironically, the geology department at that university had been one of the staunchest holdouts against Wegener's theory of continental drift.
Dr. John Imbrie, who ran the computer programs analyzing the data, was the first to hypothesize that the evident periodicity was the Milankovitch cycles, which found that the 100,000-year cycle was predominant. (Milankovitch had expected that the 40,000-year cycle of the angle of obliquity would be the dominant one; it was for the periods before about 800,000 years ago. But since that time, for reasons not yet fully understood, the 100,000-year periodicity became dominant.)
Let PQ'AQ represent the elliptical orbit of the Earth around the Sun at S (Figure 3). Looking down upon the North Pole of the Earth, the orbital motion is counter-clockwise from P to Q'A, to A to Q and back to P again. We have exaggerated the ellipse in order to simplify visualization of the processes described. As the Sun sits at one focus of the ellipse, the distance from Earth to Sun is least when the Earth is at P, the position known as perihelion, and greatest at A, the aphelion.
Let us examine the change in the amount of solar radiation that will be felt as the Earth moves from aphelion to perihelion.
An ellipse is completely described by two parameters, the length of its semimajor axis, a, and the value of the eccentricty, e, which is the factor by which a is multiplied to find the foci. Measuring from the center of the ellipse (where the semimajor and semiminor axes cross), a focus is located at a distance ae along the semimajor axis. The eccentricity e is thus always a number between 0 and 1.
With this in mind, we see that the perihelion point, P, sits at a distance (a - ae) from the Sun while the aphelion, A, is at the distance (a + ae).
Now, since the intensity of light varies as the inverse square of the distance from the source, we can calculate the maximum variation of insolation between perihelion and aphelion, and the value 4e provides a very good approximation for this flux difference.*
The present value of eccentricity for the Earth's orbit is 0.017, and the variation in insolation thus comes to 0.068, or approximately 7 percent. But the orbital eccentricity is known to pass through a complete cycle in approximately 94,000 years, varying from near 0 (a circular orbit) to 0.07. At the latter value, the difference in insolation between aphelion and perihelion becomes 28 percent.
Now, the Earth is not simply a moving point, but a solid body of more or less spherical shape. It rotates about an axis that is inclined to the plane of the ellipse by a certain angle known as the angle of obliquity. It is this inclination of the Earth's axis, which is now about 23.5 degrees, that causes the main difference in temperature between polar and equatorial regions. The Sun's rays striking the Earth obliquely are forced to pass through a much greater thickness of atmosphere, thus dissipating their warming effect, than those rays that strike in a more perpendicular direction and are thus required to penetrate a lesser amount of the atmosphere. Even without that obliquity there would be some variation in temperature between pole and equator, because of the changing angle at which the parallel rays of the Sun will strike the circular arc that represents the Earth's surface (Figure 4). An increase in the angle of obliquity tends to exacerbate this effect.
Seasonal change, that is the yearly passage from spring-summer-fall-winter, is caused by the combined effect of the orbital inclination and the yearly revolution of the Earth around the ellipse. In the course of a year, the Earth's axis of rotation will point to the same approximate direction in the distant sky, no matter where on the ellipse we find ourselves (Figure 5). However, in one annual revolution around the Sun, the axis will take up all orientations with respect to the line perpendicular to the plane of the ellipse and passing through the center of the Sun, which is known as the pole of the ecliptic. When the Earth's spin axis is pointed 180 degrees away from the pole of the ecliptic (looking down on the ellipse from the direction of the North Pole), the Northern Hemisphere experiences its shortest day, known as the winter solstice. On the same day, the Southern Hemisphere experiences its longest day, the summer solstice. The opposite situation would arise at the position 180 degrees around the ellipse.
If the axis of the Earth had no motion of its own, the seasons would always occur at the same points in the orbit. But the direction in the sky, to which the Earth's axis of rotation points, varies on a cycle of approximately 26,000 years. In the course of that cycle, the spin axis makes a complete rotation around the pole of the ecliptic, one obvious consequence of which is a change in the pole star (Figure 6). Another consequence, which was noted by the ancient astronomers, was the long-period change in that constellation in which they observed the Sun rising on the day of the vernal (spring) equinox. Later comparison of the physical dynamics of this phenomenon to the precession of a spinning top (the wobbling as it winds down) led to the name precession of the equinox for the 26,000-year cycle.
As a result of this phenomenon, we must take into account where on the ellipse the winter and summer solstices occur. When the Earth is at P in Figure 6 and the axis is turned 180 degrees away from the Sun, we will have winter in the Northern Hemisphere. That was the situation in approximately the year 1250. Today we have moved a bit on the precession cycle and find the Northern Hemisphere winter occurring at roughly the position shown in (Figure 7).
In addition to the phenomenon known as precession of the equinox, the perturbations in the Earth's orbit caused by the motion of the other planets, most notably Jupiter, cause a phenomenon known as precession of the orbit, or advance of the perihelion. The result is that the complete cycle of return to the position where Northern Hemisphere winter occurs at P takes approximately 21,000, not 26,000, years (Figure 8).
Recalling that the most important astronomical requirement for glacial advance is a string of mild summers in which the winter snow buildup is not completely erased by melt, we are now in a position to examine how the orientations of the orbit might contribute to meeting this need. It might at first appear that the occurrence of Northern Hemisphere summer at A, combined with a relatively high eccentricity, would produce the most favorable conditions.
However, we have yet to take one other consideration into account. The rate of motion of the Earth in its elliptical orbit is not uniform. As Kepler was able to demonstrate, the planets move more swiftly when near to the Sun at position P than when at position A. It turns out that, although the planet will receive exactly the same insolation in all four quadrants, the same insolation is received over a longer number of days in the two larger quadrants, and its flux density per day is consequently less.*
If winter solstice occurs at P, climatologists call the two smaller quadrants caloric winter and the two larger ones caloric summer. One sees then that another way of describing the condition described above is to say that the summer is longer and milder (at least with respect to solar insolation) than winter. The difference in length between caloric summer and winter can be as great as 33 days. At the present time, the difference is 7 days. This will vary with the eccentricity, which, as we have mentioned, has a cycle of about 94,000 years.
As the position of the winter solstice moves around the ellipse, a pair of perpendicular lines drawn through the Sun will always describe the four seasonal positions. Thus it can be seen that a cycle of 21,000 years duration will be superimposed on the longer cycle of 94,000 years duration. Let us suppose, for example, that we begin at a point in time when the winter solstice is at P and the orbital eccentricity is at a maximum. The greatest excess in the number of days of caloric summer over winter will then be experienced, and consequently the lowest flux density of the summer insolation. Assuming the proper meteorological dynamics, this should be an ideal position for the rapid advance of glaciation.
Let the rotational axis then move through one-half of its 21,000-year cycle of seasonal precession--10,500 years--bringing us to the position where the winter solstice is occurring at A. As the eccentricity will have lessened by only about one-fifth of its greatest value in this position (its cycle of change is not perfectly linear), the Earth will now experience a most intense daily flux of solar radiation during the relatively brief caloric summer, creating ideal conditions for glacial melt. The winter, however, will be longer and colder than normal insofar as the solar flux affects it. The outcome is perhaps a toss-up. Half a precessional cycle later, winter solstice occurs again at P and the eccentricity is still relatively great. Conditions for glacial advance are again good.
It will only rarely be the case, however, that the ideal situation should occur, in which the maximum of eccentricity and a winter solstice at P take place simultaneously. Further, a third cycle, the one that Milankovitch thought primary, must be considered--the variation in the angle of obliquity over a 40,000-year period. When these added considerations are taken into account, a curve can be derived of the sort illustrated for various latitudes in (Figure 9). The close relationship between the variations of average daily insolation and the estimated variation in average temperature during the last 100,000-year-plus ice age cycle is seen.
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