Researchers have generated a lot of theories to explain the mysteries surrounding the mammoths, but questions about the cause of the Ice Age have also spawned a multitude of ideas. Ice Age theories can be divided into two general groups. One group is extraterrestrial — something happened in our solar system or the Milky Way galaxy or even on the sun to start an Ice Age. The second group of theories is terrestrial. They propose that something in Earth’s complex climate changed to start an Ice Age.
Not until the mid 1800s did the scientific world finally accept that an Ice Age actually occurred. At that time, the cause of the Ice Age was first ascribed to a loss of light from the sun. The temperature of the earth is controlled by the amount of sunlight that reaches the earth. Differences in sunlight cause the cold polar and warm tropical regions. These differences in temperature power the earth’s wind system and storm movements. The theory goes that if the power of the sun could somehow be reduced, temperatures in the higher latitudes would drop and lead to an Ice Age.
However, no one knows whether the amount of sunlight changed in the past. No one was around to observe such a change, and observations are necessary for an idea to be verified as a scientific theory. It is true that the intensity of the sun does change a little, depending upon the number of sunspots, but the change is very small. As far as anyone knows, the sun has maintained a nearly constant intensity throughout Earth’s history. In meteorological jargon the assumed reliability of the solar output is called the solar constant.
Even if there were less sunlight in the past, it would not necessarily lead to an Ice Age. Colder temperatures are less able to hold water vapor. Therefore, the amount of rain or snow would be reduced, not increased.
Other scientists have proposed that the solar system moved through a dust cloud to cause an Ice Age. They suggest that perhaps this dust cloud was a dirty arm of the Milky Way galaxy. The dust between the sun and earth would block out some of the sun’s rays and lead to a buildup of ice. Unfortunately, this theory also suffers from a lack of evidence and also does not provide for the copious amount of snow needed to produce an Ice Age.
Many scientists today are concerned about the buildup of carbon dioxide in the atmosphere since more carbon dioxide brings a greenhouse effect. Increased carbon dioxide in the atmosphere absorbs more infrared radiation from the earth, heating the lower atmosphere. Conversely, if there were less carbon dioxide in the past, the climate would become cooler. Scientists believe there was about 30 percent less carbon dioxide during the Ice Age than there was at the start of the industrial revolution. This is based on measurements of carbon dioxide trapped in small air bubbles in the Greenland and Antarctica ice sheets.
Other greenhouse gases, such as methane, have increased substantially more than carbon dioxide, but they have a weaker effect on greenhouse warming. It is convenient to consider these other greenhouse gases in terms of carbon dioxide, so scientists have transposed these other greenhouse gases into carbon dioxide equivalents. The net effect of these other greenhouse gases is similar to increasing carbon dioxide another 30 percent. Thus, “greenhouse” gases have increased 60 percent since the industrial revolution. However, this has produced at most only a 1°F (0.6°C) global temperature rise. It is likely that part of this warming was not caused by the increase in greenhouse gases but due to other causes,1 but let us assume it was. Reducing carbon dioxide by 30 percent during an Ice Age would probably produce a less than one-degree drop in temperature — definitely not enough to start an Ice Age.
Mountain building can be used to explain the cold climate of an area. It is possible mountain building could have initiated an ice sheet. It is well known that as a person ascends a mountain, the temperature cools. It is also well known that mountains receive much more rain and snow than the adjacent valleys. So, as the theory goes, if the land were higher, the temperatures would be colder and the snowfall greater.
However, although the mountainous regions of North America, Europe, and Asia today are high, they have very few glaciers, and they do not have an ice sheet covering them. Another problem is that the ice sheet in North America developed over the low areas of northeast Canada. Another glitch is that the altitude of the land in the United States from North Dakota to Maine is not very high. So postulating that mountain building in the past caused an Ice Age does not help at all. Besides, the high mountains are still with us, and there are no ice sheets covering northern North America, Europe, and Asia.
Another ingenious Ice Age theory is that if the sea ice melted over the Arctic Ocean, the increased evaporation would provide the needed high latitude moisture. Due to the snow and ice buildup, the climate would then cool and the Arctic Ocean would refreeze and reverse the buildup of ice. The cycle would continuously repeat itself. This theory provides an explanation for the many ice ages that scientists claim took place in the past 2.5 million years of geological time.2
The theory has merit in that it focuses on the necessary moisture that most theories neglect. It is likely, however, that the added moisture would be insufficient to begin an Ice Age. It is true that increased evaporation from an iceless Arctic Ocean would provide more snow in northern Canada and Eurasia. However, the heat transmitted to the atmosphere from the water during the winter would most likely keep the continents too warm during the summer for snow and ice to build up. The theory fails to account for the tremendous summer cooling required for an Ice Age. Moreover, no one mentions how the Arctic Ocean ice could have melted, or whether there is any evidence of it having happened in the recent past. In the standard view, the ice on the Arctic Ocean has not melted for at least the past supposed million years of geological time.
Other theories involve increasing volcanic dust, trapped in the upper atmosphere, blocking some of the sunshine and, therefore, causing cooler temperatures and an Ice Age (figure 6.1). This theory also has merit because volcanic dust and gases do bring cooler temperatures. The problem is that each supposed ice age lasts 100,000 years. Volcanic dust and gases, on the other hand, fall out of the stratosphere in one to several years. An inordinate amount of volcanism would be required to sustain cold summer temperatures throughout such a long ice age.
One desperate theory has the West Antarctic ice sheet slipping from its undersea moorings and out into the deep ocean. As it floats around in the southern ocean, more sunlight reflects back into space and cools the earth. However, displacing the West Antarctic ice sheet from its current location could hardly change by much the amount of sunlight already reflected back to space. Moreover, if the snow and ice did somehow increase in the Southern Hemisphere by this mechanism, it would have little impact in the Northern Hemisphere. The two hemispheres generally act independently of each other with little exchange of heat or moisture between them.
A theory that gives up on most other theories is that an ice age is simply caused by chance fluctuations in the climate. Since small climate changes occur on short time scales, it is supposed that large climate changes occur over long periods of time. This assertion is backed up by sophisticated mathematics. The plausibility of this theory, however, is open to serious question. According to what is generally believed about the ice ages, ten ice ages waxed and waned regularly every 100,000 years. This defies the rules of chance. Since the theory cannot be tested, it does not qualify as a scientific theory.
Scientists abhor a theoretical vacuum. It has been difficult finding even a somewhat plausible theory to explain even one ice age. Adding to the difficulty, geologists became convinced early on that there were many ice ages. So, a mechanism that could produce more than one would be favored. One particular theory out of the many has recently become popular since the 1970s. It is called the astronomical or Milankovitch theory of the Ice Age. It is not new; meteorologists had previously rejected it more than once (see The astronomical theory of the ice ages below).
When a phenomenon cannot be explained by existing data, the theories multiply. In 1968, in a volume on the causes of climate change, Erik Eriksson counted over 60 theories on the cause of the Ice Age. Although many have merit, each has fatal flaws. After a lifetime of studying the Ice Age, J.K. Charlesworth3 commented on the status of all these theories, including the astronomical theory:
Pleistocene [Ice Age] phenomena have produced an absolute riot of theories ranging “from the remotely possible to the mutually contradictory and the palpably inadequate.”
That is not saying much for Ice Age theories; Charlesworth is essentially saying that all these theories are mammoth failures. Twenty-two years later, in 1979, Brian John, reminiscing on Charlesworth’s words, relates that the situation has not improved. In fact, he says that it is worse: “Things have become even more confused since then …”.4
Many people believe the earth travels around the sun like clockwork — never changing. However, it has been discovered that the earth’s orbit around the sun does change a little. Its path transforms from a circle to a slightly flattened circle, called an ellipse, and back again to a circle. It would take 100,000 years or so for each cycle.
Evolutionary scientists have extrapolated the eccentricity millions of years back in the past (see figure 6.2). The difference of the earth’s orbit from a circle to an ellipse is called the eccentricity. An eccentricity of zero is a circle.
The earth’s orbit itself revolves around the sun. This is especially noticeable when the orbit is an ellipse. Such a cycle is hard to visualize. Think of it as an elliptical path around the sun, and that the path slowly rotates around the sun. The orbital path would make one rotation about every 22,000 years and is called the precession of the equinoxes. In the current orbit of the earth, the sun is closest in January and farthest in July (figure 6.3). In about 11,000 years, the sun will be closer to the earth in July and farthest in January.
Many have been taught that the 23.5-degree tilt of the earth’s axis with respect to the earth’s orbital plane about the sun never changes and causes the seasons. It is true that this tilt causes the seasons. However, the tilt also changes a little with time. It wobbles back and forth from 22.1 degrees to 24.5 degrees and back to 22.1 degrees. A full cycle would take 40,000 years, assuming there were no other forces.
The slight gravitational pull of the moon and planets on the earth cause all these cyclical changes of the earth’s orbit. The variations are small with correspondingly slight changes in the amount of sunlight around the globe (figure 6.3). The change in solar radiation caused by all three orbital variables is shown in figure 6.4. Scientists postulate that a decrease in the sunlight at higher latitudes in summer, caused by changes in the earth’s orbital geometry, would cause an ice age. Alternatively, an increase in sunlight in the summer would cause an ice sheet to melt. Since the above changes in sunlight are cyclical processes, it would favor multiple ice ages repeating in a regular fashion — an attractive concept.
James Croll first proposed the astronomical theory in the late 1800s. It helped persuade scientists to believe in many ice ages as opposed to just one. According to the theory in the late 1880s, the last ice age ended about 70,000 years ago. Scientific evidence was marshaled to “prove” that this termination time was true. But, the astronomical theory was not well developed until the 1920s and 1930s, when Milutin Milankovitch, a Serbian meteorologist, worked out many of the details with more precision. According to the revised Milankovitch theory as the astronomical theory is often called, the Ice Age peaked about 18,000 years ago. Data, once again, were tweaked to “prove” this time for Ice Age maximum. Soon after Milankovitch refined the theory, it came under a withering barrage of criticism, mostly from meteorologists. It was quickly discarded in the 1950s and 1960s.
It is somewhat common in the history of science for a discarded theory to make a comeback.6 This has proven true with the Milankovitch theory. New technology applied to deep ocean-floor sediments and the persistence of several prominent scientists have revived the theory. Based on properties of the ocean-bottom sediments, oceanographers concluded there have been over 30 distinct ice ages that have repeated regularly, each completely melting during what is called an interglacial period. Some even consider the mystery of the Ice Age as solved.7
Despite the enthusiasm of most scientists toward the astronomical theory, it has a number of serious, most likely fatal, flaws. The changes in summer sunshine at high latitudes postulated by the theory are too small to generate an ice age. Heating at higher latitudes depends only partially on the amount of sunshine. Northward transport of heat by the atmosphere and ocean currents are also important, but mostly neglected by proponents of this theory. Heat transport would lessen the cooling effect caused by reduced sunshine. Between heat transport and the already small effect of reduced sunshine, the cooling would be negligible.
Meteorologists have known about this weakness in the theory for a long time. It contributed to its earlier demise. Famous astronomer Fred Hoyle8 expressed his sentiments for the Milankovitch theory by saying:
If I were to assert that a glacial condition could be induced in a room liberally supplied during winter with charged night-storage heaters simply by taking an ice cube into the room, the proposition would be no more unlikely than the Milankovitch theory.
The “night-storage heaters” are the other processes that supply heat to higher latitudes, while the ice cube represents the slight cooling due to the astronomical theory.
Data from oceanic sediments supposedly show that the 100,000-year eccentricity cycle is the most important cycle for repeating ice ages. This particular cycle, however, is the smallest of the three orbital variations by far. It causes almost no change in the summer sunshine at higher latitudes. Scientists are greatly perplexed and have been shopping around for a secondary mechanism to boost their theory.
Another serious problem is that the ice age cycles supposedly occur at the same times in both the Southern and the Northern Hemispheres. But the decreased sunlight caused by the precession of the equinoxes generally alternates between hemispheres, as shown in figure 6.4. When the Northern Hemisphere has a slightly lower intensity of sunlight during the summer, the Southern Hemisphere has an increased intensity of summer sunlight. Since the two hemispheres are generally separated climatologically, why the supposed ice ages and interglacials would cycle in phase has never been answered.
With so many scientific objections to the astronomical theory, one may ask why it is so popular. I believe it is because the apparent statistical matches from deep-sea cores have swayed most scientists. There are many problems, however, in relating properties of deep-sea cores to the astronomical theory. Accurate dates are needed, but the dating methods are not all that accurate. Unfortunately, the numbers that make up the statistics seem to have a high degree of interpretation mixed into them. One never knows how much is interpretation and how much is fact. Another reason for the popularity of the theory is that such a dramatic event as the Ice Age of the recent past demands an explanation and any explanation is better than none — even if it is the weak astronomical theory.
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