10-28-2004, 12:44 AM
WHAT CAUSES THE ICE AGES
Modern views as to the heavenly movements that influence the timing of the waxing and waning of the ice caps and the resulting rise and fall of the oceans, are still influenced by astronomical theories pioneered by the nineteenth-century glaciologists. These theories have been collected in the twentieth century under one name, the 'Milankovitch hypothesis' of ice ages.
The genius of Milankovitch:
Out of place, out of time
Milutin Milankovitch was a Serb. He was caught in the wrong country at the outbreak of the Great War in 1914 and interned. Luckily a friendly Hungarian professor had him paroled and moved from his cell to Budapest where he had access to the library of the Hungarian Academy of Sciences. Oblivious to the war, he continued his calculations and finally published his first set of predictions in 1920. The genius of Milankovitch lay in the correct combination of astronomical cycles and meticulous calculation. When he died in 1958, the theory was falling out of vogue partly because of various discrepancies between his predictions and geologists' findings. Since then, the older techniques of geologists, particularly the accuracy of carbon-dating, have been found wanting and the Milankovitch model has emerged triumphant, thus standing the test of time.
<i>A detailed description of the theory can be found elsewhere (see A.G.Dawson, Ice Age Earth, Routledge, London, 1992, chapter 13). But it is important to realise that frequent, apparently random, episodes of warming and cooling of the Earth can be explained to a great extent by the interplay of at least three celestial cycles, all running at different speeds. These cycles affect the warmth transmitted by the Sun to various parts of the Earth in a complex way. Of particular importance for the onset of glaciation is a decline in heat transmitted to northern temperate latitudes during summer with the resulting failure to melt last winter's snow. The amount of summer sun is controlled by three important heavenly cycles, which can be called respectively: the 100,000-year stretch, the 41,000-year tilt and the 23,000-year wobble.
Every year when the Earth circles the Sun, it moves alternately nearer and farther at different points of the circuit. This motion is called elliptical and the Sun lies to one end of the ellipse rather than in the middle. Over a period of approximately 100,000 years this ellipse stretches somewhat, and then shortens and fattens until it is nearly circular. The process is rather like taking a childâs hula-hoop and distorting it intermittently with two hands to make an ellipse. Over the cycle, the distance between the Earth and the Sun varies by as much as 18.26 million kilometres (11.35 million miles). Although the change in heat delivery over this cycle is relatively small, the effect on the Earth's climate is, for some reason, greater than with the other two mechanisms. At present the Sun's circuit does not particularly favour an ice age, but the onset of the next major glaciation can be predicted accurately from the cycles.
As we know the Earthâs axis of rotation is tilted at an angle to the Sun. The situation is rather like a top that will not spin upright. This is the reason for summer and winter, as the globe presents first its northern then its southern face to the Sun during one circuit. At present the tilt is about 23.5 degrees, but it varies between 21.5 and 24.5 degrees over a cycle of roughly 41,000 years. The greater the tilt, the more the seasonal imbalance in heat delivery from the Sun and the less the chance of ice remaining through the summer in temperate climates. At present we are near a neutral point between the extremes of this oscillation of tilt, thus neither favouring nor promoting an ice age.
The Earth is very like a childâs top in other ways. Not only does it spin at an angle of about 23 degrees to the Sun, but it also performs a slow pirouette as the sloped axis gyrates round itself. If you were a god accompanying the earth on its orbit round the Sun but perched up high looking down on the North Pole from directly above, you would see the pole performing a slow circle every 22,000 to 23,000 years. If you could see right through an imaginary glass globe to the South Pole you would see it performing the same circle 180 degrees out of phase. This spinning of the axis on itself is called axial precession, and all spinning tops can do it.</i>
<b>The effect of this precession is that the Earth slowly changes the face it presents to the Sun at different parts of the elliptical orbit. Precession does not change the angle of tilt, merely the direction of tilt. As a result, in the next 11,000 years or so, 21 June will become mid-winter in Europe and North America, and mid-summer in Australia. A fancy term for this slow ballet is the 'precession of the equinoxes'.</b>
At present, the Earth presents the Northern Hemisphere to the Sun (i.e. during summer) when it is at its furthest away from the Sun. Conversely, the Southern Hemisphere has its summer when it is nearest to the Sun. <b>Todayâs position of axial precession, therefore, actually favours glaciation in the Northern Hemisphere. There was a similar situation about 20,000 years ago at the height of the last great ice age, but then the position of the other two cycles happened to tip the balance towards glaciation. About 11,000 years ago the summers were warmer in the Northern Hemisphere, which should have favoured melting of the polar ice caps.</b>
The Milankovitch cycles are thus three elegant and stately celestial dances completely out of time with each other. They play out infinite yet predictable variations of heat stress on our planetary climate. In the last twenty years geologists and oceanographers have developed methods that enable them to measure indirectly, the past course and variations in the melting and freezing of the ice caps. The more refined these measurements have become, the better they fit Milankovitchâs model predictions of the waxing and waning of the ice ages of the past 2 million years
Modern views as to the heavenly movements that influence the timing of the waxing and waning of the ice caps and the resulting rise and fall of the oceans, are still influenced by astronomical theories pioneered by the nineteenth-century glaciologists. These theories have been collected in the twentieth century under one name, the 'Milankovitch hypothesis' of ice ages.
The genius of Milankovitch:
Out of place, out of time
Milutin Milankovitch was a Serb. He was caught in the wrong country at the outbreak of the Great War in 1914 and interned. Luckily a friendly Hungarian professor had him paroled and moved from his cell to Budapest where he had access to the library of the Hungarian Academy of Sciences. Oblivious to the war, he continued his calculations and finally published his first set of predictions in 1920. The genius of Milankovitch lay in the correct combination of astronomical cycles and meticulous calculation. When he died in 1958, the theory was falling out of vogue partly because of various discrepancies between his predictions and geologists' findings. Since then, the older techniques of geologists, particularly the accuracy of carbon-dating, have been found wanting and the Milankovitch model has emerged triumphant, thus standing the test of time.
<i>A detailed description of the theory can be found elsewhere (see A.G.Dawson, Ice Age Earth, Routledge, London, 1992, chapter 13). But it is important to realise that frequent, apparently random, episodes of warming and cooling of the Earth can be explained to a great extent by the interplay of at least three celestial cycles, all running at different speeds. These cycles affect the warmth transmitted by the Sun to various parts of the Earth in a complex way. Of particular importance for the onset of glaciation is a decline in heat transmitted to northern temperate latitudes during summer with the resulting failure to melt last winter's snow. The amount of summer sun is controlled by three important heavenly cycles, which can be called respectively: the 100,000-year stretch, the 41,000-year tilt and the 23,000-year wobble.
Every year when the Earth circles the Sun, it moves alternately nearer and farther at different points of the circuit. This motion is called elliptical and the Sun lies to one end of the ellipse rather than in the middle. Over a period of approximately 100,000 years this ellipse stretches somewhat, and then shortens and fattens until it is nearly circular. The process is rather like taking a childâs hula-hoop and distorting it intermittently with two hands to make an ellipse. Over the cycle, the distance between the Earth and the Sun varies by as much as 18.26 million kilometres (11.35 million miles). Although the change in heat delivery over this cycle is relatively small, the effect on the Earth's climate is, for some reason, greater than with the other two mechanisms. At present the Sun's circuit does not particularly favour an ice age, but the onset of the next major glaciation can be predicted accurately from the cycles.
As we know the Earthâs axis of rotation is tilted at an angle to the Sun. The situation is rather like a top that will not spin upright. This is the reason for summer and winter, as the globe presents first its northern then its southern face to the Sun during one circuit. At present the tilt is about 23.5 degrees, but it varies between 21.5 and 24.5 degrees over a cycle of roughly 41,000 years. The greater the tilt, the more the seasonal imbalance in heat delivery from the Sun and the less the chance of ice remaining through the summer in temperate climates. At present we are near a neutral point between the extremes of this oscillation of tilt, thus neither favouring nor promoting an ice age.
The Earth is very like a childâs top in other ways. Not only does it spin at an angle of about 23 degrees to the Sun, but it also performs a slow pirouette as the sloped axis gyrates round itself. If you were a god accompanying the earth on its orbit round the Sun but perched up high looking down on the North Pole from directly above, you would see the pole performing a slow circle every 22,000 to 23,000 years. If you could see right through an imaginary glass globe to the South Pole you would see it performing the same circle 180 degrees out of phase. This spinning of the axis on itself is called axial precession, and all spinning tops can do it.</i>
<b>The effect of this precession is that the Earth slowly changes the face it presents to the Sun at different parts of the elliptical orbit. Precession does not change the angle of tilt, merely the direction of tilt. As a result, in the next 11,000 years or so, 21 June will become mid-winter in Europe and North America, and mid-summer in Australia. A fancy term for this slow ballet is the 'precession of the equinoxes'.</b>
At present, the Earth presents the Northern Hemisphere to the Sun (i.e. during summer) when it is at its furthest away from the Sun. Conversely, the Southern Hemisphere has its summer when it is nearest to the Sun. <b>Todayâs position of axial precession, therefore, actually favours glaciation in the Northern Hemisphere. There was a similar situation about 20,000 years ago at the height of the last great ice age, but then the position of the other two cycles happened to tip the balance towards glaciation. About 11,000 years ago the summers were warmer in the Northern Hemisphere, which should have favoured melting of the polar ice caps.</b>
The Milankovitch cycles are thus three elegant and stately celestial dances completely out of time with each other. They play out infinite yet predictable variations of heat stress on our planetary climate. In the last twenty years geologists and oceanographers have developed methods that enable them to measure indirectly, the past course and variations in the melting and freezing of the ice caps. The more refined these measurements have become, the better they fit Milankovitchâs model predictions of the waxing and waning of the ice ages of the past 2 million years