How tofreeze a planet

During the Earth's early history the surface boiled with lava oceans and explodingvolcanoes, and although temperatures fell dramatically as prevailing geological processes moderated, our planet has been bathed in warmth for most of its 4.6 billion year history. Occasionally, however, a fortuitous combination of circumstances has heralded the formation of enormous ice sheets that have transformed a balmy paradise into a freezing hell. Artists' impressions and television documentaries have ensured that most of us are familiar with the last great Ice Age, when mammoths roamed the tundra and our pelt-covered ancestors struggled to eke out an existence from afrozen world. Only recently, however, have studies of ice-related rock formations around the world brought to light a far more ancient and much more terrible period of refrigeration; a time when our planet was little more than a frozen snowball hurtling through space. Long, long ago, during a geological episode that is becoming increasingly and appropriately referred to as the Cryogenian (after cryogen for freezing mixture), the Earth found itself at a critical threshold in its history. It had cooled substantially since its formation over three and a half billion years earlier and now the problem was keeping itself warm. At this time, between about 800 and 600 million years ago, the Sun was weaker and the Earth was bathed in some 6 per cent less solar radiation than it is now. Furthermore, the concentrations of greenhouse gases that are now heating up our planet-primarily carbon dioxide and methane — were not sufficiently high to hold back the bitter cold of space. Huge ice sheets rapidly formed and pushed towards the equator from both poles, encasing the Earth in a carapace of ice a kilometre thick. As the blind-ingwhite shell reflected solar radiation back into space, temperatures fell to -50 degrees Celsius and prospects for an eternity of ice seemed strong. But something must have happened to break the ice, as it were, otherwise I would not be here today to tell you about it; and in fact it seems that these 'snowball' conditions may have developed up to six times, succumbing each time to a return of warmer climes.

Just how the Earth managed to escape the clutches of the ice no one is quite certain, but it looks as if volcanoes might have been the saviours. After millions or even tens of millions of years of bitter cold, the enormous volumes of carbon dioxide pumped out by erupting volcanoes seem to have generated a sufficiently large greenhouse effect to warm the atmosphere and melt the ice. Extraordinarily, life came through this particularly traumatic period of Earth history bruised and battered but raring to go, and hard on the heels of Snowball Earth's final fling came the great explosion of biodiversity that marked the start of the Cambrian period 565 million years ago. Compared to the great freezes of the Cryogenian our most recent Quaternary ice ages come across as rather small beer. Nevertheless, although they affected smaller areas of the Earth's surface, these latest bouts of cold were crucial because they coincided with the appearance and evolution of our distant ancestors. Furthermore, they may yet have a role to play in the future of our race.

Duringrecent Earth history the Sun's output has been significantly higher than during the Cryogenian and the level of carbon dioxide and other greenhouse gases has also been higher. Why then, at the end of the Miocene period about 10 million years ago, did glaciers once again begin to form and advance across parts of the northern hemisphere? And more importantly, why, around 3 million years ago, did the southward march of the ice intensify? This remains a particularly

14 Snowball Earth: artist's impression of an ice-covered planet. During the Earth's early history, ice advanced from the poles to transform the planet into a great snowball

hot topic in the fields of Quaternary science and environmental change and a detailed analysis of competing theories is beyond the scope of this book. Suffice it to say that explanations for the twenty or so ice ages that have gripped the Earth during the last 2 million years include disruption of the planet's atmospheric circulation due to uplift of the great Himalayan mountain belt, and the drastic modification of the global system of ocean currents by the emergence of the Panama Isthmus.

Although one or both of these spectacular geophysical events may have contributed to a picture of increasing cold, the ice was already on the move, and we need to look elsewhere for the true underlying cause. What, in other words, turns ice ages on, and—just as importantly — what turns them off! This problem has intrigued scientists for many years and the solution was first put forward by the Scottish geologist James Croll as long ago as 1864 and expanded upon by the Serbian scientist Milutin Milankovitch in the 1930s. The Croll-Milankovitch astronomical theory of the ice ages proposes that long-term variations in the geometry of the Earth's orbit and rotation are the fundamental causes of the blooming and dying of the Quaternary ice ages. In order for an ice age to get going, the astronomical theory requires that summers at high latitudes in the northern hemisphere are sufficiently cool to allow the preservation of winter snows. As more and more snow and ice accumulates year on year, so the reflectivity or albedo of the surface is increased, causing summer sunshine to have even less impact and accelerating the growth of ice sheets and glaciers. But how are the northern hemisphere summers cooled down in the first place? This is where the astronomy comes in. Cooler summers at high latitudes result from a reduction in the amount of solar radiation falling on the surface, and this in turn depends upon both changes in the tilt of the Earth's axis andvariations in its orbit about the Sun.

If the Earth's axis was not tilted then we would not experience the seasons. During the northern hemisphere summer, for example, the North Pole is tilted towards the Sun, allowing more direct solar radiation to reach the surface in the northern hemisphere and raising the temperatures. In contrast, during the winter, the North Pole is tilted away from the Sun and the long, balmy days of summer are replaced by the cold and dark of a northern hemisphere winter. Now the southern hemisphere receives more direct sunlight with the result that those down under bask in warmth while the north shivers beneath gloomy skies. Although the tilt of the Earth's axis averages about 23.5 degrees, it is not constant. Like a spinning top, the Earth wobbles — or precesses--about its axis of rotation over a period of between 23,000 and 26,000 years. Furthermore, this wobble causes the amount of tilt to vary between 22 and 25 degrees over a period of 41,000 years. At times of least tilt, winters are actually milder, but more importantly, high latitudes receive less direct solar radiation and become cooler, making the survival of winter snows and the growth of ice sheets easier. On top of this there is another so-called astronomicalforczngmechanism that contributes to the onset of ice age conditions. Like all planetary bodies, the Earth follows an elliptical rather than a circular path around the Sun, whose shape varies according to cycles of between 100,000 and 400,000 years. At the moment the Earth's closest approach to the Sun occurs in January, when the North Pole is pointing away from the Sun, resulting in slightly colder northern hemisphere winters. Just 11,000 years ago, however, this closest approach—or perihelion occurred in July, giving a small temperature boost to northern hemisphere summers.

Before this gets too complicatedlet me try and draw things together. Regular and predictable cycles — known as Milan— kovitch Cycles—are recognized in the behaviour of the Earth's tilt and its orbit over periods of thousands to hundreds of thousands of years, and these cycles control the amount of solar radiation reaching the Earth's surface and therefore its temperature. At times, a number of cycles may coincide so as to depress summer temperatures at high latitudes to a degree sufficient to allow the accumulation of winter snows. On its own this could not result in the huge ice sheets that have dominated the northern hemisphere for much of the last few million years, but as the area covered by snow and ice grows, so more and more sunlight is reflected back into space, accelerating the cooling process. This — in essence — is how ice ages start. Conversely, at other times, the various cycles cancel one another out, the planetwarms as a result, and the ice sheets retreat to their polar fastnesses.

Although Milankovitch and later researchers who have addressed the issue have been able to explain the mechanics of the ice ages and their periodicity, they have been less successful in deciding why these icy episodes appeared on

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