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ICE AGE | Blowing Hot and Cold:

The True Nature of the Ice Age

The Ice Age was punctuated many times by interglacials, which could be even warmer than today

Popular representations of the Ice Age all seem to conjure up the same vista: bleak, unremittingly cold landscapes filled with snow and ice, populated by weary-looking, shaggy-coated giants lumbering from one barren province to the next. However, in truth, the Pleistocene Ice Age was a far more complex and dynamic period in the Earth's history, with much less snow than you might expect, even during its coldest depths, while at times it could be as warm, if not warmer than it is today.

The Pleistocene Ice Age, then, should not be envisaged as one monolithic block of unchanging conditions, with a descent into brutal cold at the very beginning just under 2.6 million years ago and an emergence into warmth only 11,700 years ago. In reality, the Ice Age was made up of many cold periods, called glacials, when average global temperatures could be around six degrees Celsius lower than today, punctuated by periods of warmth called interglacials, when average global temperatures could be the same as, or even a little higher than, those of our modern world.

The switch from glacial to interglacial happened many times during the Pleistocene, with perhaps 20 or so interglacials occurring over the course of the Ice Age. However, even this glacial/interglacial division makes the temperature fluctuations of the Ice Age sound more simple than they actually were, as glacials were not just periods of relentless cold and interglacials periods of continuous warmth. In reality, both glacials and interglacials could include shorter episodes of warmth, called interstadials, and episodes of cold, called stadials.

This complex sequence of temperature switches during the Ice Age is thought to have been due to changes in the Earth's position relative to the sun over time, something known as the Milankovitch Cycles. Milutin Milankovitch was a Yugoslavian mathematician whose painstaking research over three decades in the earlier twentieth century showed how changes in the shape of the Earth's orbit and the tilt of its axis could be responsible for the growth and retreat of ice sheets as they altered the amount of solar radiation received by the planet.

It is easy to see how such changes could cause switches from warm to cold when we consider the changing seasons in our modern world. These are simply due to the tilt of the Earth's axis relative to the sun determining whether the Northern Hemisphere or the Southern Hemisphere is receiving the most solar radiation. At that point in the Earth's orbit that the Northern Hemisphere is tilted towards the sun, it is summer there and winter in the Southern Hemisphere, while the opposite is true when the Southern Hemisphere is tilted towards the sun.

Simple diagram depicting how the Earth's position relative to the Sun determines the seasons

So, just as the tilt of the Earth's axis determines the changes from warm to cold on the planet during the course of a year by changing the amount of solar radiation received, so do the Milankovitch Cycles determine the changes from warm to cold over much longer periods in the same way, by changing the position of the Earth relative to the sun.

There are three main factors in the Milankovitch Cycles which change according to their own regular timing pattern, and the two with the shorter cycles both relate to features of the Earth's axis. For instance, we saw in the last post that the tilt of the Earth's axis today is about 23.45 degrees from the 'vertical', but it has not always been at this angle. Over a cycle lasting roughly 40,000 years, the Earth's axis gradually shifts between an angle of 21.1 and 24.5 degrees. The second cycle relating to the axis is how it slowly 'wobbles' as it spins, known as precession, which follows a roughly 20,000-year cycle. The third, and longest, of these cycles, though, relates to the shape of the Earth's orbit which, over a cycle of 100,000 years, stretches from a circle to an ellipse and back again.

A) Axial tilt; B) Precession; C) & D) Obliquity: shape of the Earth's orbit switches from circular to elliptical

Although only one of these cycles appears to be dominant at any one time, the interaction of all three results in changes to the amount of solar radiation received by the Earth and alterations to the length and nature of the seasons, and this is what is thought to have caused the advance and retreat of the ice sheets throughout the Pleistocene.

Thus, when all resulted in an increase in solar radiation, there was an interglacial, at the height of which, such as around 125,000 years ago, you could find hippos swimming in England's rivers, while the Earth in general had much less ice than it has today. When all conspired to decrease the amount of solar radiation, however, you had a glacial, at the height of which, such as around 24,000 years ago, lands like Ireland were completely buried under ice.

However, although at the height of glacials Ireland would often have been completely consumed by ice, we must be aware that for the majority of the time in glacials the land, although cold, would have been relatively free of ice, at least at lower elevations. We might imagine that this fairly ice-free Ireland would still have been very wintry-looking, with a thick carpet of snow blanketing the landscape, but, again, we would be wrong.

With the ice sheets greedily sequestering vast amounts of water, the climate during glacials was much drier, and no armadas of clouds would have been seen sailing in the clear blue skies. Thus, even during the harsh winters there was little snow on the ground, allowing plenty of grass to grow, which explains how great behemoths like the woolly mammoth were able to sustain themselves at these times in Ice Age Ireland.

The Pleistocene Ice Age, then, appears to have been quite different in many ways to the picture that is often conjured up in popular culture. Instead of an age of grinding greyness as we might have expected, it was a period of vibrant colour and change, when the ice sheets rocked and swayed, dancing to the music of the spheres.

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