What Causes Ice Ages On Earth
FOR HUNDREDS OF years, European people were aware of large lumps of rock, some as big as a house, lying around in places where they didn’t belong, far from the strata where such material originated. They became known as erratic boulders, shortened to ‘erratics’, and until late in the 18th Century the accepted story was that they had been dumped by the great Biblical Flood.
But in 1787 a Swiss preacher, Bernard Kuhn, suggested that these boulders had been carried to their present locations by ice, not by water. It was natural that this idea should originate in Switzerland, where the power of glaciers was clearly visible, and in the 1790s the Scottish pioneer of geology, James Hutton, reached the same conclusion after a visit to the Jura Mountains of France and Switzerland. But the idea languished until it was taken up and vigorously promoted by another Swiss, Louis Agassiz, who was born in 1807.
Agassiz wanted to study natural history, but to please his parents and obtain their financial support he qualified as a medical doctor (although he never practised), while also working for a doctorate in natural history, which he obtained in 1829. He became an expert on fossilised fish and a professor at a college at Neuchatel. It was there he encountered the Ice Age idea.
Agassiz picked up the idea from a geologist, Jean de Charpentier, who gave a talk on the topic in Lucerne at the 1834 meeting of the Swiss Society of Natural Sciences. He reported how heaps of rocky debris, known as moraines, are left behind by retreating glaciers, and speculated that the Swiss glaciers had once been joined in a single ice sheet extending across the mountains and perhaps reaching into the nearby lowlands of Europe. Agassiz thought the idea was ridiculous, and said so to de Charpentier, who was a friend of his.
The upshot was that in the summer of 1836, Agassiz joined de Charpentier on a trip into the mountains to study the evidence at first hand. He went with the intention of persuading his friend to give up this ridiculous notion of what was called ‘ice rafting’, but came away converted. Like many converts, Agassiz then became more enthusiastic about the idea than the original enthusiasts, and proposed that there had once been a great ice sheet engulfing Europe all the way from the North Pole to the Mediterranean Sea.
By the time the next annual meeting of the Society came around, at Neuchatel on 24 July 1837, Agassiz was its president, although still only 30. The audience settled into their seats expecting a dull presidential address on fossil fishes, and were astonished when he let rip with an impassioned lecture on the Ice Age, in which that very term was introduced (in German, as ‘Eiszeit’).
The talk produced a mixture of anger and disbelief. Even when Agassiz organised a field trip to show the members of the Society scars and grooves cut into the rocks by the action of boulders dragged along by glaciers, they dismissed these as damage caused by the wheels of passing carriages. This only stirred him into more proselytising.
In 1840, Agassiz presented the evidence in a book, Etude Sur Les Glaciers, written in language that could not be ignored: ‘Europe, previously covered with tropical vegetation and inhabited by herds of great elephants, enormous hippopotami, and gigantic carnivora became suddenly buried under a vast expanse of ice covering plains, lakes, seas and plateaus alike. The silence of death followed… springs dried up, streams ceased to flow, and sunrays rising over that frozen shore… were met only by the whistling of northern winds and the rumbling of the crevasses as they opened across the surface of that huge ocean of ice.’ Such language attracted attention, but in scientific terms a much more important event also occurred in 1840, when Agassiz presented his ideas to a meeting of the British Association for the Advancement of Science, held in Glasgow in September. The great geologist Charles Lyell, who was a big influence on Charles Darwin, was in the audience, and like many who heard the Ice Age theory for the first time, was unconvinced. But as a good scientist, soon after the meeting he headed into the Highlands to look for evidence in the form of ‘terminal moraines’ left behind by long-melted glaciers, and found them.
Before the year was out, the Ice Age theory had been presented to the Geological Society in London, endorsed by Lyell, and established as fact. The geologists were convinced that the Earth (or at least, Europe) had once been covered by a great ice sheet. But this raised more questions. When had the Ice Age occurred? And why? The seeds of the modern theory of Ice Ages (note the plural) were sown in a book published in 1842, just two years after the Ice Age theory came in from the cold. The author was Joseph Adhémar, a mathematician who worked in Paris, and his book was called Révolutions De La Mer. To be frank, it was a confused jumble of good and (mostly) bad ideas, but it contained one golden nugget. This was the idea that the climate on Earth is modulated by changes in the orbit of our planet around the Sun.
In the 17th Century, Johannes Kepler had realised that the orbit of our planet is slightly elliptical, with the Sun at one focus of the ellipse, which means that at one end of its orbit the Earth is slightly closer to the Sun than when it is at the other end. At present, closest approach occurs in early January, and the Earth is most distant from the Sun in July. So Northern Hemisphere summers are a tiny bit cooler than they would otherwise be, and Northern Hemisphere winters are a tiny bit warmer. But the cycle of the seasons itself is, of course, explained by the tilt of the Earth, which brings us short, cold winter days and long, hot summer days, completely overwhelming this small orbital effect. But Adhémar thought longer term.
Because the Earth travels more swiftly when it is nearer to the Sun, it spends seven days less traversing the (Northern Hemisphere) winter half of its orbit than it does traversing the summer half. In the south, winters are longer than summers. Adhémar argued that over thousands of years this extra length of winter had allowed the vast Antarctic ice sheet to grow.
But he also knew that because of a wobble of the spinning Earth (like the wobble of a spinning top), the pattern of the seasons slowly shifts around the orbit of the Earth as the millennia go by. Some 11,000 years ago, Northern winter was seven days longer than summer. And 11,000 years before that, the pattern was the same as today.
Voilà! An explanation of not one but many Ice Ages, alternating in the Northern and Southern Hemispheres. The only snag is, the idea was wrong. The actual amount of heat ‘lost’ during the seven extra days of winter is nowhere near enough to make great ice sheets grow. But it did set people thinking about the orbital influence on climate.
Enter James Croll, born in Scotland on 2 January 1821. Croll came from a poor family and was largely self-taught, reading voraciously about science while supporting himself with a succession of dead-end jobs. In 1859 he was in Glasgow, working for a temperance newspaper when he got his big break – a job as caretaker at the Andersonian College and Museum.
The college had a first-class scientific library, which Croll raided while his brother, who was living with him at the time, helped out (unofficially) with his job. In 1861, he published a paper on electricity. Then, he came across Adhemar’s book. But he already knew more astronomy than Adhemar.
In particular, he knew that the amount of ellipticity of the Earth’s orbit (its eccentricity) changes as time passes, with a rhythm roughly 100,000 years long. He also knew that cyclic changes in the tilt of the Earth affect the relative warmth of the seasons, so that although the total amount of heat received by the Earth from the Sun is the same each year, sometimes Northern Hemisphere summers are particularly cold and summers particularly hot, while at other times there is less difference between the seasons. This pattern repeats with rhythms 22,000 and 41,000 years long.
Croll published his first paper on Ice Ages in 1864, and by 1867 his abilities had been recognised and he got a job with the Geological Survey of Scotland. He developed his ideas over many years, but the final version can be summed up simply. Croll suggested that when Northern Hemisphere winters were particularly cold, snow and ice would spread across the continents, making an Ice Age.
COLD, HARD MATHS
As geologists developed techniques for dating the scars left behind by ice, and other evidence for Ice Ages in the form of past changes in flora and fauna, Croll’s idea could be tested, because it is possible to calculate when the orbital parameters made for cold winters.
By the end of the 19th Century, they showed that Croll was wrong – wrong in a way which should have grabbed attention, but didn’t.
Croll calculated that between 100,000 and 80,000 years ago the world should have been thawing out of an Ice Age. In fact, the geological evidence showed that at that time it was plunging into an Ice Age. He had got it exactly backwards. But at first nobody realised the significance – the implication that what you need to start an Ice Age is not cold winters, but cool summers. The person who made this clear was a Serbian mathematician, Milutin Milankovic.
Milankovic was born in 1879, the same year as Albert Einstein. He was intrigued by the astronomical theory of Ice Ages, and worked on how the changes in tilt and wobble had affected the amount of heat reaching the ground at different latitudes (insolation) in his spare time from being a Professor of Mathematics in Belgrade.
In 1914 he was unlucky enough to be visiting his home town in Serbia, the wrong side of what became the front line, when World War I broke out. Interned by the Austro-Hungarians, he had nothing to do except calculate, on paper using a fountain pen. It took him two years to come up with a model describing how the insolation had changed over the millennia for each band of latitudes on Earth. His results were published in 1920, and soon after reading them Wladimir Koppen, a Russian-born German meteorologist, realised that they showed how Ice Ages are associated with cool summers.
It is always cold enough for snow to fall in winter, but what matters is that sometimes summers are too cool for all the snow to melt before winter comes again. The resulting Ice Age only ends when very hot summers return to melt the ice. All of this was confirmed in exquisite detail in the 1970s, from analyses of deep sea sediments and long ice cores from Greenland and Antarctica. What we now know is that Ice Ages roughly 100,000 years long are separated by slightly warmer ‘interglacials’ about 10,000 years long, and that the present interglacial began about 10,000 years ago. If it were not for global warming, the next Ice Age would be just around the corner.