Kingdom of Frost. Bjørn Vassnes

von Humboldt (who had been Agassiz’s teacher), slaughtered the idea. But the data supporting the theory were impossible to explain any other way.²⁶

      Being from Switzerland and very familiar with the Alps, Agassiz had noticed some of the same phenomena Esmark had seen in Norway: for example, the erratic boulders and the moraine ridges. Agassiz had seen how modern-day glaciers could carry such rocks along with them, and how they formed moraines from stones and gravel. He also noticed the striations on the surfaces of the rock, which all went in one and the same direction, as if somebody—or something—had scraped the rock with some vast instrument.

      Eventually, Agassiz was able to picture how the ice had covered a much larger area than the small glaciers that still remained up in the mountains. He latched on to a term one of his friends, a half-crazy German botanist called Karl Schimper, had used in a poem in 1837: ice age. Agassiz gathered more proof and in 1840, he was able to publish his revolutionary theory: that large swaths of Europe, and perhaps other parts of the world, had once been covered in ice, and that this could explain many of the landscape formations. As he put it: “In my opinion, the only way to account for all these facts and relate them to known geological phenomena is to assume that . . . the Earth was covered by a large ice sheet that buried the Siberian mammoths and reached as far toward the south as did the phenomenon of erratic boulders.”²⁷

      Agassiz was here referring to the mammoths that had created a furor when well-preserved specimens were discovered in the Siberian tundra. The discovery of these mighty frozen creatures helped give his theory credibility. At first, it is true, people thought they were elephants, washed north to Siberia by the flood described in the Old Testament. But the French natural scientist Georges Cuvier (1769–1832) proved mammoths were a separate species, specially adapted to life in the cold Arctic.

      In spite of severe opposition—this contradicted the Bible, after all, and predated Darwin’s theory of evolution—Agassiz’s theory was eventually accepted. And now, suddenly, the traces were easy to see and people found them almost everywhere: moraines, erratic boulders, striations, U-shaped valleys, or U-shaped fjords, as in Norway. The same types of traces also appeared in other parts of the world, such as North America and New Zealand. Old stories about how the glaciers had grown or shrunk, previously dismissed as tall tales, were looked at again. Agassiz himself moved to the United States, where he garnered considerable recognition as a scientist and became a professor at Harvard. In his new homeland, he saw many signs that North America had also had an ice age. The great lake that formed when these glaciers melted was named Lake Agassiz.

      Aided by numerous ingenious methods, scientists have since discovered that there hasn’t been just one ice age, but many. In the past 800 million years, in particular, Earth has frozen and thawed, frozen and thawed in a cyclical dance. In the past 800,000 years alone, there have been at least nine ice age “episodes.” The first thorough documentation of this was in the 1960s and ’70s, with British scientist Nicholas Shackleton’s analysis of sediments from the ocean bed, together with Danish paleoclimatologist Willi Dansgaard’s ice-core drilling in Greenland. Both made use of the fact that the composition of oxygen isotopes (variants of oxygen with different atomic weights) changes in step with temperature and sea level. On that basis, it was possible to produce time series that showed how the climate had fluctuated in the last ice age periods. Among others, they showed that Earth has spent most of the past million years in ice ages and that it has been much colder than it is in our times. Even though solar radiation has increased, Earth has basically become colder. According to this pattern, we should now actually be on our way back to a new ice age. But because of what we are currently doing to the atmosphere, it is far from certain this will happen.

       WHAT CAUSES ICE AGES?

      WHY ALL THESE fluctuations, these shifts between cold and hot periods? Isn’t the temperature on Earth primarily determined by solar radiation? And in the 4.5 billion years in which Earth has orbited the sun, that radiation has, in fact, increased by as much as 30 percent. Surely this constant increase in heat from the sun shouldn’t lead to ice ages?

      There has been a lot of speculation about and research into the possible causes of these climate fluctuations—an especially burning question now, in light of global warming. Part of the answer, scientists now agree, was discovered by the Serbian engineer Milutin Milanković (1879–1958) when he managed to demonstrate links between temperature fluctuations and cyclical changes in the Earth’s movements. Two elements that come into play here are the Earth’s distance from the sun and its axial tilt.