Ice Age

The evidence Agassiz trusted most was written into the rock itself. Where a glacier moves, it drags stones frozen into its underside across the bedrock beneath, leaving long parallel grooves and polished surfaces — striations, in the technical word. It dumps ridges of rubble at its edges and snout, called moraines, and it strands isolated boulders, erratics, far from the outcrops they were torn from. To Agassiz, a valley full of these marks was as legible as a page: ice had been here, and had since gone.

The trouble was that his colleagues could read the same page differently. Taken up into the mountains to inspect the scarred rock, some of the doubters proposed that the scratches came from cart wheels, or from ordinary weathering, or from rocks tumbling in floods. The erratics could be explained by the reigning theory of the day, which held that a great flood — often identified with the biblical Deluge — had rafted boulders across the land on icebergs. This drift theory did not require whole continents of ice. It fit more comfortably with what people already believed.

By the second half of the nineteenth century, the existence of past ice ages was becoming hard to deny. Geologists across Britain, Scandinavia and North America kept finding the same signatures over enormous areas. The awkward question was no longer whether ice had spread, but why. What could cool the whole planet enough to raise ice sheets, and then let them melt away — apparently more than once?

The most durable answer came not from a geologist but from a self-taught Scottish thinker, James Croll, who in the 1860s and 1870s argued that the ice ages were driven by slow changes in the Earth's orbit and the tilt of its axis. These shifts alter how much sunlight reaches different latitudes in different seasons, and Croll reasoned that they could tip the climate into and out of glaciation on long cycles. It was an elegant idea, and it linked the frozen past to the clockwork of the solar system rather than to a single catastrophe.

The proof, when it came, arrived from the bottom of the sea and from chemistry. From the 1950s onward, scientists learned to drill long cores of sediment from the ocean floor and to read the shells of tiny fossil organisms buried in them. The ratio of oxygen isotopes locked in those shells records how much of the world's water was tied up in ice at the time the creatures lived. Layer by layer, the cores gave a continuous history of cold and warm periods stretching back through the whole span of the ice ages.

The turning point Gribbin points to came in the mid-1970s, when a study of these deep-sea records showed that the timing of the ice ages matched the orbital rhythms Milankovitch had calculated. The famous 1976 paper by James Hays, John Imbrie and Nicholas Shackleton, which called the orbital variations the pacemaker of the ice ages, is the moment the theory locked into place. Roughly a hundred and forty years after Agassiz's speech, the Ice Age was not just accepted but explained, its causes tied to the geometry of Earth's motion and confirmed by a physical archive.

Agassiz died in 1873, honored as one of the great naturalists of his century, on the wrong side of the Darwin debate and still convinced of the ice sheets he had never quite proved. He did not live to see the astronomical theory worked out, let alone the deep-sea cores that finally confirmed it a full century after his death. The scratched rocks he dragged his colleagues up the mountain to see turned out to say exactly what he claimed they said.