THE ANSWER TO THAT question was provided by Joe Kirschvink of the California Institute of Technology, in Pasadena.
In the late 1980s, through his studies of palaeomagnetism (the positioning of magnetic minerals in rocks) he became convinced of the likelihood of a tropical glaciation about 700 million years ago. Like all magnets, magnetic minerals line up with Earth's magnetic fields. The closer they are to the equator, the more horizontally they lie.
Kirschvink was particularly impressed by glacial deposits in South Australia. Their history had been carefully studied by geologist George Williams, from the University of Adelaide, and there was good evidence to suggest that the magnetic minerals had not been remagnetised since they were first laid down, their tropical signature untouched.
Beside these glacial deposits are rocks made from sediments – sand and mud – affected by tidal action. So the glaciers had clearly been at sea level, not high in the mountains as we find in the tropics today.
Williams explained this anomaly by suggesting that the tilt of the Earth's axis with respect to the plane of its revolution around the Sun (now 23.5°) was then greater than 54°.
At this angle, the poles would receive more solar radiation than the equatorial regions, and what are now the tropics would be much colder than the poles. But little supporting evidence has been found. And the strong seasonality suggested by the idea would mitigate against the formation of glaciers.
Kirschvink thought global glaciation a much simpler explanation of the riddle of Williams' deposits. It was Kirschvink, in fact, who coined the term snowball Earth. And he also presented a way out of the permanent global winter. On snowball Earth, he proposed, although most movement would cease, plate tectonics would still be inexorably churning away beneath the surface, and that would lead to volcanoes belching out gases, especially CO2.
In a complete absence of rain, with the oceans, rocks and any plants covered in ice, there would be little to soak up this outpouring of gas. Over tens of millions of years, CO2 would build up to very high levels in the atmosphere, hundreds of times the present concentration, causing an intense greenhouse effect.
In a reversal of events, all the ice would melt in less than a thousand years as the Earth experienced temperatures of between 40°C and 50°C. Although there is evidence of global glaciations first occurring more than two billion years ago, the climate appears to have been cycling from one extreme to the other through the late Neoproterozoic era.
When Kirschvink published his ideas in the early 1990s, they seemed so bizarre that they were roundly attacked, until Hoffman and his geochemist colleague Daniel Schrag took them up.
Hoffman had been working on carbonate rocks in Namibia, in southern Africa. Carbonates are laid down in the oceans as the result of a reaction between dissolved CO2 and calcium or other metal ions. They are a sign of warmer, tropical waters. But there, among those warm water rocks, about 600 million years ago, were the distinctive glacial sediments.
When Hoffman checked the source of the carbon in the carbonate, he found another odd thing. Natural carbon comes in two varieties or isotopes, a common form known as carbon-12 and the rarer, heavier carbon-13, which has an extra neutron. Plants, which fix the carbon on which life depends, show a distinct preference for carbon-12.
So sea water in which there is life has a slightly higher level of carbon-13 compared with an environment in which there is no life. These concentrations are captured in any carbonate rock formed at the time and can be used to measure living activity.
When Hoffman analysed the isotope concentrations in his Namibian rocks, he found a clear progression from an ocean full of life when the glacial period began, to one nearly devoid of life at the end. Above the glacial period was a thick band of carbonates (now known as cap carbonates), a sign of a warm Earth with plenty of carbon. Similar bands of cap carbonates have since been found around the world.
He called in Schrag to check his carbon isotope results. Schrag not only confirmed what Hoffman had found but recognised that the existence of the cap carbonates fitted nicely with Kirschvink's idea of greenhouse melting. As the water cycle restarted, rain would dissolve and wash vast amounts of acidic CO2 out of the air. This would weather the rock beneath and carry the carbon and minerals into the oceans as the ice melted. In the warming seas, the mixture would react and precipitate to form the cap carbonates.
