FEW SCIENTISTS NOW dispute the idea of unusually widespread glaciation before the rise of animals. What's at issue is its extent – whether it covered the planet completely (snowball Earth) or if parts of the ocean remained open (slushball Earth) – and what role it played in evolution. That's the mystery of snowball Earth.
"It's still highly contentious," admits Hoffman. "My own read is that about 10 per cent of the community involved in this research support complete ice cover, about 10 per cent are fiercely opposed to it, and the other 80 per cent of people are sitting on the fence. I don't think there's going to be consensus until the original antagonists cycle out of the system. After all, [the theory of] continental drift took 55 years to be accepted.
"Like any new theory, it's imperfect. So there are a lot of things we think it predicts that are inconsistent with evidence. It may be that in some cases the evidence isn't right, but very often it's because the theory isn't right," he says.
The genesis of the hypothesis has been well documented by Hoffman himself and in a book and articles by science writer, Gabrielle Walker. Interestingly, snowball Earth has roots in Australia. It was the famous Australian Antarctic explorer and geologist, Douglas Mawson, who first considered the idea of global glaciation after coming across sediments in South Australia, clearly glacial in origin and far closer to the equator than he expected.
The sediments deposited by glaciers are very distinctive. As the ice scours underlying rock, it generates mixtures of boulders, stones and silt, particles of all different sizes, flattened and scratched as they are dragged along. Another clear sign of glaciation is the smooth, often very large rocks, known as dropstones, which are carried out to sea inside icebergs and eventually tumble into the seafloor sediments.
Glacial deposits are found all over the world. Many of them seem to have formed about the same time, in the late Neoproterozoic era around 700 million years ago.
Noting this in the '60s, geologist Brian Harland, at the University of Cambridge, suggested that the Earth may have gone through a great ice age. He also recognised that these glacial sediments often occur in rocks that could only have been laid down in the tropics, and inferred that the ice cover was so complete it must have extended into low latitudes. But how could that happen?
It was a Russian geophysicist, Mikhail Budyko, who provided a mechanism. The key was the 'albedo', the extent to which the Earth reflects rather than absorbs the Sun's radiation. The impact of albedo can be dramatic.
Researchers at University of Almería in Spain have recently shown, for instance, that in their southern Spanish province, the local concentration of greenhouses – the world's greatest – has lowered the region's average temperature by 0.3°C in the past 25 years (by absorbing light), while the average temperature in the rest of the country's has risen by 0.5°C.
During glaciation, the Earth's albedo is raised by white, reflective ice. Using a simple model, Budyko was able to show that if the Earth cooled enough for ice sheets to cover half its surface, reaching a latitude of 30° North and South, a positive feedback would set in.
The planet would then be so shiny, reflecting so much solar radiation back into space, that it would begin to cool naturally and freeze over completely – and quickly. Once the feedback took over, it would only be a matter of years, perhaps less, before the Earth iced over, according to Hoffman. But if that happened, how did the planet break free of its icy shackles?
