Geologists of the nineteenth century concluded that in the last Ice Age glaciers reached much further down alpine valleys. But they did not know the half of it. Imagine their surprise if they'd discovered evidence of glacial ice near the Earth's equator. Yet this is just what, in 1986, Joe Kirschvink of the California Institute of Technology, Pasadena, discovered in Adelaide in Australia.
Kirschvink found rocks formed 700 million years ago, that were clearly deposits left by glaciers -- yet they bore a magnetic imprint which showed that, at the time, they were close to the equator.
The implication was that vast ice sheets had once stretched from pole to pole, or nearly. The Earth, it seemed, had once been an iceball. Kirschvink published his so-called 'Snowball Earth' findings in 1992.
It was not an entirely new idea. During the 1960s, geologists discovered rocks about 700 million years old all over the world bearing the signature of rough treatment from glaciers. But how could ice have blanketed the entire planet?
The Soviet scientist M. I. Budyko proposed one possible cause: runaway global cooling. Bright, white polar ice sheets reflect more of the sun's heat and light back into space than do darker land masses or open water. So as the ice sheets grow during an Ice Age, they exert a feedback effect that further cools the world. The bigger they get, the more cooling they cause and so the more they grow. Budyko's theoretical models of the Earth's climate suggested that this feedback could pass a point of no return, leaving the planet to freeze over.
On the Snowball Earth, ice is everywhere: even the oceans are frozen. Except for a few organisms clinging on around volcanos, no life can survive. The temperature is Arctic everywhere: about minus 40 °C.
But how could a Snowball Earth ever have shaken off its icy coat and returned to the blue planet we know today? Kirschvink proposed an answer. Volcanoes thrusting through the ice would have continued to disgorge gases, mostly carbon dioxide. CO2 is a greenhouse gas, and causes global warming. Today, volcanic carbon dioxide is kept in check by natural processes such as chemical weathering of rocks, which removes the gas from the atmosphere in the form of carbonate minerals. On the Snowball Earth, weathering would be suppressed because there would be no rain to wash the carbon dioxide from the skies, and no exposed rocks to react with it. So volcanic carbon dioxide would accumulate gradually in the atmosphere and warm the planet.
At some point, there would be enough of it to break the reign of ice, and the seas would thaw. Calculations suggest that a huge amount of carbon dioxide is needed to do this: about 350 times the amount in today's atmosphere. So once melting began temperatures would soar and the planet would instead become a hothouse.
The whole idea is still controversial. But it was lent strong support by a report in 1998 from geologist Paul Hoffman of Harvard University, Boston, Massachusetts, and colleagues. They found evidence from carbonate rocks in Namibia that, about 700 million years ago, biological activity almost disappeared in the surface oceans for millions of years. This is exactly what would be expected in ice-covered oceans. The researchers also showed that the carbonate rocks that typically cap glacial deposits from this time can be explained by the idea of a hothouse Earth immediately following the big freeze.
The geological record from Namibia and elsewhere implies that there may have been up to five of these freeze-thaw cycles, the last one ending about 575 million years ago. This timing corresponds to a massive diversification of the fossil record and the appearance of multicelled animals. This burgeoning of life might have been stimulated by the existence of so many evolutionary niches on a world previously smothered by pole-to-pole ice.