Once ice sheets start to grow they can advance rapidly. The white surface of the snow reflects sunlight and, more importantly, heat from the Sun (see the section "How bright is the Earth?" on pages 113-120). This prevents the land from warming. The next winter, more snow falls and extends the ice-covered area, and low summer temperatures prevent it from melting.
What matters is the cool summer, when the temperature does not rise sufficiently to melt the ice. It makes no difference how cold the winters are.
There is a very controversial theory that on four occasions between 750 million and 580 million years ago the entire Earth was covered by ice. Geologists had known since the 1960s of glacial deposits that were similar to those of the Pleistocene, but Proterozoic in age. These deposits are found on every continent, and it seemed as though there had been a time when there were glaciers at sea level near the equator. In the 1980s, Joe Kirschvink, a scientist at the California Institute of Technology, suggested how this could have happened. He called that frozen world "Snowball Earth."
The geography of the world was different then. A large supercontinent had recently broken apart due to movements of the Earth's crust, so the dry land comprised a number of small continents, where no region was very far from the sea. Rainfall increased over land, and atmospheric carbon dioxide that dissolved in the rain reacted with silicon in the rocks and was washed into the sea. As more and more carbon dioxide was removed from the air in this way its atmospheric concentration fell dramatically. Global temperatures fell due to a reversed greenhouse effect (see the section "Greenhouse gases and the greenhouse effect" on pages 94-104) and large ice packs formed in polar regions. The ice then extended farther south, the white surface reflecting solar radiation and preventing the temperature from rising, until eventually the sea ice reached the equator. Average temperatures over the whole world fell to about -58°F (-50°C) and all of the oceans were frozen to a depth of more than half a mile (800 m).
Once the world was in this condition the climate was highly stable, but beneath the ice the solid Earth remained active. Heat from the interior of the Earth prevented the oceans from freezing completely, and volcanoes continued to erupt, releasing gases, including carbon dioxide. These entered the atmosphere.
The glacial climate was extremely dry. No rain fell, and without rain to dissolve it and bring it down to the surface, the carbon dioxide released by volcanoes gradually accumulated. It continued accumulating for about 10 million years. By that time the concentration reached about 10 percent of the lower atmosphere by volume. This was high enough to trigger a massive greenhouse effect. The atmosphere grew warmer and within a few centuries all the ice had melted.
While the Earth was covered in ice, its surface reflected solar radiation. Now, with the ice melting, the darker surface absorbed solar radiation—the effect that had contributed to the cooling was strongly reversed. Warming continued and the average temperature rose to more than 120°F (50°C).
The extremely high temperature caused large amounts of water to evaporate. It condensed and produced long and intense rainfall, and the rain washed the carbon dioxide from the air to the surface. George Williams, an Australian geologist at the University of Adelaide, found that all over Australia, and later all over the world, the ancient glacial deposits are covered by a layer of cap dolostones. Dolostone (mainly magnesium carbonate, MgCO3) is a sedimentary rock formed when the calcium in lime-
Will there be more ice ages?
stone (calcium carbonate, CaCO3) is replaced by magnesium. These particular dolostones that cap the glacial deposits appear to have formed from limestone that was precipitated over a very short period.
As the prolonged rain reduced the atmospheric concentration of carbon dioxide, the greenhouse effect diminished. Temperatures fell and gradually the climate stabilized.
The "Snowball Earth" hypothesis remains highly controversial. Other scientists maintain that the cap dolostones can be explained in other ways, and some climatologists have pointed out that if the world were ever to freeze over there is no way it could warm up again. Once ice covered all of the oceans the condition would be irreversible. Nevertheless, the scientists who propose the idea point to many lines of evidence they say support it.
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