SIMON TERREY / SCIENCE PHOTO LIBRARY SIMON TERREY / SCIENCE PHOTO LIBRARY
Snowball Earth. Computer artwork of the Earth over 600 million years ago as global glaciation begins. It is thought that an unusual amount of land near the equator disturbed the carbon cycle. This land was free of ice, and so large amounts of carbonate rock could form as rain washed carbon dioxide out of the atmosphere. Carbon dioxide helps keep Earth warm by a natural greenhouse effect, and as levels fell Earth cooled and runaway glaciation formed kilometre-thick global sea ice for millions of years. Volcanoes restored the carbon dioxide which raised temperatures. The ice melted in hundreds of years of violent storms. Most life was killed, but the survivors exploded in evolutionary diversity.
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