THREAT: Tectonic movement and orbital disruptions lead to a permanent global freeze, sterilizing the planet.
REALITY: The Earth's deepest freeze lifted with the retreat of massive glaciations about 640 million years ago. Geologists have seen traces of it from South Australia to the Norwegian Arctic, and some scientists see evidence that ice reached the equator. Ultimately, volcanoes probably reemitted enough heat-trapping CO2 to melt the ice.
Not to worry. Snowball Earth episodes likely need hundreds of thousands or millions of years to develop. Civilization evolved in an atmosphere about four degrees Celsius warmer than the previous ice age. Since industrialization, it's up about about another 0.8 degree, heading higher.
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