The Cryogenian Big Freeze
Animals Appear After the Thaw
Published in The EarthSphere Blog
Prologue
Previously in the Forgotten Origins series, we traced the rise of eukaryotes, an early step in the development of complex life forms. We also questioned why evolution seemed to stagnate for a billion years after that step forward.
Hot and Cold
Earth has run hot and cold for 4.5 billion years, but a warm planet is the norm. Earth spent about 670 million years of its history in glacial periods, so cold spells account for 15% of its existence. Meaning Earth’s normal state is a hot ice-free planet. Technically, we are currently in a cold spell but rapidly moving back to hot Earth conditions because of Anthropocene warming. We are now emerging from the last ice age, which was quite cold with massive glaciers covering much of the northern hemisphere. But 720 million years ago, our planet plunged into an unimaginable deep freeze during a period called the Cryogenian.
Atmospheric carbon dioxide (CO2) is measured in parts per million (ppm), and currently, Earth averages about 420 ppm of CO2 in its atmosphere. I once had someone comment that CO2 certainly couldn’t make much of a difference in temperature at such low concentrations. After all, even a change of 100 ppm is just a…
