Hot enough to boil oceans and vaporize rock. The highest terrestrial temperatures occurred more than four billion years ago, when a Mars-size proto-planet smashed into the Earth. (The debris from this collision formed our moon.) Within a millennium, the surface air temperature had dropped from a high of about 3,700°F down to 3,000°. Then the planet went into a period of slower cooling that lasted a few tens of millions of years. As the atmosphere thickened with heat-trapping water clouds and carbon dioxide and a shell of solid rock formed around the Earth’s core, conditions stabilized at 440°.
The warmest weather we’ve had in recent times—since mammals diverged from the tree of life—came about 55 million years ago, during a period known as the Paleocene-Eocene Thermal Maximum. In just a few thousand years, global surface temperatures increased by 5° to 10°, with parts of North America experiencing a tropical climate and spring-like average temperatures in the Arctic.
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