What’s the hottest the Earth has ever gotten?

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.

[Excerpted from Popular Science]

As for the heat wave that is currently baking the US, expect more of it in the future.