Artist’s impression of Hadean Earth. Image via The Hadean Era

By Michon Scott and Rebecca Lindsey, via NOAA Cimate.gov
Our planet probably experienced its hottest temperatures in its earliest days, when it was still colliding with other rocky debris (planetesimals) careening around the solar system. The heat of these collisions would have kept Earth molten, with top-of-the-atmosphere temperatures upward of 3,600 degrees Fahrenheit (7,538 C.)
Even after those first scorching millennia, however, the planet has sometimes been much warmer than it is now. One of the warmest times was during the geologic period known as the Neoproterozoic, between 600 and 800 million years ago. Another “warm age” is a period geologists call the Paleocene-Eocene Thermal Maximum, which occurred about 56 million years ago.

Cartoon by Emily Greenhalgh via NOAA Climate.gov.