About 55 million years ago, an intense heat wave hit the planet. Earth's surface temperature surged by 9 degrees Fahrenheit (5 degrees Celsius). Then, after a relatively short time, the heat subsided, only to be followed by at least two similar, but smaller heat waves.

Based on chemical clues preserved in rocks, scientists believe a surge of carbon dioxide warmed the planet. But where did all of this greenhouse gas come from?

A team of scientists is proposing that it came from the melting of permafrost, frozen soil packed with organic matter, after cycles in the Earth's orbit warmed up the areas near the poles. The melting released a massive amount of carbon into the atmosphere, keeping reflected sunlight from escaping and causing the heat wave....