For the majority of Earth’s history, the planet has been hotter than today.

Hotter periods make up some 70 percent of the past two and a half billion years, and are called Greenhouse Earth. They can last hundreds of millions of years, with CO2 levels 10–20 times higher than today, and no ice anywhere on the planet.

During a Greenhouse interval, Earth actually explodes with life.

The age of dinosaurs happened during a Greenhouse. Land animals covered the continents. Reptiles swam in Arctic seas. Birds, mammals and flowering plants first appeared.

It’s been colder the other 30 percent of the past two and a half billion years, called Icehouse Earth. Life struggles in the most severe of these times.

Within Icehouse periods, Earth has shorter cycles of glacials and interglacials.

Glacial periods last about 80,000 years, when ice sheets cover large parts of the continents. Interglacials last for 20,000 years or less, and ice retreats toward the poles.

We’re living in a mild interglacial of a long-term Icehouse now. Temperate climate for many millennia has allowed the human population to expand to what it is today.

Human activity may be accelerating warming, but historical climate patterns suggest that within a few thousand years we could enter another glacial period, when ice would slowly advance again from the poles.

Why we shift from Icehouse to Greenhouse, and glacial to interglacial, are important concepts, which we’ll explore on another EarthDate.