Greenhouse and icehouse Earth

Throughout the history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse earth and the icehouse earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years. There are five known glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, and oceanic and orogenic changes due to tectonic plate dynamics. Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth.

A "greenhouse earth" or "hothouse earth" is a period in which there are no continental glaciers whatsoever on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions.

There are several theories as to how a greenhouse Earth can come about. The geological record shows CO₂ and other greenhouse gases are abundant during this time. Tectonic movements were extremely active during the more well-known greenhouse ages (such as 368 million years ago in the Paleozoic Era). Because of continental rifting (continental plates moving away from each other) volcanic activity becomes more prominent, producing more CO₂ and heating up the Earth's atmosphere. Earth is more commonly placed in a greenhouse state throughout the epochs, and the Earth has been in this state for approximately 80% of the past 500 million years, which makes understanding the direct causes somewhat difficult.

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