Throughout Earth's climate history (Paleoclimate) its climate has fluctuated between two primary states: greenhouse and icehouse Earth.[1] Both climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur as alternate phases within an icehouse period and tend to last less than 1 million years.
Earth is now in an icehouse state, and ice sheets are present in both poles simultaneously.
A "greenhouse Earth" is a period during which no continental glaciers exist anywhere on the planet.[6] Additionally, 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.[7] Earth has been in a greenhouse state for about 85% of its history.[6]
We can live in even the warm areas of an Earth like that, but it'd be an Earth that's warmer than humanity has ever experienced.