Data compiled by the international Ocean Drilling Program reveal four distinctive Earth’s climate states, Hothouse, Warmhouse, Coolhouse, and Icehouse. The important take home message is: For the past 3 million years, Earth’s climate has been in an Icehouse state characterized by alternating glacial and interglacial periods. Modern humans evolved during this time, but greenhouse gas emissions and other human activities are now driving the planet toward the Warmhouse and Hothouse climate states not seen since the Eocene epoch, which ended about 34 million years ago.
Westerhold et al. (2020). An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science, 369, 1383-1387.