The Early Eocene period is one period in the geologic past that stands out as distinctly warmer than today, particularly at high latitudes. During the Early Eocene Period, 54–48 million years ago, fossil remains of plants and animals believed to inhabit warm environments were found at much higher latitudes and the poles had little or no ice. The Eocene period occurred far enough in the past that continents were in slightly different positions, with different mountain chains and shallow seas in some places that do not exist today.

The Early Eocene was characterized by high carbon dioxide levels, inferred to be between 1,000 and 2,000 parts per million. Scientists think that increased volcanic activity was an important cause of these high levels of carbon dioxide. Temperatures during the Eocene can be reconstructed from geochemical measurements of ocean sediments and from vegetation types preserved on land. The reconstructed global mean surface temperature for the Early Eocene is 9 to 14°C higher than today. As seen by proxy evidence and model simulations, this warming was widespread across the globe. There is good agreement between model simulations incorporating high CO 2 concentrations and proxy evidence, providing strong support for the role of CO 2 in maintaining the high temperatures of the Early Eocene.