According to "Past Climate Changes on Earth," what changes to the atmosphere caused temperatures to increase, making the Eocene such a warm time in Earth's history?

The most important factor was the accumulation of greenhouse gases, especially methane and carbon dioxide.

Explanation:

The Eocene climate began with warming climate at 56 million years ago (Mya) and finalized at around 49 Mya ago. It is believed that during this period the greenhouse gases increased the temperature on the Earth's surface. This hypothesis is supported by evidence that indicates the melting of the ice cap in the poles.