What clues indicate that the earth has changed its postion in its orbit?

Some 56 million years ago, during the transition between the Paleocene and Eocene epochs, Earth caught a fever. In a span of scarcely 20,000 years-not even a rounding error in most measures of geologic time-massive amounts of carbon dioxide flowed into the atmosphere, and average temperatures rose by five to eight degrees Celsius. The planet was transformed. Crocodiles basked on Arctic beaches lined with palm trees, and steamy swamps and jungles stretched across much of the midlatitudes. Such "hyperthermal" events periodically come and go throughout Earth's history, but this one was particularly intense for unclear reasons. For decades, researchers have puzzled over what triggered this Paleocene-Eocene Thermal Maximum (PETM), peering through the lens of the past to better understand our planet's present-day warming. A surge in volcanic eruptions likely played a role, perhaps aided by a comet impact. But a new study suggests the PETM may have been instigated by subtle shifts in Earth's orbit around the sun.