Eocene Thermal Maximum 2 (ETM-2), also called H-1 or the Elmo (Eocene Layer of Mysterious Origin) event, was a transient period of global warming that occurred approximately 53.7 million years ago (Ma).[1][2][3][4] It appears to be the second major hyperthermal that punctuated the long-term warming trend from the Late Paleocene through the early Eocene (58 to 50 Ma).[5]
Source: Eocene Thermal Maximum 2 – Wikipedia
Tim B.
https://www.theatlantic.com/science/archive/2018/04/are-we-earths-only-civilization/557180/
“Fifty-six million years ago, Earth passed through the Paleocene-Eocene Thermal Maximum (PETM). During the PETM, the planet’s average temperature climbed as high as 15 degrees Fahrenheit above what we experience today. It was a world almost without ice, as typical summer temperatures at the poles reached close to a balmy 70 degrees Fahrenheit. Looking at the isotopic record from the PETM, scientists see both carbon and oxygen isotope ratios spiking in exactly the way we expect to see in the Anthropocene record. There are also other events like the PETM in the Earth’s history that show traces like our hypothetical Anthropocene signal. These include an event a few million years after the PETM dubbed the Eocene Layers of Mysterious Origin, and massive events in the Cretaceous that left the ocean without oxygen for many millennia (or even longer).”