It’s true the Earth has experienced periods of global warming in its past. The largest such warming event in the past 90 million years - since the time dinosaurs roamed Earth - was the Paleocene–Eocene Thermal Maximum, about 56 million years ago. Average global temperatures increased by 4–5°C over a period of 3,000–10,000 years. Human beings were definitely not walking the Earth back then, but today scientists are able to piece together evidence of how and why this ancient global warming happened.

Spoiler alert? It was caused by greenhouse gas emissions, likely stemming from carbon cycle feedbacks – the processes that unlocked and released more and more CO2 from the Earth’s surface as it warmed, and volcanism - the eruption of volcanoes.

Dustin Harper is a marine geologist postdoctoral researcher in the Department of Geology and Geophysics at the University of Utah. He, along with U Geology professor Gabe Bowen, published a study where they examined tiny, microscopic shell fossils taken from drilling cores in the ocean floor, that revealed important information about ancient sea surface temperatures and the levels of atmospheric CO2 . They found sea surface temperatures were closely linked with levels of atmospheric CO2 during this period.

This helps us to understand the sensitivities of our planet and the feedback mechanisms that can kick in during periods of rapid global warming triggered by greenhouse gas emissions, which is what we are experiencing today – with anthropogenic climate change - albeit at a much faster rate. Like, 4 to 10 times faster than occurred during those ancient hyperthermal events.
https://wilkescenter.utah.edu/podcast/25-climate-sherlocking-dustin-harper/