The Paleocene-Eocene Thermal Maximum (PETM) is a period in Earth's history that occurred about 55.5 million years ago. The thing that attracts interest to it is how it was a period of global warming. The more scientists study it, the more we are learning about what is happening today. There is good news along with the bad from a recent study. The good news was that the environment adjusted and eventually returned to normal after the global warming episode. While it is good to hear of the adjustment, it is also important to understand the environment took about 200,000 years to fully recover. Left to itself, things will get bad but it won't end the world, But, we may be stuck with this situation for a while.
The PETM is very interesting because of the similarities to today. What has been found is the emission levels were in the ballpark of today's manmade emission levels. The thinking is what happened back then might be a good model for what we can expect to see today. The problem is that it was already much warmer back then when the big emissions came along. In fact, it was so warm there were no ice caps. So it isn't a perfect analogy.
What was seen is the temperature rose by 5 to 8 degrees C (9 to 15 F). That isn't enough to destroy the world, but it sure would cause a lot of devastation. Analysis of sediment cores has indicated there were two pulses of carbon release. It is thought the second one occurred in response to the rising temperature caused by the first. Does that mean we can expect to see something like that today? As the temperature increases due to manmade emissions, can we expect the natural environment to become a CO2 source instead of sink? That would be doubly bad because nature currently removes roughly half of all manmade emissions. If it became a source, it would not only be adding CO2 itself, but would no longer be removing that half of our emissions.
What they have been able to piece together about the PETM is there was a changing climate where some areas became drier and others became stormier. Continent-scale mass migrations have been identified, probably as a result of the changing climate. Some extinctions occurred, but not enough to be a mass-extinction event. The oceans became more acidic.
In other words, pretty much what we are already seeing today. History really does repeat itself for those that don't learn its lesson.