The period of the highest CO2 concentrations is called the Palaeocene-Eocene Thermal Maximum, PETM for short. Sometime around 56 million years ago the amount of carbon dioxide in the atmosphere increased dramatically over a period of 4,000 years, and created climactic effects that lasted for 100,000 years.
There is no comparable period of time in the intervening 56 million years.
The amount of carbon released during the PETM is roughly twice what mankind has released during the Industrial Age. But the rate at which humans are pumping carbon dioxide into the atmosphere is unprecedented:
Washington Post wrote:The anthropogenic release outpaces carbon release during the most extreme global warming event of the past 66 million years, by at least an order of magnitude,” writes Peter Stassen, an Earth and environmental scientist at KU Leuven, in Belgium, in an accompanying commentary on the new study.
So the rate at which carbon dioxide is being introduced into the atmosphere is unprecedented in Earth's history. We are introducing TEN TIMES as much carbon dioxide per year compared to the PETM.
Washington Post wrote:Given that the current rate of carbon release is unprecedented throughout the Cenozoic, we have effectively entered an era of a no-analogue state, which represents a fundamental challenge to constraining future climate projections,” the study concludes.
So we are introducing 10 times more carbon dioxide per year than a period of time that changed the climate for 100,000 years,