Let me see if I can help you figure that out. There a two things you may not be thinking about.
First is it’s not the total change in temperature that’s really the problem right now, it’s the rate of change.
You are right that in the past it was significantly hotter.
Many of the cyclic changes in temperature start with orbital changes, movement of the continents, and changes in Earths albedo which cause secondary effects like melting permafrost releasing greenhouse gases when warming or more snow when cooling.
This PBS video explains the cycles very well
The difference is (and it’s hard to spot on that graph) is those higher temperatures took 1,000s of years to change. This gave life a while to adapt.
NASA puts our current temperature changes at somewhere between
10-20 times faster than those historical changes.
https://earthobservatory.nasa.gov/Features/GlobalWarming/page3.php
Incidentally the faster historical changes caused by massive volcanism dumping huge amounts of gasses into the air correlate to extinction events.
There is currently another extinction event underway, if you weren’t aware.
The second issue is what’s changed since this last time the Earth hit these temperatures. The two costliest hurricanes in history were Katrina and Harvey costing a combined $250 billion in damages. They were stronger than normal in part due to increased water temperatures in the gulf due to global warming.
What would the cost of those hurricanes have been if they occurred the last time gulf waters were so warm, 100,000s of years ago?
If you said $0 because no humans lived there you’d be correct. The last time the world was this warm there weren’t:
- 700Million people living within 10 meters of altitude of the rising oceans
- 7.5Billion mouths to feed from farms located in places that worked well when the climate was cooler
So TLDR: the climate is changing faster than life with have time to adapt and the last time it was this warm there weren’t 7B people to feed.