Those charts of temperatures through the ages makes me wonder how do they know the temperatures when there were no thermometers?
That particular chart was based on ice core data in Greenland and assumptions on thickness of ice at different depths (years into the past) represented different temperatures. Like you, I'd be skeptical, except that the graph corresponds to written histories of life in those warming and cool periods.
For example, Pevensey Castle in the UK was on the shore during the Roman Warm Period. There were stories of people tying their boats to the castle during high tide. There was also a notorious history of prisoners being put into the ground floor when the tide was out, then drowning when the tide came back in. Then the Little Ice Age moved the coastline away from the castle, and the Modern Warm Period is moving the coastline back to the castle. Yet, to this day, the coastline is about a mile away, suggesting the Modern Warm Period isn't as warm as the Medieval Warm Period (hint, the castle didn't move).
Also, Ephesus was a port city from the Minoan Warm Period to the Roman Warm Period (when the Apostle Paul wrote the letter Ephesians in the Bible). Today it is the 3 miles from the coast (hint, the city didn't move).