>> For example the hottest period ever was during time of Pangaea supercontinent, which was largely desert.
It was almost the hottest period, but not the hottest. Pangaea was dry because of its unique geography which prevented moist air from coasts to move around the continent. You picked up a bad example.
>> Which historical data?
Mostly geological data. Btw. historical temperature data also comes from geological data. You can search Wikipedia for specific information.
Moist air from coasts exists thanks to warm oceanic currents. These currents might change even in current geography so that even despite average temperature rises, the currents move farther from areas suitable for agriculture. Or, on the other hand, the water becomes hotter and produce strong hurricanes/typhoons more regularly.
It was almost the hottest period, but not the hottest. Pangaea was dry because of its unique geography which prevented moist air from coasts to move around the continent. You picked up a bad example.
>> Which historical data?
Mostly geological data. Btw. historical temperature data also comes from geological data. You can search Wikipedia for specific information.