This isn't true. For most of history we just don't have the resolution to see changes over a few hundred years. But we do have recent examples of much quicker

This isn't true. For most of history we just don't have the resolution to see changes over a few hundred years. But we do have recent examples of much quicker temperature changes. During the Younger Dryas the global average temp dropped about 5°C over a few decades and about 1000 years later, rose again just as quickly.