You are referring to the Medieval Warming Period between 800 - 1300AD.<quoted text>The natural cycle happens roughly every 100,000 years when the Earth’s orbital shifts and we get more sunlight. Also, those changes always happened over a period of several centuries. The last natural warming was about 1,000 years ago, and this warming we have now is much more rapid.
The Earth's axis shifts every 26,000 years. It literally wobbles. Megalithic observatories found in Ethiopia that are approximately 7000 years old pointed to the stars in Orion when they were originally built. The wobble in the Earth's axis has moved these reference points to where they are today. The stones have been dated to appr. 5000BC when they were aligned with the stars in Orion's belt.