(Medieval Optimum) - (Little Ice Age) = 4 degrees C
Well, what can you say about the Medieval warm period when the vikings colonized Iceland, Greenland and reached America?! And the drop of the temperatures during the Little Ice Age? The drop was about 4 degrees Centigrade with CO2 levels unchanged.
You must admit: the main driver of the climate is the Sun, which is a variable star, whose activity periodicity is very complicated.
If you are familiar with the theory of oscillations, you should know what 'beat mode' is. When you have a system with more then 2 base frequencies, the picture gets very, very interesting - with huge maxima and huge minima...