Answer to Question 1
There have been many such observations, for example, the Maunder Minimum,
which roughly coincides with the Little Ice Age.
Answer to Question 2
A difference of 20 C is a big, big temperature difference. It is true that there is
evidence for swings i temperature of this great an amount, but most models of ice ages
suggest that the difference is just a few degrees in the mid-latitudes and perhaps 8 to 10 C
near the pole (see References 15).