What causes unpredictable short term temperature swings?

The evidence that we have records for, as from the Little Ice Age, points to a strong interconnection between the solar activity states and the terrestrial climate. During the Little Ice Age the solar activity was low (The Maunder Minimum), consequently the climate was cold. The phenomenon became known as the Maunder Minimum of solar activity, referring to a 70-year period between 1645 and 1715. During this entire period only about 50 spots were observed on the face of the sun as opposed to the typical 40,000–50,000 spots for the same time frame in a high-activity (or 'normal') period. 

But what causes these large variances in solar activity?

Modern plasma physics defines such variances as the most natural occurrences in the universe, with the Sun being electrically powered by electric plasma currents that power the galaxies and every sun within them. Since the electric-power density that affects our sun is evidently determined by countless factors in our galaxy of 200 to 400 billion stars, each of which is a sun that is powered by the electric energy streams that pervade the galaxy, and considering further that the entire galaxy is constantly in motion, it would be surprising if we wouldn't see major and minor fluctuations happening.