What happens when the Earth’s environment freaks out

The last major freakout of the Earth environment, I have just read, was in 1816, which was called the “Year Without a Summer”.

What happens during a period of unusually low solar output and then a string of massive volcanoes spewing hundreds of tons of dust into the atmosphere?  Read this article at Wikipedia.

A quote from one small section of the article:

In July and August, lake and river ice were observed as far south as Pennsylvania. Rapid, dramatic temperature swings were common, with temperatures sometimes reverting from normal or above-normal summer temperatures as high as 95 °F (35 °C) to near-freezing within hours.