When I give talks on climate and climate change, I often get questions about volcanoes and their impact on our climate. The Washington Post had a recent article on the subject, mentioning the famous eruption of Tambora in 1815, which in 1816 led to the year without a summer in the eastern US. It probably had impacts on Illinois but we had no widespread observations in place at the time.
The one I remember the best was Mount Pinatubo. The following summer after that eruption was exceptionally cool across the US and around the world.
Other significant events mentioned in the article were Krakatoa (western Pacific) in 1883, El Chichon (Mexico) in 1980, and Mount St. Helens (US) in 1982. All of these had some degree of a cooling effect on the atmosphere that lasted 1 to 3 years.
Volcanoes release massive amounts of sulfur dioxide (SO2) into the atmosphere, which undergoes a chemical reaction to produce sulfate aerosols. These aerosols, or tiny particles, are very efficient at reflecting sunlight back into space like a giant shade tree. As a result, the earth is cooled.
Volcanic eruptions also produce CO2. However, the amount release per year on average is far less than human activity and is equal to the CO2 emissions of Florida or Ohio (Source American Geophysical Union article).
For us in Illinois, the large volcanic eruptions in the tropics tend impact our weather the most. Due to how the atmosphere’s circulation works, it is easier for the sulfate aerosols from those events to reach higher into the atmosphere and circulate around the world.
Scientific American article on volcanoes and climate