From NASA - volcanic vs solar activity: https://pubs.giss.nasa.gov/abs/st07500u.html

"volcanos erupted causing a cold period that lasted from the 1600s to the late 1800s"

Correlation vs causation - while the volcanic winter phenomenon is well-known, there are those that propose that solar activity is the primary driver of climate on Earth:

https://www.amazon.com/Neglected-Sun-Precludes-Catastrophe-I...

http://www.breitbart.com/big-government/2016/08/12/winter-is...

Not to say that the greenhouse effect is not real, or that anthropogenic CO2 does not cause warming - just that the magnitude of the effect may well be greatly over-hyped.