Mt Tambora erupted in April 1815, one of the largest, if not the largest, eruptions in recent history. It precipitated three years of significant climate change (massive and sustained cooling) due to the ash emissions which travelled around the world. 1816 was the (in)famous "year without a summer" which caused crop failures world wide, and was instrumental in Napoleon failing to take Moscow after his armies battled relentless snow.
It is also possible that there was a comet passing by around this time: A comet set the city Moscow on fire in 1812?So the Cs were right, we just didn't latch on to that particular bit of data.