The Earth has likely been fully enveloped in ice and snow before, you should know that there were several ice ages and the one our ancestors survived was one of the smaller. Damn if we had another snowball earth we would have never existed. When a megavolcano erupts it can spew it ashes so high that it envelops the whole planet and blocks sunlight and that is what causes cooling on a global scale.
Snowball Earth is actually a highly disputed theory. But if it happened my main point still stands: the climate changes took place over tenthousands of years. Small local changes through vulcanic eruptions may have happened faster. Now we have global changes in the timeframe of decades. This is new.