The Freeze was a human-engineered catastrophic event that occurred seven years prior to the boarding of Snowpiercer. It was an attempt to reverse the effects of global warming, however, it went disastrously wrong and instead caused a massive drop in temperatures which froze the Earth; Snowpiercer carries the only survivors of humanity.[1]
It is revealed that the temperature outside the train is -119 degrees Celcius in the TV series.