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 using CW-7, 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 during its passing of Yukon, Canada is around -120 degrees Celcius.