The Freeze was a human-engineered catastrophic event that occurred seven years ago during 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 so severely that no life could survive; Snowpiercer and Big Alice carry the only known survivors of humanity.[1]
It is revealed that the temperature outside the train during its passing of Yukon, Canada is around -120 degrees Celsius.
While traveling through Yukon, a class 3 avalanche breaks a window to the Cattle Car, killing all of the cows and workers. Later, Bojan Boscovic is sitting in a room, when a Breach Team opens a door to the part of the exposed Cattle Car and feels The Freeze on his face, before the Team closes the door.
After realizing Andre Layton escaped from The Drawers, Melanie Cavill tortured Josie Wellstead in The Hospitality Car, by using a hose filled with The Freeze and shattered one of her fingers. Later, with help from Bess Till, she freezes the rest of her hand to escape and attempts to kill Melanie by flooding the room with The Freeze. But Melanie escapes and Josie suffers from severe frostbite.
During the Revolution, the Jackboots executed prisoners, such as Walter Flemming, by making them breathe in The Freeze, with the Lungs of Ice.
At the end of the Revolution, 7 cars were separated from the train and sent onto a siding. The occupants froze to death in 13 minutes.
In Season 2, Melanie discovers snow falling despite the fact that it is supposedly too cold for snow, suggesting that the planet is beginning to recover from the Freeze and to come alive again. After further study, Melanie determines that CW-7 is actually breaking up in the upper atmosphere and falling to the Earth, meaning that the planet will potentially be suitable for colonization again in the survivors' lifetime rather than in a thousand years.