Meaning of かくせんそう in Japanese

かりに核戦争が起こったとすれば、人類は滅亡するだろう。
If a nuclear war were to break out, mankind would perish.

世界的な全面核戦争が起これば、地球規模でこの「核の冬」が数カ月間も続くと言われています。
If there's a world-wide all-out nuclear war it's said that there will be a "nuclear winter" all over the planet for several months.