It is said that if a full-scale global nuclear war were to break out, this 'nuclear winter' would continue for several months on a planetary scale.
Literal
Global [attributive-な] full-scale nuclear-war [subject-が] break-out [if-ば], earth-scale [at-で] this 'nuclear winter' [subject-が] several-months [emphatic-も] continue [quotative-と] is-said-[passive-polite-ています].
Formal scientific/explanatory register. ~と言われている is a standard impersonal construction for 'it is said that X,' attributing the claim to a general authority without naming anyone. The ~ば conditional marks a hypothetical situation. 数カ月も ('as much as several months') uses the emphatic も to stress duration — 'as long as several months.' 地球規模 'global scale, planetary scale' is a kango compound. 核の冬 ('nuclear winter') is a calque of the English scientific term.