【正确答案】It is unlikely that environmental changes will extinguish the entire human race, but it is difficult to maintain/sustain the existing civilization

【答案解析】(“环境变化导致全人类灭亡”为句子主语，当主语太长时，常用代词it代替放句首，而把真正的主语放置于句尾。extinguish消除，消灭。“全人类”可译为entire human race。)