因为没有上下文,所以对prospect一词的理解有点迷惘,暂且不管prospect确切词义,其实这个长句就是：prospect is equally so( prospect同样如此）,of letting that fear prevent us from acting是prospect的定语,
when the failure to act could mean untold misery for future generations and perhaps environmental catastrophes which threaten our very existence.是状语,就是在这个条件下（when.),PROSPECT IS EQUALLY SO.