这个观点是：人们没能发现已经在海洋中发生的巨大变化,因为他们仅仅回顾了很短的一段历史.
The notion 是主语,that people have failed to detect the massive changes which have happened in the ocean because they have been looking back only a relatively short time into the past这一整个同位语从句作表语,在这个从句中,people have failed to detect the massive changes which have happened in the ocean because they have been looking back only a relatively short time into the past是一个因果句,because they have been looking back only a relatively short time into the past是因果句中的从句,people have failed to detect the massive changes which have happened in the ocean是主句,在这个分句中,which have happened in the ocean是定语从句,修饰限制the massive changes .