求助，曼哈顿里面有一处不明白，求指教

KK要努力过GMAT

再看chapter4 modifiers那里
Right: Citizens of many countries are expressing concern about the environmental damage caused by the widespread release of greenhouse gases, DAMAGE THAT may be impossible to reverse.
这句话是对的，但是我不晓得后面的“DAMAGE THAT may be impossible to reverse.” 是什么结构？同位语？独立主格？修饰什么？

希望NN们指教~！