In other words, either the deep heat or fire under the earth beds will lead to natural calamities like volcanoes, earthquakes and tsunamis which will one day end the world or the melting of the ice from the snowy mountains due to global warming will shrink the world and one day the entire world will die of the icy water.
But I feel that the poet is pointing more towards the emotional and sentimental side of the issue. As per him, fire denotes deep passion and the burning desire, while ice is highlighted for icy cold and hatred side of the emotions. He is favoring both the arguments to be equally valid. It is either the deep burning passion or the cold hatred jealousy factor which push people to walk on the path of destruction.
The poet is very much sure of this destruction of the humanity. He first talks about the destruction because of fire, and in case this fails, then ice will the world is full of hatred means the sweet love among people for each other has vanished, and this hatred will end humanity one day.
The author means that the love is full of passion for oneself and in this race of climbing up; people tend to hate each other. And the worst part is that these negative emotions are so strong that it surpasses the sweet relationships of love and humanity.