I have an urban fantasy novel set in an undisclosed time in the future. A lot of the cities are going to be rundown or destroyed. The weather is going to be weird, and the sea levels are going to be higher. Is it necessary to explain how this happened? It isn't central to my plot. There isn't a big war going on or a fight for life or death because of the weather. It's just there. But should I include it anyway for the sake of a backstory? Any advice appreciated.