I am writing a novel based on earth, though not the time or place we live in now. I'm looking for maps that show what the earth might look like in different situations, such as if the water level rose 20 feet world wide, how would that effect livable space? How would the world be different? I am also interested in focusing mostly in Europe and Asia, rather than north America. I was wondering if there is a program that anyone knows of that would allow me to play with earth to create what I'm looking for. Say an adjustable world map or something like that.

If there is nothing available, would anyone know of a way to do something like this simply? I'm not looking for specific detail right now, I just need to get some basic mapping ideas before I move forward with this.