I'm writing a story where a total nuclear WWIII occurs in the early 21st century. in a matter of minutes all of the world's nuclear powers unleashed their entire arsenals on each other in an event known as the Second Wave. Although it caused an unbelievable amount of damage in the short term (crop failure, a nuclear winter, annihilating our biggest cities, etc.), the biggest impact of it all came far after the end of the war.
After the initial nuclear blasts scattered fallout into our atmosphere, much of it fell on top of our ice caps as snow in the nuclear winter that followed. Over the course of the next 50 years, the radiation it created heated up the snow and ice; destabilizing the ice caps and accelerating sea level rise to levels that (at our current rate) we thought we wouldn't see until a few hundred years from now (most of the Earth's original coastlines are submerged, the Midwest is severely flooded by the expanded Mississippi River, the entire Florida peninsula is completely gone from the map, etc.).
Could a scenario like this really occur, and if so, how much damage could an event like this actually cause?