system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If the planets oceans raise 8ft, what happens to the volume of atmosphere being replaced? Does it compress or does the atmosphere grow wider?
gpt
Technically, the atmosphere has to *contract* around the planet.  The water that's added to the oceans are locked in glaciers, which are 'piled' up above sea level, to an extent.  Since water expands as it freezes, it's actually taking up more volume than it will once it melts.  

However, to address the opposite side of your question, the combined diameter of the earth plus atmosphere would simply grow slightly if mass was added.