Sea levels rise and lower when ___ form and

ice sheets ... melt

Explanation:

The sea levels depend primarily on the global climate. If the global climate is experiencing cooling down and it enters into an ice age, the low temperatures will cause lot of water from the sea to freeze and create enormous ice sheets. This will cause drop in the sea levels. On the other hand, when the global climate is warming up, the ice sheets melt, which results in enormous amounts of water to return back to the sea. This results in an increase of the sea levels.