Eustasy refers to what aspect of the environment?

Basically global sea level. Eustasy compares all the water in the oceans to a gigantic bathtub. Water entering or leaving the system changes the total water levels. There are three major ways the level changes. 1) Glacier melt, 2) plate tectonics, and 3) thermal expansion. As the Earth warms more glaciers melt. Plates are constantly moving and sometimes they can decrease the level by forming a gap or they can increase the level by forming a ridge. As for thermal expansion as the ocean heats up it takes up more space. So the same number of molecules expand as they heat up.

https://en.wikipedia.org/wiki/Eustatic_sea_level