Isostatic adjustment is a corrective factor that scientists use to account for the fact that the basins in the ocean have increased in size since the last cycle of glaciers came to an end. This is not the result of glaciers melting, but instead the recovery of the Earth's surface from the once heavy, thick ice sheets that covered the majority of Europe and North America.
The sheets that lay on the Earth's surface were several kilometers thick, and that significant weight caused major shifts in the planet's structure. As a result, part of the Earth's mantle is still emerging from beneath the oceans into land areas that previously featured glaciers, with the effect that the Earth has some land surfaces that are rising and some ocean depths still falling in relationship to the Earth's center.
This means that glacial isostatic adjustment, or GIA, is leading to a shift in sea level change of -0.3 millimeters per year. This is a minuscule correction magnitude, but the uncertainty involved is a minimum of 50 percent. In general, the ocean is actually gaining room over time, and the purpose of the isostatic adjustment is to make sea level reflect oceanographic phenomena only.