We evaluate the sea level changes caused by changing the age/area relationship of the seafloor by altering the thermal structure and hence the elevation of the world seafloor. We do this by having a passive-margined (and therefore continuously aging) ''Atlantic-type'' ocean open at a uniform rate at the expense of an active-margined (and therefore constant age) ''Pacific-type'' ocean. This run of the model was specifically constructed to answer the question of how much sea level would change if the Atlantic opened at a uniform rate. Therefore, it specifically excludes the other major processes that change sea level. These would include changes in ice volume, other changes in plate generation/consumption rates, continental orogeny and epei rogen, lithospheric stretching, lithospheric flexure, and thermally induced subsidence at passive margins and amplification thereof caused by water and sediment loading. The isostatic effects of continental flooding and oceanic unloading are accommodated by application of balanced global isostasy to an Earth model consisting of 121 independent columns. The lithospheric portions of the columns are linked to the hypsographic curve. Sea level changes are viewed from two different observational positions: (1) geologically stable and continually emerged continental plateau and (2) geologically stable and continually submerged shelf break. The modeled sea level that results from changing the age/area relationship of the seafloor since 160 Ma peaks at ~85 Ma and is ~30% less for the emerged observer (34 m) than for the submerged observer (48 m). Furthermore, the peak is in phase with, but only ~1/4 (48 m versus 200+ m) of that portrayed by either the ''eustatic'' curves inferred from seismic stratigraphy or the isostatically readjusted positions of marine strandlines of the stable platform of North America. We therefore conclude that the other 3/4 of sea level rise that accompanies the opening of the Atlantic must be partitioned among the other causes suggested above. ¿American Geophysical Union 1991