We have developed a one-dimensional numerical computer model (simulator) to describe methane hydrate formation, decomposition, reformation, and distribution with depth below the seafloor in the marine environment. The simulator was used to model hydrate distributions at Blake Ridge (Site 997) and Hydrate Ridge (Site 1249). The numerical models for the two sites were conditioned by matching the sulfate, chlorinity, and hydrate distribution measurements. The constrained models were then used to investigate the effect of changes in seafloor temperature and sea-level on gas hydrate stability. For Blake Ridge (site 997), changes in hydrate concentration are small. Both the changes in seafloor temperature and sea-level lead to a substantial increase in gas venting at the seafloor for Hydrate Ridge (site 1249).