TIt takes years for methane to come out of the atmosphere. In that time, methane is something like 50 times more potent of a greenhouse gas than CO2. If all the methane hydrates get dumped in an incredibly short time period (could be a couple of decades in geological timescales) the effects would be unbelievable. Fuck 4C warming. It could be 10C. Maybe more. We have no idea what would happen.