Atolls, in an undeveloped state, will always be just a few feet about sea level. They are the result of an equilibrium between deposition and erosion. If the sea level were to drop, an undeveloped atoll (without walls, paving, etc) would erode downwards because of greater wind exposure. Attempting to fix the level of an atoll by paving it will not work in the long term, regardless of what the climate does.