This map only shows land as lost when it is turned to open water. There are large areas of land, such as the area east of New Orleans, that have lost most of their elevation since 1956, but are still just barely above water. They will disappear from the map as go below water, creating the impression of sudden land loss when the reality is that a long term process of subsidence. If you fly over the area or try to walk on this land, it is clear that it is already lost and is just awaiting the final death of the marsh to disappear into open water.