The sea is warming and as the water warms its expansion is causing the sea level to rise. In the future, it is expected that sea level rise will also result from melting glaciers in the northern and southern hemisphere. Each species of mangrove lives within a certain range of the tide. Over a very long period of time as new plants grow from seeds each species will find a new home with a suitable tide.
However the land where a lot of mangroves could move to has been developed. Around our towns and cities these areas will be protected by seawalls.
Land owners will not want to lose their land to the sea. The saltmarsh that lives behind the mangroves will be under greater threat from sea level rises.