5.1 Land Loss

A rise of a few mm per year by the sea, although not threatening spectacular inundation is still extremely important. Direct land loss of low lying areas can rapidly (decadal to centennial periods) damage or destroy coastal ecosystems. Rising sea levels inundate wetlands and other low-lying lands, erode beaches, intensify flooding, and increase the salinity of rivers, bays, and groundwater tables. Some of these effects may be further compounded by other effects of a changing climate.

Coastal wetland ecosystems, such as salt marshes and mangroves are particularly vulnerable to rising sea level because they are generally within a few feet of sea level (IPCC, 2007). Wetlands provide habitat for many species, play a key role in nutrient uptake, serve as the basis for many communities’ economic livelihoods, provide recreational opportunities, and protect local areas from flooding.

As the sea rises, the outer boundary of these wetlands will erode, and new wetlands will form inland as previously dry areas are flooded by the higher water levels. The amount of newly created wetlands, however, could be much smaller than the lost area of wetlands – especially in developed areas protected with bulkheads, dikes, and other structures that keep new wetlands from forming inland. The IPCC suggests that by 2080, sea level rise could convert as much as 33 percent of the world’s coastal wetlands to open water. (IPCC, 2007). Tidal wetlands are generally found between sea level and the highest tide over the monthly lunar cycle. As a result, areas with small tide ranges are the most vulnerable.

In addition to inundation, long-term sea level rise can cause erosion and shoreline retreat by creating a sediment budget deficit. Coastal erosion is caused by a variety of factors, which broadly fall into two categories. First, sand often migrates along the shore, causing some areas to erode and others to accrete. Second, rising sea level causes virtually all shores to erode. Using a model first developed by Danish coastal engineer Per Bruun, coastal geologists have estimated that a one-meter rise in sea level will cause beaches to erode 50 to 100 meters from New England to Maryland, 200 meters along the Carolinas, 100 to 1000 meters along the Florida coast, and 200 to 400 meters along the California coast. These model calculations are roughly consistent with the observed rate of erosion. (http://www.cmar.csiro.au/sealevel)