Environmental and economic implications of rising sea level and subsiding deltas: the Nile and Bengal examples.
The sea level could increase 0.2 or 2.2 by 2100 hinging on the effects of carbon dioxide and chlorofluorocarbons on atmospheric temperature changes, parallel heat transfer to and thermal expansion of the ocean, more precipitation, and melting of polar ice. Local sea level increases also depend on subsidence. Normally deposition of sediment equalizes delta subsidence. Yet many activities change this balance including channeling, diverting, or damming rivers; destruction of mangrove forests; and removal of groundwater or hydrocarbons. Large increases in local sea level are a problem especially for low lying deltas with a large human population. 80% of Bangladesh comprises the Bengal Delta. Annual floods can cover as much as 35% of the entire country. India has diverted flow from the Ganges causing rising salinity in coastal streams in Bangladesh. Considerable drilling of shallow and deep wells has resulted in subsidence of twice the normal rate. Destruction of mangrove forests increases coastal erosion in Bangladesh. In Egypt, almost all the productive land and most of the population are located in the Nile Delta. The coast has an almost endless band of 1-5 m dunes which could protect Egypt against a sea level rise, but the Aswan Low and High dams have prevented the deposition of sediment in the Delta leading to coastal erosion. They have also ceased the freshwater influx to the delta causing increased salinization of soils. This in turn increases the demand for drilling shallow and deep wells and the increased removal of groundwater hastens subsidence. By 2100, local sea level rises of these 2 deltas may be as high as 3.3 4.5 m. This could result in the loss of 26% of habitable land in Egypt and 34% in Bangladesh. Rising sea levels would worsen environmental and economic effects. Future coastal and fluvial planning for all areas with low lying deltas and large rivers should consider these scenarios.