Climate change threatens the Sundarbans

Md. Tareq Mahmud, The New Nation 1 Jun 08;

The Sundarbans mangrove forests, the largest of such forests in the world (over 10,000 km2 of land and water, more than half situated in India, the rest in Bangladesh), lie within the delta of the Ganges, Brahmaputra and Meghna rivers on the Bay of Bengal. The site is intersected by a complex network of tidal waterways, mudflats, and small islands of mangrove forests.

Mangroves are made up of salt-adapted evergreen trees. They are restricted to the inter-tidal zone along the vast coastlines of tropical countries and extend landward along tidal rivers.

Mangroves act as natural buffers against tropical cyclones and also as filtration systems for estuarine and fresh water. They also serve as nurseries for many marine invertebrate species and fish.

The Sundarbans mangrove forests are wellknown for their biodiversity, including 260 bird species, Indian otters, spotted deer, wild boar, fiddler crabs, mud crabs, three marine lizard species, and five marine turtle species. But they also host threatened species such as the estuarine crocodile, Indian python and the most iconic Bengal tiger.

For these reasons, the Sundarbans National Park, India, and the Bangladesh part of the Sundarbans were inscribed on the World Heritage List in 1987 and 1997, respectively.

According to the IPCC, sea-level rise is the greatest threat and challenge for sustainable adaptation within south and southeast Asia.

The consequences in terms of flooding of lowlying deltas, retreat of shorelines, salinitization and acidification of soils, and changes in the water table raise serious concerns for the wellbeing of the local population.

In addition to global sea-level rise (or eustatic sea-level rise, i.e. the change in global average sea level brought about by an alteration of the volume of the world ocean), there is a continuous natural subsidence in the Sundarbans, which causes a sea-level rise of about 2.2 mm per year. The resulting net sea-level rise rate is 3.1 mm per year at Sagar.

Additional sources of stress, not related to climate change, include the diversion of upstream freshwater inflow of the Ganges by the Farraka Barrage in India since 1974 to alleviate the rapid siltation in the Port of Calcutta. This barrage diversion induced a decrease of 40% of the dry season flow.

The joint action of sea-level rise, increased evapotranspiration, and lower freshwater flow in winter will also result in increased salinity in the area threatening the conservation of the Sundarbans mangroves.

In the Sundarbans, as in many protected areas worldwide, conservation is threatened by several external factors and, again, climate change should be viewed as one source of stress among others. Altogether these factors could lead, in the case of a 45 cm rise in global sea level, to the destruction of 75% of the Sundarbans mangroves.

Further destruction of the Sundarbans mangroves would diminish their critical role as natural buffers against tropical cyclones. The Bay of Bengal is heavily affected by tropical storms: about 10% of the world's tropical cyclones occur in this area and 17% of these sweep the land in Bangladesh.

No matter whether the frequency or intensity of cyclones change in the future due to climatic disturbances, exposure of the region to the devastating effects of storms will increase if the mangroves cannot be conserved successfully.

Sea-level rise is typically a process that cannot be entirely prevented through site level strategies.

However, the following measures could help in increasing the adaptive capacity of the Sundarbans mangroves against the adverse effects of sea-level rise:

· conservation of remaining mangrove forests in protected areas;

· restoration or rehabilitation of mangrove forests through re-planting selected mangrove tree species, for example along freshwater canals of reclaimed land (successfully practiced on Sagar Island).

Such measures make sense both ecologically and economically. A project of the United Nations Development Programme (UNDP) has evaluated the cost of building 2,200 km of protective storm and flood embankments that would supposedly provide the same level of protection as the Sundarbans mangroves. The capital investment was estimated at about US$294 million with a yearly maintenance budget of US$6million49 - much more than the amount currently spent on the conservation of the mangrove forests in the area.