Climate change and human interference may destroy much of Sundarbans in India and Bangladesh as well as 25 other World Heritage sites, says a new UNESCOreport.
"Sea-level rise is the greatest threat and challenge for sustainable adaptation within South and Southeast Asia. A 45 cm rise in global sea levels would lead to the destruction of 75 percent of the Sundarbans mangroves," the report warned.
The report, "Case Studies on Climate Change and World Heritage", features 26 examples - including Sundarbans (in India and Bangladesh), Sagarmatha National Park (Nepal), the Tower of London, Timbuktu in Mali and the Great Barrier Reef in Australia.
The World Heritage List has 830 sites across the world.
Along with global sea level rise, there is a continuous natural subsidence in the Sundarbans, causing a rise of about 2.2 mm per year. The resulting net rise rate is 3.1 mm per year at Sagar, the biggest delta of Sundarbans, the report added.
The consequences in terms of flooding of low-lying deltas, retreat of shorelines, salinitisation and acidification of soils, and changes in the water table raise serious concerns for the well being of the local population.
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 Kolkata. This barrage diversion induced a 40 percent decrease of the dry season flow.
Jointly, the sea level rise and lower freshwater flow in winter will also result in increased salinity in the area, threatening the conservation of the Sundarbans mangroves.
Spread over 10,000 sq km, the Sundarbans, the largest of such forests in the world, lie within the delta of the Ganges, Brahmaputra and Meghna rivers in the Bay of Bengal. A complex network of tidal waterways, mudflats and small islands of mangrove forests intersect the site.
Famous for its beauty, it is home to at least 260 bird species, Indian otters, spotted deer, wild boar, fiddler crabs, mud crabs, three marine lizard species and five marine turtle species. They also host threatened species such as the estuarine crocodile, Indian python and the iconic Bengal tiger.
For these reasons, the Sundarbans National Park in India and the Bangladesh part of the Sundarbans were inscribed on the World Heritage list in 1987 and 1997 respectively.
Underlining that climate change will constitute one of the major challenges of the 21st century, UNESCODirector General Koïchiro Matsuura in the report has called for "an integrated approach to issues of environmental preservation and sustainable development".
The publication stressed that these mangroves have been acting as protective buffers against tropical cyclones that are common in the Bengal basin. About 10 percent of the world's tropical cyclones occur in this area of Bay of Bengal, and 17 percent of them 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," the report said.
In the Sundarbans' case study it was mentioned that a project of the UN Development Programme (UNDP) has evaluated the cost of building 2,200 km of protective storm and flood embankments which would supposedly provide the same level of protection as the Sundarbans mangroves.
The capital investment was estimated at about $294 million with a yearly maintenance budget of $6 million - much more than the amount currently spent on the conservation of the mangrove forests in the area.
The UN body has said that though the sea level rise cannot be entirely prevented, conservation of remaining mangrove forests in protected areas and re-planting selected mangrove tree species along freshwater canals of reclaimed land would help in restoration.
The UN body is also worried that the melting of glaciers around the world is affecting the appearance of sites inscribed for their outstanding beauty and destroying the habitat of rare wildlife species such as the snow leopard in the Sagarmatha National Park, Nepal.
The report also examines the effects of climate change on the marine World Heritage sites. By 2100 AD, 70 percent of the world's deep-sea corals are expected to be affected due to rising temperatures and increased ocean acidification.
The Great Barrier Reef in Australia would be subjected to frequent bleaching events, cases in which corals turn white and may die due to rising sea temperatures. At least 58 percent of the world's coral reefs - home to hundreds of thousands of fish species - are considered to be at risk.