Sundarbans is a mangrove area in the delta formed by the confluence of the Ganges, Brahmaputra and Meghna Rivers in the Bay of Bengal. It spans from the Hooghly River in India’s state of West Bengal to the Baleswar River in Bangladesh’s division of Khulna. It comprises closed and open mangrove forests, land used for agricultural purpose, mudflats and barren land, and is intersected by multiple tidal streams and channels. Four protected areas in the Sundarbans are enlisted as UNESCO World Heritage Sites, viz. Sundarbans National Park, Sundarbans West, Sundarbans South and Sundarbans East Wildlife Sanctuaries.
Despite these protections, the Indian Sundarbans were considered endangered in a 2020 assessment under the IUCN Red List of Ecosystems framework. The Sundarbans mangrove forest covers an area of about 10,000 km2 (3,900 sq mi), of which forests in Bangladesh’s Khulna Division extend over 6,017 km2 (2,323 sq mi) and in West Bengal, they extend over 4,260 km2 (1,640 sq mi) across the South 24 Parganas and North 24 Parganas districts. The most abundant tree species are sundri (Heritiera fomes) and gewa (Excoecaria agallocha). The forests provide habitat to 453 fauna wildlife, including 290 bird, 120 fish, 42 mammal, 35 reptile and eight amphibian species. Despite a total ban on all killing or capture of wildlife other than fish and some invertebrates, it appears that there is a consistent pattern of depleted biodiversity or loss of species in the 20th century, and that the ecological quality of the forest is declining.
Despite preservation commitments from both governments, the Sundarbans are under threat from both natural and human-made causes. In 2007, the landfall of Cyclone Sidr damaged around 40% of the Sundarbans. The forest is also suffering from increased salinity due to rising sea levels due to climate change and reduced freshwater supply. In May 2009 Cyclone Aila devastated the Sundarbans with massive casualties. At least 100,000 people were affected by this cyclone. The proposed coal-fired Rampal power station situated 14 km (8.7 mi) north of the Sundarbans at Rampal Upazila of Bagerhat District in Khulna, Bangladesh, is anticipated to further damage this unique mangrove forest according to a 2016 report by UNESCO. Climate change is expected to both continue to negatively effect both natural systems and human populations in the region, resulting in further ecosystem degradation and climate migration. Experts examining the region recommend further focus on mangrove restoration and management and advocating for adaptation of human populations, through processes like managed retreat and investments in resilient infrastructure.
The Sundarbans are at the mouth of the Ganges, which carries water from the rain-fed highlands of central India to Bengal. The area is flat and low-lying, with an average elevation of 0.1 m (4 in) above mean sea level. The highest point in this region of Bangladesh is found on an island in the Bay of Bengal, about 10 km (6.2 mi) from the mainland; it rises to 5 m (16 ft).
The Sundarbans are subject to violent cyclones coming off the Bay of Bengal. The sea level around the Sundarbans is unusually high, possibly due to silt deposits from the Ganges. This has also contributed to the mangroves growth. The tidal range in the region is heavy; storm surges of 4–5 m (13–16 ft) above normal tides are common.
The area’s climate is humid, with temperatures reaching 42 °C (108 °F) in summer and 10 °C (50 °F) in winter. Temperatures at the coast average 1 °C higher than inland areas, low humidity and high rainfall year-round. Humidity is high throughout the year, with an average annual rainfall of 1,100 mm (43 in) across the Sundarbans. Cyclones and tropical storms are a major threat to Sundarbans landscapes. The coasts are particularly vulnerable to infrastructural damage from storm surges generated by cyclonic storms.
The floods spread across the Sundarbans after tides rise close to 8 metres (26 ft) above normal. In this context, a storm surge of 4 m (13 ft) caused extensive damage after Cyclone Aila struck Bangladesh in 2009. The total area affected by the cyclone was about 200 km2 (77 sq mi), 90% of which lies in West Bengal and 10% in Bangladesh.
Geology and hydrology
Satellite image of the Sundarbans, with India on top and Bangladesh on bottom.
The Sundarbans were formed in a delta of three major rivers: the Ganges, Brahmaputra and Meghna rivers. The origin of this sediment is primarily from bhanga or old highland material from central India. However, most of the infill in the Sundarbans Delta is derived from erosion by the Ganges. The main channel location has been stable since 1839 when it diverged from its previous bank near Dhulian Hill. This bank lies to the west of the present location of the channel.
The delta has been formed due to massive deposition of sediments over a long period of time, which is attributed to tectonic uplift. The infill sediments are composed of mainly coarse-grained sands, silts and clays, with some fine sands. Mangrove forests are found in the moist low-lying areas. A number of different soil types are found in this area with each soil being derived from local parent material. The soils are generally rich in iron but low in nitrogen, phosphorous and humus. The variation in soil properties is mainly due to variations in porosity and texture. The main soil types are Typic Deltist, Ombric Typic Medipedust, Histosol and Histosol Ultisol.
The Sundarbans’ rainfall is highly seasonal. Though the annual average rainfall amounts to 650 mm (25 in), it is concentrated during the monsoon season from June to September. The highland areas surrounding the Sundarbans receive a total of 1,700 mm (67 in) of rainfall a year while the coastal belt receives an average annual rainfall of 600 mm (24 in). The highest mean monthly maximum temperature recorded was 43.6 °C (110 °F) in 2017. The mean minimum temperature is 21.5 °C (70 °F) while the mean annual temperature is about 17 °C (62 °F). The average high tides are 2.06 m (7.9 ft) while the average low tides are 0.88 m (2 ft).