Biodiversity and climate of Bangladesh

Bangladesh is dominated by the low-lying Ganges Delta, but has highlands in the north and southeast. The Ganges delta is formed by the confluence of the Ganges (local name Padma or Pôdda), Brahmaputra (Jamuna or Jomuna), and Meghna rivers and their respective tributaries. The Ganges unites with the Jamuna (main channel of the Brahmaputra) and later joins the Meghna, finally flowing into the Bay of Bengal. The alluvial soil deposited by the rivers when they overflow their banks has created some of the most fertile plains in the world. Bangladesh has 57 trans-boundary rivers, making water issues politically complicated to resolve – in most cases as the lower riparian state to India.

Most parts of Bangladesh are less than 12 m (39.4 ft) above sea level, and it is estimated that about 10% of the land would be flooded if the sea level were to rise by 1 m (3.28 ft). Sylhet Division, Chittagong Division and parts of Rangpur Division, Mymensingh District and Gazipur District feature topographically hilly areas, parts of regional mountain ranges and highland formations that include the Garo, Khasi and Tripura Hills, the Bhawal highlands, theChittagong Hill Tracts and the Arakan mountains.

In southeastern Bangladesh, experiments have been done since the 1960s to 'build with nature'. Construction of cross dams has induced a natural accretion of silt, creating new land. With Dutch funding, the Bangladeshi government began promoting the development of this new land in the late 1970s. The effort has become a multiagency endeavor, building roads, culverts, embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some 27,000 acres (10,927 ha) to 21,000 families.

With an elevation of 1,052 m (3,451 ft), the highest peak in Bangladesh is Saka Haphong, in Mowdok range in the Chittagong Hill Tracts, in the southeastern part of the country.

Straddling the Tropic of Cancer, Bangladeshi climate is tropical with a mild winter from October to March, and a hot, humid summer from March to June. The country has never frozen at any point on the ground, with a record low of 4.5 °C in the south west city of Jessore in the winter of 2011. A warm and humid monsoon season lasts from June to October and supplies most of the country's rainfall. Natural calamities, such as floods, tropical cyclones, tornadoes, and tidal bores occur almost every year,combined with the effects of deforestation, soil degradation and erosion. The cyclones of 1970 and 1991 were particularly devastating. A cyclone that struck Bangladesh in 1991 killed some 140,000 people.

In September 1998, Bangladesh saw the most severe flooding in modern world history. As the Brahmaputra, the Ganges and Meghna spilt over and swallowed 300,000 houses, 9,700 km (6,000 mi) of road and 2,700 km (1,700 mi) of embankment, 1,000 people were killed and 30 million more were made homeless, with 135,000 cattle killed, 50 km2 (19 sq mi) of land destroyed and 11,000 km (6,800 mi) of roads damaged or destroyed. Two-thirds of the country was underwater. There were several reasons for the severity of the flooding. Firstly, there were unusually high monsoon rains. Secondly, the Himalayas shed off an equally unusually high amount of melt water that year. Thirdly, trees that usually would have intercepted rain water had been cut down for firewood or to make space for animals.

Bangladesh is now widely recognised to be one of the countries most vulnerable to climate change. Natural hazards that come from increased rainfall, rising sea levels, and tropical cyclones are expected to increase as climate changes, each seriously affecting agriculture, water and food security, human health and shelter. It is believed that in the coming decades the rising sea level alone will create more than 20 million climate refugees. Bangladeshi water is contaminated with arsenic frequently because of the high arsenic contents in the soil. Up to 77 million people are exposed to toxic arsenic from drinking water.Bangladesh is among the countries most prone to naturalfloods, tornados and cyclones. Also, there is evidence that earthquakes pose a threat to the country. Evidence shows that tectonics have caused rivers to shift course suddenly and dramatically. It has been shown that rainy-season flooding in Bangladesh, on the world’s largest river delta, can push the underlying crust down by as much as 6 centimetres, and possibly perturb faults.

A major part of the coastline is marshy jungle, the Sundarbans, the largest mangrove forest in the world and home to diverse flora and fauna, including the Royal Bengal Tiger. In 1997, this region was declared endangered. The Magpie Robin is the National Bird of Bangladesh and it is common and known as the Doyel or Doel . It is a widely used symbol in Bangladesh, appearing on currency notes and a landmark in the city of Dhaka is named as the Doyel Chatwar (meaning: Doyel Square). The national flower of the country is white-flowered water lily, which is known as Shapla. The national fruit is jackfruit (Artocarpus heterophyllus), which in Bengali is known as Kathal. In late 2010, the Bangladeshi government selected the Mango tree as the national tree.