Straddling the Tropic of Cancer, Bangladeshi climate is tropical with a mild winter from October to March, a hot, humid summer from March to June. 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 theBrahmaputra, the Ganges and Meghna spilt over and swallowed 300,000 houses, 9,700 kilometres (6,027 mi) of road and 2,700 kilometres (1,678 mi) of embankment 1,000 people were killed and 30 million more were made homeless with 135,000 cattle killed, 50 square kilometres (19.3 sq mi) of land destroyed

and 11,000 kilometres (6,835 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 intercept rain water had been cut down for firewood or to make space for animals.
Bangladesh is now widely recognized 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 change, each seriously affecting agriculture, water & 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 natural floods, tornados and cyclones.