West Bengal - Climate
The climate of the State is tropical and humid except in the northern hilly region which is close to the Himalayans. The temperature in the mainland normally varies between 24°C to 40°C during summer and 7°C to 26°C during the winter. The average rainfall in the State is about 1750 mm with considerable variation among the districts ranging between 1234 mm in Birbhum to 4136 mm in Jalpaiguri.
Presently the State is facing inadequate and erratic rainfall in successive years mainly in South Bengal Districts. Further, more than 80% of annual precipitation occurs during monsoon and that too erratically in respect of time and space causing drought like situations in summer and flood during monsoons. The onset of monsoon is delayed. There is an overall warming with minimum temperatures increasing faster than the maximum temperatures. Cyclone frequency of severe cyclonic storms is increasing over Bay of Bengal and the sea level is rising globally, however, the level of rise is higher across coast line of West Bengal, mostly due to subsidense of the land mass near the coast and also may be due to devlopmental activities, leading to submergence of islnads in the eastern region of the coast.
Long term observations between 1900-2008 show an increasing trend in the frequency of tropical cyclonic storms (48-63 kts or 88-117 kmph) as well as formation of severe cyclonic storms (64-90 kts or 118-167 kmph) in the Bay of Bengal during the period 1900-2008. Further, Storm surges form when heavy winds produced by tropical cyclones generate the disturbances in the ocean. As these surges propagate into the shallow regions, they amplify and produce large variations of sea level at the coast. Category 5 tropical cyclones can produce storm surges in excess of 6m (20 feet). Because the storm surge occurs ahead of the eye of the storm, the surge will reach coastal areas long before the cyclone makes landfall.
The key projections on the availability of rainfall in a mid century scenario (2021-2050s), does not suggest any major deficits in West Bengal with respect to the base line scenario, however, projections indicate a high regional variability in annual asell as seasonal rainfall across the state. The annual rainfall in Darjeeling Himalayan region and the coastal region is projected to increase in the mid century scenario. However, in the Himalayan region there is likely to be decreasing trend in winter rain fall. The increase in extreme rain fall events is likely to lead to higher run offs, and lower recharge of underground water, and frequent flash floods.
With increase in winter temperatures, decrease in yields of Rabi crops is anticipated such as wheat, horticultural crops such as potato and that of oil seeds including mustard. Further, as the annual average temperatures is projected to exceed by more than 1oC in the future a reduction in rice yields with respect to current levels is expected. Availability of quality seeds might be a concern in the future. In the coastal areas, increase in sever cyclones together with the continuous sea level rise is likely to inundate areas further inland rendering the soils excessively saline and making agriculture unviable in the region. Excess summer temperatures leading to heat stress, shortage in feed, increase in pests and diseases and higher levels of humidity are are likely to reduce milk production in livestock, including egg production in poultry as livestock suffer heat stress.
Specific health issues that are linked to climate are the morbidity and mortality rates due to - cyclones in the coastal areas, respiratory diseases arising from inhalation of pollutants from heavy vehicular traffic in urban areas and from burning of fossil/fuel wood for cooking in rural areas. Incidences of sunstrokes from intense heat are also quiet high in West Bengal, though number of cases between 2009 and 2010 showed a decline.
As relative humidity and temperatures increase, in areas like Kolkata, increase in vegetation growth is likely to provide more conducive habitats for malaria. Thereby malaria is likely to emerge as the most menacing disease agent for this mega city. Similarly, water borne diseases might increase with increase in flood frequency in plains and intensity of cyclones in South 24 Parganas as potable water availability gets affected. Since the pollutant load bearing capacity of the atmpsphere increases with increase in temperature, the respiratory diseases incidences are also likely to increase in the state unless source control is carried out. Malnutrition levels amongst the children may also increase, with less availaibility of food grains due to decrease in yields of staple crops with increase in temperature.
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