Bangladesh is among the most disaster prone countries in the world and will become even more so as a result of climate change. Floods, tropical cyclones, storm surges and draughts are likely to become more frequent and severe in our country. These changes will threaten the significant achievements that Bangladesh has made over the past 25 years in increasing income and reducing poverty. Over the last 40 years, the Government of Bangladesh, with the support of development partners, has invested over 10 billion US$ in disaster management to make country less vulnerable from natural disasters. In addition, Government at different times have developed state-of-the art warning systems for floods, cyclones and storm surges and are still trying to expand the community-based disaster preparedness.
Setting the Scene
Bangladesh in general is subject to floods because much of the country occupies the deltaic floodplains of the Brahmaputra/Jamuna, Ganges/Padma, and Meghna Rivers, which is the second largest river system in the world. This research aims to observe the climatic trend (temperature, rainfall and humidity) for last 57 years, evaluate the impact of climate induced flood vulnerability and predict the future climatic condition. The unpredictability of climate factors poses new challenges for different livelihood groups to understand and adapt to floods and its impacts. The challenge, Bangladesh now faces is to scale up different projects and investments to create a suitable environment for economic and social development of the country and to secure the well-being of our poorest and the most vulnerable groups including women and children.
Variation of Climatic Parameter
Climate Change refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from that in the United Nations Framework Convention on Climate Change, which defines “climate change” as: ‘change in climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural variability observed over comparable time periods”. In Bangladesh there are four prominent climatic seasons, namely, winter (Dec.-Feb.), Pre-monsoon (March-May), Monsoon (June-Sep.) and Post-monsoon (Oct.-Nov.). The monsoon has its onset during the first week of June and in the first week of October. However, the onset and withdrawal dates vary from year to year.
In all points of the world, one-year, one decade, one country differs from another. Temperature variations from year to year and from epoch to epoch generally increase towards high latitudes. Rainfall variations are greatest in low latitudes, where the heaviest individual falls occur, rain and snowfall varies most in and near the warm and cold deserts of the tropics and Polar Regions.The most recent assessment by the Intergovernmental Panel on Climate Change projected that global average surface temperature would increase by between 1.4 to 5.8°C over the period 1990 to 2100, while sea-levels could rise by between 9 and 88 centimeters.The increasing frequency and intensity of extreme weather events – including droughts, storms and floods - could result in crop damage and land degradation. Crop yields vary considerably according to temperature.Rainfall is the single most important climatic parameter influencing agriculture of our country since some 75 to 80% of the cultivated land is non-irrigated. This is the free source of water directly and most uniformly available to crop and the foliage. But it can be utilized most efficiently by reducing its harmful effect and increasing beneficial outcomes.
Maximum and Minimum Temperature
Bangladesh has a tropical monsoon type climate, with a hot and rainy summer and a pronounced dry season in the cooler months. January is the coolest month of the year, with the temperature ranging 13.5°C to 26.5°C, and April the warmest month, with the temperature ranging 33°C and 36°C. In rare cases the temperature goes down less than 50°C but never touches freezing point.It is evident that, our region has been getting warmer. According to the climatic situation, Bangladesh may be divided into following climatic sub-regions: a) South-eastern zone, b) North-eastern zone, c) Northern part of the northern region, d) North-western zone, e) Western zone, f) South-western zone, and f) South-central zone.
The annual mean temperature over Bangladesh has a slight increasing trend during the whole period of 1961-90, but the trend is not statistically significant. Karmakar and Shrestha (2000) reported that the present 5-year running average trends of climate elements continue, the annual mean maximum temperature was likely to rise by 0.48°C and 0.88°C by 2050 and 2100 years respectively whereas the annual mean minimum temperature was likely to decrease by 0.66°C and 0.11°C by 2050 and 2100 years respectively. But the overall annual mean temperature over Bangladesh was likely to increase by 0.21°C and 0.39°C by 2050 and 2100 years, respectively. In some places in Rajshahi and Kusthia districts the maximum temperature in summer season rises up to 40°C or more. After April, temperature decreases slightly during the summer months, which coincides with the rainy season.
Scarce and Surplus Rainfall
Monsoon and winter seasons are separated by two transitional seasons namely pre-monsoon and post-monsoon. However, life-giving rain comes during south-west monsoon accounting for over 70% of the total annual rainfall. It is to be noted that the high lands situated to the east and north of Bangladesh play their due role to this monsoonal rain. Hussain (2001) found that the mean annual rainfall was 2387.20 mm from 1975 to 1995. May to September were the highest rainfall months when the rainfall more than 300 mm in over 63.80% of the years and always more than 50 mm white May to June could be useful for kharif (April to September ) sowing. The prediction models for drought of different meteorological variables in Bangladesh. Sylhet is the heavy rainfall region in Bangladesh. In the month of April and May, Sylhet receives 340 mm and 650 mm rain respectively. Lalkhan, the north-east border of Sylhet receives 560 mm and 600 mm rainfall in the month of April and May respectively. The social and economic conditions of million of the people of the subcontinent are largely dependent upon this rainfall. More than 70% of Bangladesh's annual rainfall occurs in the monsoon (June- September) season. The mean rainfall during the monsoon season ranges from 1000 mm to 3000 mm in the country. The maximum rainfall occurs at Sylhet in the north-eastern part and along the coastline in the southern part and with a minimum in the west central part.
Only for individual months of November, December January and February Bangladesh falls in aridity condition. In March, whole country except Sylhet, Rangamati, Chittagong and Cox's Bazar falls in arid zone. The annual most of the monsoon precipitation occurs during July and August. Sylhet shows very high monsoon precipitation and Rajshahi shows a relatively low monsoon precipitation compared to the other stations. Amin (2004) analyzed the period between May to October was surplus period for rainfall and generally no problem with agricultural and other water-base activities in Bangladesh. The months from November to March and in some instant up to April were deficit period for rainfall.Rainfall at 50% and 80% probability with respect to crop demand was almost scarce throughout the crop season. The irrigation water requirement during different months of the crops growing period is a function of rainfall deficits in those months for planning an irrigation water supply system. Thus, rainfall deficit information for different areas and periods can greatly help in determine optimal water release from a reservoir in accordance with demand, when rainfall exceeds evaporation, soil moistures reserves are recharged till filed capacity is reached and any further rainfall is termed as surplus. When rainfall is less than evaporation, soil moisture is utilized and rainfall deficit conditions occurred. The rainfall deficits are functions of time.In the year 1957, 1958, 1972 and 1979 longer dry season than the normal is recorded in Bangladesh. In 1966 and 1967, however, the duration of dry season is less than the normal. During the drought years (1957, 1958, T966, 1967, 1972 and 1979) the highest values of duration of dry season are observed in the northern and western parts of the country.
State of Humidity
In north region of Bangladesh, the annual average humidity showed increasing trend. Annual average maximum humidity over the period of study is 216.75% and annual average minimum humidity over the period of study is 71.92%. Analyzing the last 57 years (1960-2017) average humidity data, we find the average annual humidity is fluctuating. From 1960 to 2006 the average rainfall is augmenting but after 2006 to present the average rainfall is gradually decreasing.
By bringing greater fluctuations in crop yields, local food supplies and higher risks of landslides and erosion damage, can adversely affect the stability of food security. There are some policy related guidelines which can help to take decision to improve rice production in Bangladesh. Different studies are needed to assess the impacts of climate change; develop a flood level and risk map based on future projected climatic parameters; zoning of flood plain depends on various level of vulnerability; develop climate resilient cropping patterns suited to different regions of the country depends on monsoon flooding; assess the drainage capacity of all river and canal to investigate structural and nonstructural causes of water logging within the catchment area; develop early warning system and trained the vulnerable people to reduce the vulnerability.
The Writer is an Environmental Analyst & Associate Member, Bangladesh Economic Association.