RISING sea levels will definitely be the biggest threat for Bangladesh among all the external drivers related to climate change. Sea level rise affects the coastal zone of Bangladesh and its geometry in a number of ways including inundation, erosion and saline water intrusion into the water table. Climate change induced potential rise in sea level will aggravate the risks to coastal zones. In particular, the poorest people living in the vulnerable regions are the most susceptible to climate-induced risks and will continue to suffer the most. It is therefore important to know about the level of impact due to sea level rise (SLR) on Bangladesh.
Results from a recent collaborative study of BUET with Centre for Environmental and Geographic Information Services (CEGIS) and Institute of Water Modelling (IWM), funded by the Department of Environment (DOE), showed that trends of water level rise of the 18 tidal gauge stations were determined for a period of 30 years. It can be found that the overall trend in the coastal zone is 6-20 mm per year, which is about 2 to 5 times higher than the global average. Spatial variation of sea level rise was found along the coast, and the highest trend has been observed in the Chittagong and Cox's Bazar region compared to other two coastal sub-zones of the country.
Several recent studies, including the Intergovernmental Panel on Climate Change (IPCC) reports, provide sea level rise projections for the world. Over the last century, global mean of sea level rise was stated as 19 ± 2 cm. Higher rapid sea-level rise is now projected to be 28-98cm by 2100. In the 21st Conference of Parties (COP21) in Paris, a historic agreement to combat climate change in order to achieve a low carbon, resilient and sustainable world was signed by the representatives of 195 nations in Paris. The purpose of this historic agreement is to reduce the level of greenhouse emissions to restrict the increase in global average temperature within 2 degree Celsius above the pre-industrial level. However, different projections scenarios of IPCC showed that the increase of average temperature from the pre-industrial period is about 0.78 degree Centigrade. For very high emissions, IPCC predicts a global rise of sea level will be 52-98 cm by the year 2100 even with aggressive emission reduction; a rise of 28-61 cm is predicted. IPCC reports also note that a collapse of the marine-based sectors of the Antarctic ice sheet, if initiated, could potentially add up to a further several tenths of a metre of sea level rise, independent of scenario, and that estimate could be around 0.5m. This additional amount, when combined with the upper limit of the highest-emission-scenario (RCP8.5) likely range, gives a value of sea level rise by 2100 of around 1.5 m, not including potential local variations or subsidence.
Considering all these risks, it is very important to know the probable areas which are under threat of inundation in the coastal zone of Bangladesh with sea level rise in the near future. A four year long research project entitled 'High End Climate Impact and Extremes (HELIX)' is currently on-going at BUET, where the European Union provide funding under the Seventh Framework Programme FP7/2007-2013, to carry out the study for assessing impacts of the extreme climate change in Bangladesh. Under the HELIX project, coastal inundation due to sea level rise has been assessed using coastal modelling. A widely used flow simulation model Delft3D has been set up for a part of the complex Bangladesh delta.
After validation, the model has been simulated to prepare an inundation map and to calculate affected areas and affected population for 0.5m, 1m and 1.5m sea level rise. According to the Coastal Zone Policy report of the Government of Bangladesh, 19 districts out of 64 are in the coastal zone, covering a total of 147 Upazilas. Of these 19 districts, 12 districts are contiguous with the sea or lower estuary directly. The coastal zone covers 47,201 sq.km land area, which is 32 percent of the total landmass of the country. According to the Bangladesh Bureau of Statistics (BBS) report of 2003, the total population living in the coastal zone is 35.1 million, i.e. 28 percent of the total population of the country.
The area under sea water inundation will be changed under different sea level rise conditions. It has been found that 0.5m, 1m and 1.5m SLR will inundate about 4.3 percent, 8.4 percent and 11.3 percent of the coastal areas of Bangladesh. The number of affected people will be 2.5 lakh, 6 lakh and 8 lakh for the 0.5 m, 1m and 1.5m SLR. The biggest threat of SLR is on the largest mangrove forest of the world, the Sundarbans, as it is not protected by the coastal polders. It has been found that about 12 percent, 43 percent and 60 percent of the Sundarbans will be inundated for the 0.5m, 1m and 15m SLR. As the Sundarbans is very important from ecological, environmental, geological and economic point of view for Bangladesh, its inundation patterns with different SLR conditions have profound impact.
Note: The scenarios presented in this article are indicative of the expected range of changes of the coastal inundation. Mr. Abdur Rahman Tazkia worked as a Research Assistant at BUET to simulate the coastal model and prepared the figures.
The writer is Professor of the Institute of Water and Flood Management (IWFM) at the Bangladesh University of Engineering and Technology (BUET), Bangladesh.