Land subsidence key to Bangladesh’s rising water levels: Dr AKM Saiful Islam

Recent research has revealed that sea levels are rising – more rapidly than previously anticipated – in the Ganges-Brahmaputra-Meghna delta. 

According to the research, conducted by Mélanie Becker of the University of La Rochelle in France, and her colleagues, a sea-level rise almost twice as much as previously predicted could seriously affect parts of Bangladesh and India.

Researchers from La Rochelle Université, Université des Antilles and the Bangladesh University of Engineering and Technology (Buet), among others, were involved in the study. 

The research was carried out from 2014-2018, as part of a project, and published in the Proceedings of the National Academy of Sciences of the United States of America on January 6, 2020. 

Among others, Professor AKM Saiful Islam of the Institute of Water and Flood Management at the Bangladesh University of Engineering and Technology (Buet) co-authored the article. 

Kamran Siddiqui of The Business Standard interviewed him about the research in Dhaka on Wednesday. 

The Business Standard (TBS): How do you assess sea-level rise?

AKM Saiful Islam: There are different models to assess sea-level rise. Depending on assessment models, predictions also differ slightly. The average sea level of long term period is defined as "mean sea level." The difference between future mean sea level and the current level is known as sea-level change. However, now we predict according to the current phenomena on the basis of research findings. 

TBS: What are the global predictions about climate change?

AKM Saiful Islam: The latest report of the Intergovernmental Panel on Climate Change (IPCC), an intergovernmental body of the United Nations, has projected that the global mean sea level will rise from 43 to 84 cm – with Representative Concentration Pathway (RCP) of greenhouse gases at 4.5 – by 2100, relative to 1986–2005 figures. However, in the case of an extreme rise in temperature, the sea level rise could be 1m at RCP 8.5, by 2100. This is the average prediction. However, the situation varies by region. Parts of the Bay of Bengal experience different phenomena regarding sea level rise, compared to other parts of the world; we determined this through our new research. The new analysis found that depending on the region of the delta, sea-level rise could reach 85 to 140 cm (RCP 4.5) by 2100 – nearly double the IPCC estimate. 

TBS: What are the factors due to which Bangladesh's sea levels could rise almost twice as much as predicted?   

AKM Saiful Islam: Greenhouse gas emission is the main cause of temperature rise which impacts melting ice. It is the global phenomenon that increases sea levels. However, we found that the situation is direr in the Ganges-Brahmaputra-Meghna delta. Land subsidence is a new factor in this region behind the increasing water level as well as climate change. And, subsiding land is the consequence of a lacking proper sedimentation process. We have built 139 polders in our coastal belt in last 30-40 years which are barriers to the flow of sediment from rivers to plain land. Now the sedimentation is occurring in rivers which also badly affect the navigability of rivers. In such a way, river water levels are increasing, threatening plains. 

TBS: How does land subsidence play a role in increasing water levels? 

AKM Saiful Islam: Our research provides a robust estimate of water-level changes in the Ganges-Brahmaputra-Meghna delta, driven by continental fresh water dynamics, vertical land motion, and sea-level rise. Through an unprecedented set of 101 gauges, we reconstructed water-level variations since the 1970s and showed that the water level across the delta has increased slightly faster, 3 mm/year, than the global mean sea-level rise of 2 mm/year. By combining satellite altimetry and water-level reconstructions, we estimated that the maximum expected rates of delta subsidence since the 1990s range from 1 to 7 mm/year. By 2100, even under a greenhouse gas emission mitigation scenario at RCP 4.5, the subsidence could double the projected sea-level rise, causing it to reach 85 to 140 cm across the delta.

TBS: What are the consequences of sea-level rise?

AKM Saiful Islam: According to Buet's research, if climate change occurs at  its current rate, 42 percent of the Sundarbans will be submerged. Rising saline water, internal migration, and the destruction of the Sundarbans will result from such changes in the sea level. The coastal people will be the main victims of the consequences. A significant number of foreign tourists visit Bangladesh to see the Sundarbans. The tourism industry will be negatively affected. We also have to make necessary preparations for a large number of climate migrants. We have to determine an alternative livelihood for coastal people. Fresh water reservation will also be a big deal.    

TBS: How do we find a way out of the land subsiding?

AKM Saiful Islam: Because of increasing use of fresh water in the upstream, the supply of fresh water has been greatly affected in Bangladesh's dry season for years. Now land subsiding is a new phenomenon because of a lacking proper sediment management plan. An embankment is constructed along a river, while a polder is something different from an embankment and can be built anywhere. In Bangladesh's coastal belt, we cannot remove the polders because thousands of people are living around those areas. However, we must think about managing the river's sediment. Additionally, we have to build more shelters – taking into account future high-intencity cyclones. These are the local actions but we also have to continue our global efforts to mitigate carbon emissions according to the Paris agreement. The target of binding the temperature increase level to two degrees Celsius must be achieved.