Ganges-Brahmaputra-Meghna delta in peril due to sea-level rise, says study

The water level in the Ganges-Brahmaputra-Meghna (GBM) delta, covering a part of West Bengal and two-thirds of Bangladesh could rise up to 85 to 140 centimetres by 2100, said a latest study.

It noted that being one of the most vulnerable regions in the world, the GBM delta presents a major challenge for climate change adaptation of nearly 200 million inhabitants. It is often considered as a delta mostly exposed to sea-level rise and exacerbated by land subsidence. 

The study was published in the journal Proceedings of the National Academy of Sciences in January 2020 noted that it clearly appears that during the 1968 to 2012 period, the relative water level (RWL) in the GBM delta has increased at approximately the same pace, sometimes even slightly faster, than the global mean sea-level.

It stressed said the relative water level (RWL) changes are triggered by three factors including river and water flows, the sinking of land and the sea-level rise. 

The lead author of the study, Melanie Becker of the French National Centre for Scientific Research (CNRS), explained to Mongabay-India that “it’s the total water level (WL) changes that are effectively felt by the delta’s populations.” 

The study done by a group of researchers from France, India, Bangladesh, United States of America and China considered six regions in the delta. They found that each region shows its own water-level rise in the range of 85 -140 cm by 2100. 

Becker stressed that the findings can “plausibly be used by decision and policymakers to better plan adaptation scenario.”

The delta’s vulnerability

The Ganges–Brahmaputra–Meghna (GBM) delta is the largest in the world with an area of about 150,000 square kilometres and covers about two-thirds of Bangladesh (about 100,000 square km) and a part of West Bengal. The low-lying Bangladesh delta plain, with at least 10 percent of the land standing below one metre above mean sea level, has one of the highest population densities in the world, reaching more than 1,000 people per square km.

It’s a place prone to frequent climate disasters. Floods occur with regularity, triggered by heavy monsoon rainfall, the sinking of land and inland flooding. Yet, the delta is poorly instrumented region as well as a poorly studied region. 

The study emphasised that previous studies prior to this mostly used single measurements of water level and were not in a position to account for the contribution of each of the drivers behind the sea level rise.

To tide over these gaps in knowledge of the delta, these researchers took monthly readings of water levels and sea level from “an unprecedented set of 101 gauges” spread across the delta and reconstructed the regional water levels changes since the 1970s. They found that, between 1968 and 2012, water level across the delta increased slightly faster, about three millimetres per year than the global mean sea-level rise of about two millimetres per year. 

The researchers then zeroed in on what subsidence is doing to the delta. By combining satellite altimetry and water-level reconstructions, they estimate that maximum expected rates of delta subsidence since the 1990s range from one to seven millimetre per year.

The past studies, focussing on local relative sea-level trends, pin the sea-level rise between 6-21 mm per year over the last 30 years, and are commonly mentioned in the scientific literature. By taking water-level readings from as many gauges as they did (the past studies used readings from much less number of gauges), and aggregating the results, the researchers said they were able to weed out a wide range of local processes and instrumental errors and provide a robust estimate of the sea-level rise in the six deltaic regions they considered.

“Regional relative water level considers both land processes (subsidence, tectonic movement, sedimentation etc.), and ocean processes (thermal expansion, ice melting etc.), while calculating the water level. Many previous studies used only a few near coast tidal gauge records to calculate the local sea level rise which was much higher than our studies,” study’s co-author A.K.M. Saiful Islam, who is a professor at the Institute of Water and Flood Management, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh, told Mongabay-India.

“One strong implication of our results is that, over the past 45 years, the delta might not have subsided as fast as local estimates have suggested before,” Islam explained.  

If the subsidence continues as suggested by the paper, the researchers estimate that, by 2100, even under a greenhouse gas emission mitigation scenario, the projected sea-level rise could reach up to 85 to 140 cm across the delta.

“Sea level rise along with subsidence will make the delta more vulnerable in the future,” Islam cautioned.

By analysing and estimating the processes behind the vulnerability of the delta, the paper stitches together a better picture of what’s in store for the delta. 

“This paper puts together the subsidence story with the sea level story in a comprehensive fashion,” Robert James Wasson, Emeritus Professor Australian National University, Adjunct Professor James Cook University and the National University of Malaysia, told Mongabay-India.

For Wasson, who was not involved in the research,  the main takeaway from the study is that “subsidence will exacerbate sea level rise by twice.”

“Sea level rise, subsidence and more intense cyclones provide a great threat to the people living there,” Wasson added.

The work was partly funded by the Band-Aid, a project of Belmont Forum via National Science Foundation in the United States of America and the French National Research Agency by the DELTA project via the French National Research Agency. 

Adaptation measures

By any yardstick, the people in the delta are in peril, right now itself, not to speak of future. As sea level continues to rise, the ultimate goal is to reduce greenhouse gas emissions.

“Understanding and forecasting of the sea level rise critical thresholds along low-lying heavily populated tropical coastlines are among the most vital societal issues. Our team has been working together for many years on the vulnerability of the tropical deltaic coasts where there is a current knowledge gap for priority-populated areas and more specifically those of Bangladesh,” Becker emphasised.

The researchers suggested that a number of short-term and long-term adaptation measures can be planned and implemented: rising of coastal polders (embankments) and other infrastructures, the building of new cyclone shelters, improving early warning systems for cyclones, storm surges and tidal flooding, developing salt-tolerant varies of rice, expanding coastal afforestation programme known as greenbelt, mainstreaming climate change in development planning, implementing drinking water and sanitation programs, capacity building of government organisations and so on.

It’s not just Bangladesh that is imperilled. “West Bengal of India also will have more sea-level rise in the future due to land subsidence rate of 1.5 mm/year,” noted Islam.

The trends in the sea-level rise along the West Bengal coast consistently point to a higher sea-level rise. This will exacerbate the already-worsening situation of storm surges, flooding and coastal erosion, and other hazards.

CITATION:

Becker, M., Papa, F., Karpytchev, M., Delebecque, C., Krien, Y., Khan, J. U., … & Calmant, S. (2020). Water level changes, subsidence, and sea level rise in the Ganges–Brahmaputra–Meghna delta. Proceedings of the National Academy of Sciences.

Banner image: A large part of India’s population is dependent on River Ganga. Photo by Bibek Raj Pandeya/Wikimedia Commons.