Water-level in the Ganges-Brahmaputra-Meghna, the largest and most densely populated delta in the world -- and located in the Bengal region of the Indian subcontinent, could rise as much as 85 to 140cm by the end of this century, a new study has warned.
The Ganges-Brahmaputra-Meghna delta as seen from the European Space Agency (ESA) Envisat satellite. The image was taken on November 8, 2003 and covers an area of around 633 x 630 km with a spatial resolution of 300 m. Photo: The European Space Agency (ESA)
According to the report published by researchers from the CNRS, IRD, BRGM, La Rochelle Université, Université des Antilles and Bangladesh University of Engineering and Technology, the region which covers two thirds of Bangladesh and part of eastern India, is one of the most vulnerable places to climate change and already regularly prone to flooding, favoured by intense monsoon rainfall, rising sea levels, river flows and land subsidence.
However, the researchers have cautioned that the extent and impact of water level rise in the region remains poorly known.
According to them, it is difficult to disentangle these various factors.
Forecasts carried out so far have been based on highly localised measurements of water level.
To overcome these difficulties, the researchers analysed monthly readings from 101 gauges measuring water and sea levels across the delta.
By aggregating the data over geographical areas in order to filter out local effects and offset quality differences between gauges, they obtained robust estimates for water-level changes. Between 1968 and 2012, water level increased by 3 mm/year on average, slightly more than global mean sea level rise (2 mm/year during the same period).
They then estimated the contribution of land subsidence, obtained by subtracting from absolute sea level (3) the measurements of relative water level obtained previously.
According to their calculations, maximum land subsidence in the delta between 1993 and 2012 was between 1 and 7 mm/year.
Although not negligible, these values are lower than some local measurements (e.g. 1-2 cm/year in Dhaka) that have been used as a reference until now.
The researchers warn that if subsidence at the same rate, and even under a greenhouse gas mitigation scenario, water-level rise in the delta, depending on the area, could reach 85 to 140 cm by the end of the century compared to the period 1986-2005. This is twice as high as the projections provided in the latest IPCC report, which did not take land subsidence into account.
They hope the study should help to improve forecasts of water level in the Ganges-Brahmaputra-Meghna delta, and thus lead to refine impact studies and possible adaptation scenarios for its 200 million inhabitants