Imma Perfetto
Cosmos science journalist
Climate change and human interventions are threatening the water and food security of one of the most densely populated regions in the world – the Bengal delta in Bangladesh – and it’s a threat likely to be experienced around the world.
As sea levels climb and weather grows more extreme, and as humans disrupt freshwater flows through dams and embankments, saltwater is being pushed further into freshwater rivers and underground water sources, according to new research.
A study published in the journal Ecological Indicators analysed nearly 2 decades of data from more than 50 monitoring stations in coastal Bangladesh, which revealed a consistent rise in salt levels in rivers and estuaries, particularly since the mid-2000s.
Dr Mohammad Hoque, a hydrogeologist from the University of Portsmouth and co-author of the study, says what’s happening in the Bengal Delta is not just a local crisis.
“It’s a signal of what’s coming for low-lying coastal areas around the world.
“Salinity is rising faster and reaching farther inland than many people realise, and it’s happening quietly with major consequences for water security, agriculture, and livelihoods.
“This study helps us understand the mechanics behind it, and underscores the urgency for coordinated, global action.”
The Bengal delta, also known as the Ganges-Brahmaputra-Meghna delta, is where the Ganges, Brahmaputra and Meghna rivers converge in Bangladesh and the Indian state of West Bengal and flow out to the Indian Ocean.
The delta, which covers an area of about 100,000km2 and is home to more than 100 million people, plays a central role in supporting agriculture and sustaining local communities amid freshwater limitations.
The research team used advanced statistical methods to track long-term trends in salinity while distinguishing these from short-term weather or seasonal changes.
They found that western parts of the delta, already prone to effects from tides, showed the fastest increases in salinity.
The data suggests that the combination of sea-level rise, reduced freshwater flow, and increasingly frequent storm surges are contributing to the inland movement and retention of saltwater.
And since 2007, many parts of the delta have seen sharp increases in salinity linked to powerful cyclones and storm surges – such as Cyclones Akash and Sidr – which extend the inland reach of salt water.
The researchers introduced a new conceptual model called the Offshore Controlled Estuarine and Aquifer Nexus (OCEAN) Salinisation framework, which highlighted how offshore features like steep underwater slopes and restricted tidal flows can trap salt in low-lying coastal zones.
Along with declining freshwater river flows caused by water diversion and extraction by humans, coupled with return flows from irrigation, aquaculture, and industrial activities which introduce additional salinity into the river system, the Bengal delta is the perfect storm to escalate salinity.
“While the focus is on Bangladesh, the study’s implications are global,” says Dr Sean Feist, former PhD researcher at the University of Portsmouth and first author of the paper.
“Coastal regions from the Mekong Delta in Vietnam to Louisiana’s wetlands in the United States face similar pressures.
“As sea levels continue to rise, the risk of agricultural land turning salty, drinking water becoming undrinkable, and shallow groundwater becoming permanently brackish grows ever more serious.”
This sea levels rise is “driven by the thermal expansion of oceans and meltwater contributions from land ice, alongside shifts in ocean circulation patterns, which collectively contribute to regional sea-level variations and climate changes,” the researchers write in their study.
According to the IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate, “…sea level will continue to rise for centuries due to continuing deep ocean heat uptake…and will remain elevated for thousands of years.”
Co-author Dr Ashraf Dewan from Australia’s Curtin University says the study highlights that the creeping salinisation of deltas is a slow-moving but deeply disruptive force.
“Without urgent investment in salt-tolerant agriculture, better water storage, and strategic planning across entire river basins, the disruptive impacts of salinity are likely to intensify.”
The findings emphasise the urgent need to rapidly reduce global greenhouse gas emissions to halt the increase in global average temperatures.
“The Bengal Delta is on the frontline of climate change, but it is not alone,” says Dewan.
“The patterns observed here are emerging in many of the world’s great coastal regions. What happens next depends on how quickly we respond.”
