A new study has found that the Ganges-Brahmaputra, Brahmsoani and Mahanadi deltas are sinking faster than the regional sea level is rising, with Calcutta experiencing land subsidence at rates equal to or exceeding the regional average.

The assessment spanning 40 delta regions worldwide has identified the three in eastern India among 19 where land subsidence — largely attributed to the over-extraction of groundwater — affects over 90 per cent of the delta area.

Scientists say the subsidence will enhance the risk of flooding in the delta regions and underscore the need for more investments in groundwater management and the creation of resilient infrastructure.

In 13 deltas, including the Brahmani, Godavari and the Mahanadi in India, the Yellow River in China, the Nile in Egypt, the Mekong in southeast Asia and the Po in Italy, the average subsidence rates exceed the current global sea level rise rate of 4mm per year.

“The dominance of subsidence over sea-level rise is striking. In every delta that we monitored, at least some portion is sinking faster than the sea surface is rising,” said Robert Nicholls, a climate scientist at the University of East Anglia in the UK and co-author of the study.

The study, published on Wednesday in the journal Nature, has identified Calcutta, alongside other coastal cities such as Alexandria (the Nile delta), Bangkok (Chao Phraya), Jakarta (Ciliwung) and Dongying (Yellow River) with subsidence rates equal to or exceeding delta-wide averages.

Scientists say their analysis suggests that the land subsidence dominates even when compared to future worst-case scenarios of sea level rise, implying that for millions of coastal city residents, the immediate threat is not just sea level rise but their sinking ground.

“If land is sinking faster than the sea is rising, investments in groundwater management, sediment restoration and resilient infrastructure become the most immediate and effective ways to reduce the exposure (risk),” Leonard Ohenhen, the study’s lead author at the University of California, Irvine, said in a media release.

The study’s observations about Calcutta are in line with an independent 2023 study by Indian scientists that measured subsidence in the city ranging from 4mm to 12mm per year between 2003 and 2011.

Geophysicist Bhagwan Singh Chaudhary at Kurukshetra University and his collaborators who had used satellite imagery for the 2023 study found the emergence of new subsidence zones west of the Hooghly river as a response to a rapid decrease in groundwater level there.

“But we also noticed some encouraging signals — aggressive and large-scale recharge of groundwater can help slow down the land subsidence,” Chaudhary, who was not associated with the global deltas study, told The Telegraph. Between 2017 and 2021, the Indian study found that subsidence rates decreased by 2mm to 3mm specifically in areas with large-scale groundwater recharge measures.

The global study has estimated that worldwide, thesubsidence of the delta regions is set to place some 236 million people at enhanced risk of flooding.

Besides flooding, Chaudhary said, long-term high subsidence rates could also put structures such as buildings and other infrastructure at an enhanced risk of damage. “This may require revision of building design codes,” he said.