Satellite data show India’s major deltas sinking due to human activity

The researchers were motivated by the lack of high-resolution data of river deltas’ subsidence worldwide even though they support more than 340 million people.

They used interferometric synthetic aperture radar data from the European Space Agency’s Sentinel-1 satellite collected in 2014-2023. The study covered 40 major deltas around the world, including six in India, at a spatial resolution of 75 m.

Then, the team used a random forest machine learning model that correlated the subsidence rates with three stressors: groundwater storage (already measured by the NASA-German GRACE satellites), sediment flux, and urban expansion.

The Ganges-Brahmaputra, Brahmani, Mahanadi, Godavari, Cauvery, and Kabani deltas were all confirmed to be sinking, with more than 90% of the Ganges-Brahmaputra, Brahmani, and Mahanadi deltas’ total area affected. In the Ganges, Brahmani, Mahanadi, Godavari, and Kabani deltas as well, the average rate of land subsidence exceeded the rate of regional sea-level rise.

The team also found that 77% of the Brahmani delta and 69% of the Mahanadi delta were sinking at more than 5 mm/year. Even under the worst future climate scenario, the 95th-percentile subsidence rates in the Godavari delta were expected to exceed the projected rate of global sea-level rise.

In Kolkata, the subsidence rates equalled or exceeded the delta’s average because the weight of the city and its resource consumption were actively accelerating its descent relative to the sea.

The effects of such subsidence include worse coastal and river flooding, permanent loss of land, intrusion of saltwater that contaminates freshwater sources and degrades agricultural land (which can increase competition for dwindling resources and drive migration), and damage to ports and transport networks.

The analysis also indicated that the Ganges-Brahmaputra and Cauvery deltas are particularly affected by unsustainable groundwater extraction while the Brahmani delta bears the brunt of rapid urbanisation. The sinking of the Mahanadi and Kabani deltas is driven by a combination of groundwater extraction, drop in sediment flux, and population pressure.

The study also found that the Ganges-Brahmaputra delta has shifted from being a “latent threat” in the 20th century to an “unprepared diver” in the 21st, meaning risk has increased significantly while the institutional capacity to manage it has stagnated.

The study was published in Nature on January 14.

“All deltas, by their inherent nature, subside over time as recently deposited sediments or in situ organic material compact under their weight, a process further influenced by isostatic adjustments and tectonic activity,” the team wrote in its paper,” the team wrote in the paper.

“However, human interventions have accelerated subsidence rates in many of the major deltas of the world, transforming a gradual geological process into an urgent environmental crisis.”

The team also acknowledged that among other issues, the groundwater storage trends might be off for small deltas due to limitations in the GRACE data, that the sediment flux data aren’t up to date, and that, “although the 40 deltas represent a substantial portion of global delta area and population, they are not globally representative”.