Every year, hundreds of thousands of Hindu pilgrims flock to Sagar, a teardrop-shaped island in the Hooghly Estuary, 60 miles south of Kolkata. A temple stands on its southern tip, where the island faces the open sea. Its location is revered, a place where the sacred Ganges River flows into the Bay of Bengal and eventually merges with the Indian Ocean. In recent years, Hooghly has also become known as the estuary of disappearing islands: three have been submerged in the past few decades. Some villages in Sagar have been forced to retreat from the encroaching sea. Visiting journalists often describe the region’s first climate refugees, who blame the loss of land on rising seas.
But science now tells a more complex story. In fact, researchers believe a major cause of subsidence and shoreline loss in the Hooghly Estuary is that dams on rivers flowing into the Bay of Bengal have greatly reduced the flow of sediment. When sediment continues to flow and tides allow it, mangrove islands grow—one of them, New Island, has tripled in size. Dredging and protective walls for the local harbor have also changed the estuary’s hydrodynamics, increasing the erosion of some islands, says Tuhin Ghosh, director of the School of Oceanographic Studies at Jadavpur University in Kolkata. And in Sagar, sand dunes near the temple were leveled to make way for a concrete road. “When you remove the natural barriers and flatten the land, the water will naturally continue to rise,” says Ghosh. “Sea level rise is only part of the picture.”
Like many coastal communities, the residents of Sagar Island are on the front lines of climate change. However, their vulnerability is shaped as much by processes in the Ganges-Brahmaputra Delta, which stretches from eastern India and Bangladesh, ranging from tectonic shifts and river movements to human interventions such as dams, urbanization, and groundwater extraction, as well as by emerging cuisine. “Deltas are shaped by multiple drivers on different timescales,” says Robert Nicholls, director of the Tyndall Centre for Climate Change Research in the UK, who is part of an international project on deltas. While climate change often dominates the story, it’s important to understand all the factors that cause oceans to encroach on the world’s deltas, he says. Otherwise, like a patient who has only tested one condition, “you might think it’s all a disease and you can’t solve the problem.”
Development may be a more important factor than climate change in the coming decades.
Although sea level rise will be the main force driving coastal flooding in the medium to long term, scientists say key factors in the short term are local changes related to human activity. Development has made some of the world’s most fertile and populated deltas more vulnerable to sea level rise. In the Mekong Delta in Vietnam, upstream hydropower projects, coupled with sand mining to provide concrete for the construction of the region’s growing cities, have more than halved sediment flow, leading to land subsidence, saltwater intrusion, and erosion. In the Volta Delta in Ghana, erosion rates increased sediment flow after the construction of dams in the 1960s. And the Mississippi Delta lost 2,000 square miles of land over the past century as the U.S. Army Corps of Engineers built a massive network of levees to control flooding, and oil companies swept canals to transport rigs and other equipment to and from the Gulf of Mexico.
New research shows similar changes in the Ganges-Brahmaputra Delta, home to more than a fifth of the world’s 500 million people in delta regions. Parts of the delta’s coastal margin are shrinking, particularly in the west, while areas in the east are stable or growing. New estimates of land subsidence suggest that effective sea level rise in the region could be up to 70 percent higher than current projections in some areas. And recent research suggests that development related to agriculture and other activities may be a more important factor than climate change in the coming decades.
All of this underscores that while global sea level rise is not within the direct control of coastal communities, development decisions, particularly those that strengthen the capacity of natural defense systems, can. These include encouraging mangrove growth, restoring river flows to increase land-forming supplies, and reducing reliance on groundwater, which exacerbates land subsidence. “There’s a tendency to write off deltas [to sea level rise],” says Nicholls. “But how you develop them has the power to shape the future.”
The remains of brick buildings and coconut tree stumps in Dhablat on the southeastern shore of Sagar Island. The village faces the open sea, a boundless horizon of deep blue flash under the midday sun. The village lost two primary schools to rising waters, and half of the village is now gone. The remaining houses lie behind the embankments. Most of the young men now take seasonal jobs in Kolkata or elsewhere.
A little further inland, I meet Jaba Das, whose family was relocated by the government 15 years ago from the nearby island of Ghoramara and is now largely in hiding. In that time, she has seen roads and electricity improve. Brick kilns have sprung up across the water on the mainland. Her children now work on the outskirts of Kolkata. Despite frequent cyclones and floods, she has no desire to leave. Recent surveys show that nearly a fifth of households on the Indian side of the Sundarbans—the swamp and mangrove fingers of the delta—have a migrant who left for better prospects.
The Sundarbans are a network of tidal channels, mudflats, and mangrove forests, the largest such ecosystem in the world. It was built over thousands of years by sediments deposited by the Ganges and Brahmaputra rivers, which flow north from the Himalayas. In the 16th century, an earthquake tilted the basin east, reducing the flow of Ganges water to the western Sundarbans. Then British colonizers arrived, felling the forests and building the port city of Kolkata. In the second half of the 20th century, the Indian government harnessed rivers for irrigation and power, building thousands of dams on numerous rivers upstream, as well as the controversial Farakka Barrage—a massive, 7,500-foot-long water diversion barrier. Towns and farms were often expanded into wetlands. “There were hundreds of human interventions,” says Sugata Hazra, professor and former director of the School of Oceanography at Jadavpur University. “They have changed the hydrosphere.”
The Ganges-Brahmaputra Delta saw sediment load halved between 1960 and 2008.
No change was more significant than the supply of sediment. The Ganges-Brahmaputra Delta, often described as the world’s largest sediment distribution system, saw a decline of 1 billion to 500 million tons annually from 1960 to 2008, according to a 2018 study. The decline is projected to fall to 79 million to 92 million tons annually by 2100, the study said. This sharp reduction in sediment further opens the way for rising seas. A study last year analyzing satellite imaging data found that the Sundarbans lost 137 square kilometers (53 square miles) of mangrove forest from 1984 to 2018, much of it at the southernmost edge. There was also some accretion—62 square kilometers (24 miles), although some of this was temporary or seasonal.
Sediments, says Hazra, are the delta’s natural defense against the sea, and “now sea levels are rising, and we have no sediments to counteract it.” Other human activities also cause land subsidence, particularly oil and gas exploitation and groundwater extraction, which lead to soil compression. Studies have found land subsidence rates of up to 2.2 centimeters—0.8 inches—per year in the Bangladeshi capital of Dhaka, likely due to groundwater extraction and the weight of urban infrastructure. This rate is not as severe as areas like Jakarta in Indonesia, which is subsiding so rapidly—at a rate of 10 inches per year—that authorities are planning to build a new capital elsewhere. However, one study projects that at current rates, the Ganges-Brahmaputra Delta could subside by as much as a foot by 2050, worsening the situation with a relative sea level rise.
These projections don’t account for development changes, which may further reduce the rate of decline. For example, the expansion of megacities like Kolkata and Dhaka, which have a combined population of 35 million people, means more urban infrastructure and flood defenses, which can trap more sediment onshore. “What we found,” says Nicholls, “was that your development may be more important than how climate change evolves.”
Delta communities around the world face challenges similar to those in the Ganges-Brahmaputra Delta. In the Mekong Delta, groundwater extraction for aquaculture, agriculture, and drinking water is thought to have contributed to a submergence of 0.4 to 1.2 inches per year—far greater than sea level rise. A 2019 analysis also highlighted the contribution of sand mining along the river, with an estimated 17 million cubic meters of sand removed in 2018 alone. Last year, seawater went further upstream in the delta than ever before, staying for four months instead of the usual. The intrusion was blamed on reduced freshwater flow due to upstream dams in China and the deepened riverbeds that had been mined from sand.
In the Volta River Delta in Ghana, the historical sediment transport of 71 million cubic meters per year is thought to have been reduced by 90 percent after the construction of the Akosombo Dam in 1965, threatening the stability of the east coast. Defensive walls and groynes have helped slow the trend in some areas, but other parts remain at risk. Coastal sand mining and proposed oil and gas exploration could worsen the problem, according to the Delta, Vulnerability and Climate Change: Migration and Adaptation (DECCMA), an international initiative that examined conditions in the Volta, Ganges, and Mahanidhi Deltas in India. In the Indian Sundarbans, researchers have developed risk maps based on climate hazards as well as socioeconomic factors such as dependence on marginal farming or fishing. Among the districts at highest risk is Sagar Island.
Better science is critical to finding solutions, say Kolkata’s Hazra and Ghosh, who were part of the Deccma project. Improved understanding of the role of sediment supply has increased interest in nature-based solutions. In the Mississippi Delta, sediment diversion projects aim to restore the wetlands’ natural landbuilding capacity. In other delta regions, momentum is growing to bolster “soft” defenses. Although Vietnam can do little about China’s dams, its Mekong Delta plan includes ending sand mining, restoring mangroves, and reducing shrimp farming expansion in the wetland to ensure erosion and subsidence.
Bangladesh has a New Delta Plan that includes mangrove clearing and a pilot project to test controlled flooding, which would allow sediment to replenish the coast. Traditional farmers used to practice some form of controlled flooding, says Munsur Rahman, a professor at the Institute of Flood and Water Management in Dhaka, but agricultural intensification changed all that. He says it’s not easy to persuade farmers to sacrifice short-term productivity for long-term sustainability, a challenge Vietnam also faces.
With most of the world’s deltas under threat, compromises seem inevitable, and local authorities must rethink development decisions regarding urban expansion and groundwater exploitation, says Hazra. This requires “a better scientific footprint in policy and new paradigms for thinking about development.”