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Story Publication logo January 14, 2021

As World’s Deltas Sink, Rising Seas Are Far from Only Culprit

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People bathe in the Ganges overlooking the Howrah Bridge in West Bengal. Image by ABIR ROY BARMAN / Shutterstock. India, 2019.
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Climate change threatens millions who live in the world's river deltas, including the Ganges...

Children play with old rafts over flooded fields in rural Bangladesh. Image by Amir Jina / CC. Bangladesh, 2009.
Children play with old rafts over flooded fields in rural Bangladesh. Image by Amir Jina/CC. Bangladesh, 2009.

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 at the southern tip where the island faces the open sea. The location is revered, a place where the holy Ganges River flows into the Bay of Bengal, eventually merging with the Indian Ocean. In recent years, Hooghly also has become known as an estuary of vanishing islands: Three have been submerged over the past few decades. Some villages in Sagar have been forced to retreat from the advancing sea. Visiting journalists often describe inhabitants here as the region’s first climate refugees, attributing the loss of land to sea level rise.

Yet science is now telling a more complex story. Indeed, a major cause of subsidence and loss of coastline in the Hooghly estuary, researchers say, is the construction of dams on rivers that flow into the Bay of Bengal, sharply reducing the supply of sediments. Where sediment still flows and tides allow, mangrove islands are growing — one of them, New Island, has tripled in size. Dredging and protective walls for the local port also have altered the hydrodynamics of the estuary, increasing 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 have been levelled for a concrete road. “If you remove the natural barriers and flatten the land, of course the water will come further up,” says Ghosh. “Sea level rise is only part of the picture.”

Like many coastal communities, the residents of Sagar Island are on the front line of climate change. But their vulnerability is shaped as much by processes in the Ganges-Brahmaputra Delta that cradles eastern India and Bangladesh — ranging from natural tectonic shifts and river movements to human interventions like dams, urbanization, and groundwater extraction — as it is by rising seas. “Deltas are shaped by multiple drivers on different timescales,” says Robert Nicholls, director of the Tyndall Center for Climate Change Research in Britain, 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 lead to seas encroaching into the world’s deltas, he says. Otherwise, like a patient tested for just one condition, “you might think it’s all one disease, and you won’t be able to solve the problem.”

Development may be a more important factor than climate change in the next few decades.

Although sea level rise will be the most important force driving coastal inundation 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 supply concrete to build 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, rates of erosion increased after the construction of dams in the 1960s slashed the flow of sediments. And the Mississippi River Delta has 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 carved canals to transport rigs and other equipment to and from the Gulf of Mexico.

New research is highlighting similar changes in the Ganges-Brahmaputra Delta, home to more than one fifth of the 500 million people who inhabit the world’s delta regions. Parts of the delta’s coastal edge are shrinking, especially in the west, while areas to the east are stable or growing. New estimates on land subsidence suggest effective sea level rise in the region could be up to 70 percent higher than current projections in some areas. And recent research suggests development related to agriculture and other activities may be a more important factor than climate change in the next few decades.

All this underscores that while global sea level rise is not within the direct control of coastal communities, development choices are, especially those that strengthen the capacity of natural defense systems. These include fostering mangrove growth, restoring river flows to increase the supply of land-building sediment, and reducing dependence on groundwater, which worsens land subsidence. “There’s a tendency for deltas to be written off [to sea level rise],” says Nicholls. “But how you develop has the power to shape the future.”

The remains of brick buildings and the stumps of coconut trees dot the beach at Dhablat on the southeastern shore of Sagar Island. The village looks out over the open sea, a limitless horizon of deep blue flashing under the noon sun. The village has lost two primary schools to rising waters, and half the village is now gone; the remaining houses lie behind embankments. Most young men now leave for seasonal jobs in Kolkata or elsewhere.

A little inland, I meet Jaba Das, whose family was relocated here by the government 15 years ago from the nearby — and now largely submerged — island of Ghoramara. 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. But she has no desire to leave, despite frequent cyclones and flooding. Recent surveys show that nearly one-fifth of households on the Indian side of the Sundarbans — the marsh and mangrove fringe of the delta — have a migrant who left for better prospects.

The Sundarbans is a network of tidal channels, mudflats and mangrove forests, the largest such ecosystem in the world. It was built up over thousands of years by sediments deposited by the Ganges and the Brahmaputra rivers, which flow from the Himalayas to the north. In the 16th century, an earthquake tilted the basin east, reducing the flow of Ganges water to the western Sundarbans. Then came British colonizers, who cut down the forests and built 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. Cities and farms expanded, often into wetlands. “There have been hundreds of human interventions,” says Sugata Hazra, professor and former director of the school of oceanography at Jadavpur University. “They’ve changed the hydrosphere.”

The Ganges-Brahmaputra Delta saw sediment load halved from 1960 to 2008.

No change has been more important than sediment supply. Often described as the world’s largest sediment dispersal system, the Ganges-Brahmaputra Delta saw sediment load halved from 1960 to 2008, from 1 billion to 500 million tons annually, according to a 2018 study. The decline is projected to continue, falling to 79 million to 92 million tons a year by 2100, the study said. This steep 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 on the southernmost edge. There was some accretion, as well — 62 square kilometers (24 miles), although some of this was transient or seasonal.

Sediments, says Hazra, are the delta’s natural defense against the sea and “now sea levels are rising and we don’t have the sediments to counter it.” Other human activities also cause land subsidence, especially oil and gas exploitation and groundwater extraction, which lead to compression of soil. Studies have found land subsidence rates as high as 2.2 centimeters — 0.8 inches — a year in the Bangladesh capital of Dhaka, likely due to groundwater extraction and the weight of urban infrastructure. This rate isn’t as severe as areas like Jakarta in Indonesia, which is sinking so fast — at a rate of 10 inches a year — that authorities are planning to build a new capital elsewhere. But one study projects that at current rates, the Ganges-Brahmaputra Delta could sink by up to a foot by 2050, worsening relative sea level rise.

Those projections don’t account for development changes that might further accelerate sinking. For example, expanding megacities like Kolkata and Dhaka, which have a combined population of 35 million, mean more urban infrastructure and flood defenses that may trap more sediment on land. “What we found,” says study co-author Nicholls, “was that how you develop may be more important than how climate change unfolds.”

Delta communities around the world are facing challenges similar to those unfolding in the Ganges-Brahmaputra Delta. In the Mekong Delta, groundwater extraction for aquaculture, farming, and drinking water is thought to have contributed to sinking 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 advanced farther up the delta than ever before, and remained for four months instead of the usual one. The intrusion was blamed on reduced freshwater flow due to upstream dams in China and deepened river beds that had been mined of sand.

In the Volta River Delta in Ghana, the historic sediment transport of 71 million cubic meters a 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. Defence 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 aggravate the problem, according to research from the Deltas, Vulnerability, and Climate Change: Migration and Adaptation (DECCMA) project, an international initiative that studied conditions in the Volta, the Ganges, and the Mahanidhi Delta 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, says 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 natural land-building capacity of the wetlands. In other delta regions, momentum is growing to strengthen “soft” defences. Although Vietnam can do little about China’s dams, its Mekong Delta Plan includes ending sand mining, restoring mangroves, and reducing shrimp farm expansion in the wetlands to staunch erosion and subsidence.

Bangladesh has a new delta plan that includes mangrove afforestation and a pilot project to test controlled flooding that would allow sediments to replenish the coast. Traditional farmers used to practice a form of controlled flooding, says Munsur Rahman, professor at the Institute of Flood and Water Management in Dhaka, but agricultural intensification changed all that. He says it won’t be easy to persuade farmers to sacrifice short-term productivity for long-term sustainability, a challenge that Vietnam also faces.

With most of the world’s deltas under threat, trade-offs seem inevitable, and local authorities must rethink development choices around urban expansion and groundwater exploitation, says Hazra. That requires, he says, “a better science imprint on policy and new paradigms of thinking about development.”

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