The Ganges-Brahmaputra Delta (GBD) is the world’s largest and most densely populated delta system, spanning approximately 100,000 square kilometers across Bangladesh and the Indian state of West Bengal. It is formed by the confluence of the Ganges, Brahmaputra, and Meghna rivers, which discharge into the Bay of Bengal. Home to over 170 million people, the region has an extremely high population density, often exceeding 1,000 individuals per square kilometer. The delta’s low elevation, with much of the land sitting less than two meters above mean sea level, leaves its vast population highly exposed to multiple environmental threats.
Acute Weather Events and Storm Surges
The Bay of Bengal is a major basin for powerful tropical cyclones, which pose an immediate risk to the delta’s coastal communities. The semi-enclosed, funnel-shaped geography of the Bay, combined with a vast, shallow continental shelf, acts to amplify the height of incoming storm surges. These massive walls of water are driven by the cyclone’s powerful winds, pushing seawater toward the coast.
The delta’s low elevation means a storm surge of only a few meters can inundate huge areas, leading to widespread fatalities and infrastructure destruction. Storm surges are the most devastating type of flood event because they frequently breach coastal embankments, or polders, which are the primary defense for coastal populations. For instance, Cyclone Sidr in 2007 generated a surge up to three meters high and killed thousands, while Cyclone Aila in 2009 displaced millions of people in both India and Bangladesh.
Chronic Threat of Sea Level Rise and Salinity
A slow but relentless environmental threat is the long-term rise in global sea levels, which pushes saltwater further inland throughout the delta system. The primary effect is widespread salinity intrusion, contaminating both surface water in rivers and groundwater aquifers. This contamination is exacerbated during the dry season when reduced upstream freshwater flow allows the tidal influence to dominate the river system.
The contamination severely impacts food security, as saltwater destroys vast areas of arable land, particularly wet-season rice paddies. High salinity zones are projected to expand significantly by 2050, fundamentally altering the delta’s agricultural capacity. Furthermore, the intrusion makes freshwater sources unusable for drinking and sanitation purposes, creating a public health crisis. The central and western parts of the delta are particularly susceptible to increased salt intrusion year-round.
Geomorphological Instability
The physical structure of the delta contributes to its vulnerability through geomorphological instability. Riverbank erosion, often called bankline migration, causes the rapid loss of land and property as the powerful Ganges and Brahmaputra rivers continually shift their courses. This natural process of avulsion, or sudden river course changes, can result in the complete destruction of villages and cultivated fields.
A compounding factor is land subsidence, the gradual sinking of the delta surface, which magnifies the impact of global sea level rise. While the delta naturally compacts under sediment weight, human activities accelerate this sinking. Groundwater extraction for agriculture and urban use contributes to subsidence rates that can reach 3 to 7 millimeters per year in certain areas. Furthermore, the construction of earthen embankments has blocked the natural deposition of river sediments inside protected areas, leading to a loss of land elevation by 1.0 to 1.5 meters compared to surrounding areas.
Societal and Migration Pressures
The cumulative impact of acute weather events, chronic salinity, and land loss places immense pressure on the delta’s dense population, leading to vast climate-induced migration. Millions are forced to relocate when their land becomes unlivable or their agricultural livelihoods are destroyed by floods, erosion, or saltwater contamination. This mass movement primarily takes the form of internal migration, with people moving from vulnerable rural areas toward urban centers like Dhaka and Kolkata.
The influx of environmental refugees places an immediate and intense strain on the infrastructure and resources of the destination cities. Urban areas experience a spike in population density, intensifying the scarcity of clean water, adequate housing, and sanitation services. This heightened competition for resources exacerbates socio-economic instability and vulnerability, creating new challenges. The consequences of this forced displacement also include extreme stress on public services, including healthcare and education, across the entire region.