Historical and Projected Future Hydrological Characteristics of the Mangrove Forest in the Ganges Delta—A Review

Abstract

Mangrove forests protect coastlines from erosion, enhance biodiversity, store carbon, and support coastal communities. These ecosystems rely on hydrological conditions. This paper reviews past, present, and future hydrological characteristics of Bangladesh’s Sundarbans to guide restoration and sustainable development. It examines historical and projected hydrological indicators, addressing knowledge gaps and suggesting strategies. Renowned for productivity, biodiversity, and socio-economic benefits, the Sundarbans depend on seasonal freshwater from the Ganges River. However, threats from climate change and human activities, including reduced freshwater flow due to India’s Farakka Barrage on the Ganges, rising salinity, cyclones, and pollution, endanger these ecosystems. The primary threat is mangrove destruction for alternate land use and reduced sediment supply due to upstream dam construction. Sea-level rise is a secondary concern, as a healthy Sundarbans delta could naturally accrete with adequate sediment input and mangrove growth. Sustainable management practices are critical, including maintaining upstream water flow, minimizing deforestation, and rehabilitating degraded areas. Alternative livelihoods and strategies addressing salinity rise are essential. Long-term approaches should adopt adaptive management and ensure sustainable resource use. Policy actions must regulate human activities, mitigate cyclone impacts, ensure freshwater availability, halt harmful industries, and promote awareness and surveillance. Protecting mangroves to reduce CO₂ emissions and advancing research are vital.

1. Introduction

1.1. Mangrove Forests and the Sundarbans

Mangroves are salt-tolerant trees and shrubs in brackish intertidal regions along tropical and subtropical coastlines [1]. These species have developed unique adaptations to survive in wet and saline conditions. Globally, mangroves cover approximately 15.2 million hectares across 123 countries and territories [2], comprising 0.7% of the world’s tropical forests. Asia holds the largest share of mangrove coverage (37%), followed by Africa (21%), North and Central America (15%), and South America (12.6%). The remaining 12.4% is distributed among Australia, Papua New Guinea, New Zealand, and the South Pacific Islands [2,3,4]. A mangrove ecosystem is a transitional area where freshwater and saline water converge, thus connecting terrestrial and marine environments.

The Sundarbans is a deltaic mangrove forest formed approximately 7000 years ago by the deposition of sediments from the foothills of the Himalayas through the Ganges River system. It is situated in the Southwest of Bangladesh and south of West Bengal of India [5] (Figure 1). The Sundarbans stretches across the Bay of Bengal delta in Bangladesh, shaped by the confluence of the Ganges, Brahmaputra, and Meghna rivers (known as the GBM River). It lies between latitudes 21.52° N and 22.5° N and longitudes 89.3° E and 90.3° E [6]. The Sundarbans, spanning approximately 6000 km² in the Southwestern region of Bangladesh and approximately 4000 km² in India’s West Bengal State, stand as the largest continuous mangrove forest globally [7,8] and the world’s second-largest basin [9]. The Bangladesh part of this forest is located south of the districts of Khulna, Bagerhat, and Satkhira along the coast of the Bay of Bengal (Figure 1). It constitutes 44% of the forest area in the country [10], which is important for its floristic, economic uses, and wildlife. Resting upon the active delta of the GBM river system, this forest boasts an incredibly diverse array of life, hosting a staggering 300 floral and 1760 faunal species [11,12]. Approximately 10 million people residing in coastal areas depend directly or indirectly on the Sundarbans mangrove for various purposes, including agriculture, fishing, farming, human settlement, gathering housing materials and food, as well as employment opportunities in forestry activities [13]. In 1997, UNESCO designated three sanctuaries within the Sundarbans as World Heritage Sites.

1.2. Climate and Vulnerability of the Sundarbans

The yearly rainfall spans from 1500 to 2000 mm, with monthly averages ranging from 16 mm in December to 344 mm in July. It follows a single peak pattern, with 83% occurring between May and September. Relative humidity consistently exceeds 70% throughout the year. The mean annual relative humidity ranges from 70% in Satkhira to 80% in Patuakhali [14]. Evapotranspiration remains supreme, driving the biophysical processes that recycle 67.7% of the total annual rainfall back into the atmosphere [15]. The monsoonal patterns notably shape its impact. The Nor’westers, known locally as Kalbaishaki, are brief yet intense events characterized by powerful winds, heavy rainfall, and thunder, often followed by a rapid temperature drop during the pre-monsoon season. Tropical cyclones, often accompanied by storm surges, are frequent phenomena in the Sundarbans region during monsoon and post-monsoon.

Mangroves are one of the most affected ecosystems globally, facing substantial degradation. Over the past two decades, numerous countries in the Asia-Pacific area have witnessed a staggering loss of 50% of their mangroves, which have been either degraded or repurposed for other activities [16]. According to this investigation, mangrove ecosystems face heightened vulnerability to the impacts of climate change, notably sea-level rise. On a worldwide scale, mangrove coverage declined by nearly 20% from 1980 to 2005 [17], and this trend persists with an annual reduction rate of 1–2% [18]. In Cambodia, mangrove forests have declined due to human activities and are further threatened by future sea-level rise caused by climate change [19]. Significant alterations in forest health and structure have been observed due to various factors, including direct human intervention, developmental activities, extreme weather events, and the gradual onset of climate change impacts [20]. With global annual coverage loss rates of 1–2%, there is a risk that no mangrove forests will remain by the end of the 21st century if the current trend persists [18]. Agriculture, fishing, farming, and settlement pressures steadily erode the Sundarbans mangrove ecosystem [21]. Building dams, barrages, and embankments along the rivers to divert upstream water and flood control purposes has diminished the freshwater inflow into the Sundarbans mangrove forest significantly impacting its biodiversity [22]. Note that the physical expanse of the Indian Sundarbans has remained relatively stable in recent times; shifts in forest vigor and composition have occurred due to various influences such as human activities, upstream developments, climate fluctuations, and occurrences of severe weather events [20]. The Sundarbans ecosystem is threatened by both natural and human-made factors [23]. It faces various climatic stressors, including rising sea levels, frequent tropical cyclones, elevated salinity levels, shifts in seasonal patterns [24,25], and human interventions like the Farakka Barrage [26]. It is vulnerable to tropical cyclones because of its geographical location and intricate biodiversity. Elevated salinity levels, warmer temperatures, and decreased rainfall during both pre-monsoon and post-monsoon seasons seem to be driving a decline in the extent and vitality of mangroves [27,28].

Climate change projections indicate a heightened intensity of tropical hurricanes and cyclones, coupled with larger storm surges and more extreme waves [29]. The Sundarbans’ ecological landscape is swiftly transforming, driven by a multitude of factors including the Farakka Barrage, deforestation, cyclones, and industrial pollution [30]. Within the Sundarbans region of India–Bangladesh, ref. [31] observed a 1.2% decline in forest cover between 1973 and 2000 due to deforestation and coastal dynamics, including erosion and deposition. Ref. [26] noted a 5.1% reduction in the Sundarbans area between 1975 and 1989, followed by a 4.5% decline from 1989 to 2000, attributed to coastal erosion and the impact of multiple cyclones between 1977 and 1988. In 2007, approximately 1900 km² of the Sundarbans area in Bangladesh suffered the impacts of tropical cyclone Sidr. The damaging effects are exacerbated by various human activities [2,32].

1.3. Justification and Review Process

Despite significant hydrological impacts on the Sundarbans ecosystem, a comprehensive review of the mangrove forest’s historical and projected hydrological indicators is missing to understand their ecological and socio-economic consequences. The major historical hydrological indicators include surface and groundwater systems, water and soil salinity, tidal dynamics, tropical cyclones and storm surges, and erosion and accretion, while the major projected hydrological indicators comprise temperature, rainfall, extreme events, sea-level rise, and saline water intrusion. A review of these indicators would highlight the interplay between the natural processes and human interventions, which have profoundly influenced the forest’s hydrology and biodiversity. It would provide insights into how climate change impacts the delicate balance of this critical ecosystem. The review can inform sustainable management strategies, ensuring the livelihood of millions of people who depend on the Sundarbans for resources and protection against natural disasters. Understanding the hydrological patterns is essential to predict future challenges, including salinity intrusion and freshwater scarcity that threaten the forest’s health and productivity. Finally, the review can be a crucial tool for policymakers and conservationists to implement adaptive measures that preserve the Sundarbans’ ecological integrity and its global significance as a biodiversity hotspot.

The literature search policy for this review involves a comprehensive approach to gather relevant studies on the historical and projected hydrological characteristics of the mangrove forests in the Ganges delta. The search prioritizes peer-reviewed journals, academic books, government reports, and credible datasets focusing on the Sundarbans’ hydrological changes, climate impacts, and socio-economic aspects. It uses key terms such as “Ganges delta hydrology”, “Sundarbans mangroves”, “climate change impacts”, and “socio-economic significance” across multiple databases like Scopus, Web of Science, and Google Scholar. Additionally, the search policy ensures the inclusion of interdisciplinary studies to capture the interplay of ecological, hydrological, and human factors, emphasizing relevance, quality, and geographical focus to support sustainable management insights.

This review paper was structured into five major sections, in addition to an introduction describing the mangroves and their general characteristics and a concluding section. The second section reviews the physical characteristics and ecological importance of mangrove forests. The third section highlights the historical hydrological conditions of mangrove forests, while the fourth section addresses the projected future hydrological conditions. The following section identifies the challenges facing the Sundarbans’ ecosystem and the necessary actions to address challenges, followed by an overall summary and concluding remark section.

2. Physical Characteristics and Ecology

2.1. Landforms

The rivers of the Ganges delta constantly shift, reshaping the landscape and keeping the region dynamic. The eastward migration of the Ganges and Brahmaputra rivers impacts sedimentation and reduces freshwater inflows. The Sundarbans’ topography, characterized by a complex network of rivers, channels, and estuaries (Figure 2), is heavily influenced by seasonal rainfall and sediment deposition [33]. The Bay of Bengal experiences semi-diurnal tides; with approximately 70% of the Sundarbans lying within 1 m of sea level, making it prone to frequent tidal flooding [34].

Geomorphological changes are tilting the Bengal Basin, diverting freshwater eastward and increasing salinity in the Indian part of the Delta [35]. The Sundarbans, composed of numerous islands, remains an active and hydrologically dynamic Delta, subject to heavy sedimentation, tidal forces, and cyclonic activity. In Bangladesh, the Sundarbans are divided into eastern, central, and western regions, while the Indian portion is located in the Southeastern region. The central area experiences significant sediment deposition, new land formation, and both erosion and accretion, while the western region stretches to the Harinbhanga River [36,37]. Satellite data (1989–2010) show that the forest cover in northern Khulna and Chandpai ranges reduced by 3.61% due to changes in the river course [5]. Human activities, including dam and embankment construction, deforestation, and pollution, have compounded the natural processes, reducing freshwater inflows and impacting the region’s hydrology and ecology [38]. The Sundarbans’ ecological balance is threatened by increased siltation due to reduced downstream flow [39]. Since the 1960s, the coastal areas of Bangladesh have been transformed with a vast network of embankments and polders (Figure 2) designed to mitigate flooding, regulate water levels, and enhance agricultural productivity. While these structures have effectively reduced the frequency and scale of coastal flooding, they have also caused notable changes in flood patterns, coastal dynamics, and subsidence rates (e.g., [40]. While natural factors such as tidal changes and freshwater influx shape the region’s sedimentation, human interventions like upstream water diversion and coastal polder construction (Figure 2) have impacted hydromorphological changes more than climate change [41].

2.2. Ecological Importance

Mangrove forests, known for their biodiversity and productivity [42,43], play a crucial role in providing ecosystem services along coastlines [44,45]. The Sundarbans, where the Ganges meets the Bay of Bengal [46], is a key transitional zone featuring a complex network of creeks and rivers that cover 30% of the area [47]. Situated between 0.9 m and 2.1 m above sea level, the dynamic region is shaped by sediment deposition and influenced by various environmental stressors [48], with hydrological processes driving its formation and ecosystem services [49].

The Sundarbans offers various ecosystem services, such as soil formation, hydrological regulation, climate moderation, natural disaster protection [50], sediment trapping, coastal protection, nursery grounds for fish, wood and food resources, oxygen production, waste recycling, and carbon sequestration [51]. The mangroves also filter suspended sediments, protecting offshore ecosystems like coral reefs and seagrass beds. As natural barriers against cyclones and tidal surges, the Sundarbans help stabilize shorelines and protect coastal communities. Several studies [52,53] demonstrate that mangroves reduce wave energy and storm surges, with [54] showing a 66% reduction in wave energy within the first 100 m of forest width and a potential reduction in storm surges by 5–50 cm per kilometer of mangrove width. A 100 m stretch of dense mangroves is estimated to reduce wave heights by 20–25% [55]. Additionally, mangroves serve as vital carbon sinks, storing carbon in peat soils and ocean sediments and mitigating greenhouse gas emissions [56]. The Sundarbans are significant carbon sinks, with an annual net ecosystem productivity of 249 ± 20 g of carbon per square meter [57] and have absorbed an estimated 415 million tonnes of carbon dioxide, valued at USD 79 billion on the international market [58].

Figure 2. Coastal area of Bangladesh showing polders, river systems, main fluvial inflow inlets and tide gauging locations (source: [59]).

Figure 2. Coastal area of Bangladesh showing polders, river systems, main fluvial inflow inlets and tide gauging locations (source: [59]).

Thus, the mangrove forests in the Sundarbans, where the Ganges meets the Bay of Bengal, are vital for ecosystem services. This region, characterized by a complex network of waterways and low elevation, benefits from mangroves’ ability to trap sediments, protect coastlines from storm surges, and serve as nursery grounds for marine life. They act as effective natural barriers against cyclones, reducing wave energy significantly. Additionally, mangroves are important carbon sinks, with the Sundarbans alone storing vast amounts of carbon dioxide, contributing to greenhouse gas mitigation.

2.3. Knowledge Gaps

Despite extensive research on the geomorphology, hydrology, and ecosystem services of the Sundarbans, several critical knowledge gaps persist. Studies focusing on how local communities adapt to environmental changes, particularly in social, cultural, and economic dimensions remain limited [60]. While the importance of biodiversity is widely acknowledged, species-specific dynamics, like salinity’s impact on the Bengal tiger and key fish species, are still underexplored [61]. Long-term predictive models for climate change impacts, such as sea-level rise and extreme weather events, require greater specificity and regional focus [62]. Furthermore, the cumulative effects of anthropogenic activities across time and space lack comprehensive evaluation. Soil microbiota’s role in carbon sequestration and mangrove health, as well as broader economic valuations of ecosystem services such as fisheries, tourism, and water purification, have been inadequately studied [60]. Understanding the Sundarbans’ interconnectedness with adjacent ecosystems, including coral reefs and offshore fisheries, remains a significant research gap. Enhanced hydrological models integrating both the natural and anthropogenic factors are crucial to better predicting changes in sedimentation, salinity, and tidal patterns. Evaluations of conservation policies and strategies for managing socio-environmental conflicts are still insufficient. Furthermore, the health impacts on local populations resulting from changes in water quality, salinity, and exposure to disaster need more thorough investigation [60]. Addressing these knowledge gaps would significantly enhance the understanding of the Sundarbans’ ecological and socio-economic systems, facilitating the development of more effective management and conservation strategies.

3. Historical Hydrological Indicators

3.1. The Surface Water System

The Sundarbans mangrove forest spans 6017 km², with 68.85% land and 31.15% water bodies. Key river systems contributing to its hydrology include the Passur, Shibsa, Arpangasia, Baleshwar, Malancha, Shella, and Raimangal rivers [63]. The forest is bordered by the Hariabhanga, Raymongal, and Kalindi rivers to the west; the Baleswar River to the east; the Bay of Bengal to the south; and various small rivers, canals, and settlements to the north. Approximately 4038 km² consists of forest, while over 115 km² includes marshlands, interconnected through a network of 450 rivers that form approximately 12,000 km of waterways [51,64].

The hydrology of the Sundarbans is largely influenced by the Ganges River, which undergoes significant seasonal fluctuations. The Baleshwar River, receiving more freshwater than the Shibsa and Passur rivers, is fed by both the Gorai River and the GBM River system. The Gorai River supplies vital freshwater to the Sundarbans, with approximately 16% of its flow entering the Haringhata-Baleswar estuary system and the remainder joining the Passur Basin. Seasonal variations result in low flow during the dry season (January–April) and increased salinity levels, while rainfall during the wet season (June–September) boosts discharge and reduces salinity [65].

Freshwater from the Ganges River is crucial for the regional economy and the mangrove ecosystem. However, since the construction of the Farakka Barrage in 1975, the Ganges flow has diminished, leading to increased salinity in the Sundarbans, particularly during the dry season [66,67]. A Ganges Water Sharing Agreement made in 1996 did not resolve the issues, as sediment blockage hindered flow in the Gorai River. The Gorai River Restoration Project (1998–1999) attempted to dredge the river to restore ecological balance, but the freshwater flow continued to decline from 1985 to 2010 [65]. Since 2004, salinity in the Gorai River has surged significantly impacting coastal areas such as Rampal, Mongla, and others, where prawn farming exacerbates saline water intrusion from the Bay of Bengal [68]. Consequently, the decline in freshwater flow has severely affected agriculture, fisheries, drinking water, and the mangrove ecosystems.

Several studies (e.g., [41,69] suggest that human interventions, such as dam construction, embankments, and upstream water diversions, have significantly influenced hydromorphological changes, surpassing the impact of climate change in some cases. Conversely, others (e.g., [35,70] emphasize climate-driven factors, including rising temperatures, sea-level rise, and salinity, as key drivers of ecological and hydrological transformations since the mid-1990s. Reduced freshwater inflows due to human activities were identified as major contributors to the increases in salinity and sedimentation [71,72].

Thus, the key rivers such as the Passur, Shibsa, Arpangasia, and Baleshwar influence the hydrology of the Sundarbans, while the Ganges River is vital for seasonal flow through it. The declined Ganges flow is however increasing salinity, particularly in the dry season. Restoration efforts, like the Gorai River Project, aim to improve the ecosystem, but declining water flow continues to negatively impact agriculture, fisheries, and coastal health, worsened by prawn farming and saline water intrusion from the Bay of Bengal.

3.2. The Groundwater System

Historically, groundwater levels in the Sundarbans have shown significant seasonal variation, primarily driven by monsoon rains, which typically occur from June to September. The hydrology of groundwater is intricately linked to the flow of the Ganges River, as both river water and rainfall are essential for sustaining and balancing groundwater conditions. During the wet season, aquifers receive considerable recharge, replenishing groundwater levels [73]. However, this recharge often falls short of compensating for the depletion that occurs during the dry season, exacerbated by increased irrigation demands in agriculture. The extraction of groundwater for agricultural use, coupled with diminished freshwater inflows due to upstream water diversion and the plausible impacts of climate change, has led to a significant decline in groundwater levels. This decline poses a growing concern for water availability and disrupts the natural hydrological balance within the ecosystem.

The reduction in groundwater levels has also serious implications for salinity intrusion, a critical issue in the Sundarbans. Under normal conditions, the flow of freshwater toward the sea helps to limit the landward encroachment of seawater. As noted by [74], decreasing groundwater levels shift the hydraulic gradient, permitting saline water from the Bay of Bengal to move inland, particularly during the dry season when freshwater availability is at its lowest. This intrusion compromises groundwater quality, threatening drinking water supplies, agricultural productivity in the adjoining polders, and the health of the mangrove ecosystem.

Table 1. The hydrological dynamics, challenges, and solutions of the groundwater system in the Sundarbans region.

Aspects	Details
Seasonal fluctuations	Groundwater levels fluctuate seasonally, influenced by monsoon rains (June–September).
Groundwater recharge	Wet season recharge from aquifers is insufficient to compensate for depletion during the dry season.
Impact of over-extraction	Over-extraction for agriculture and reduced freshwater inflows lead to significant groundwater depletion.
Link to river flow	Groundwater hydrology is influenced by the Ganges River flow, critical for maintaining water balance.
Salinity intrusion	Declining groundwater levels increase salinity intrusion, especially during the dry season, affecting agriculture and ecosystems.
Environmental implications	Reduced freshwater flow allows seawater to encroach inland, compromising water quality and ecosystem health.
Key solution	Sustainable groundwater management is essential to mitigate depletion and salinity issues.

summarizes the main points regarding the hydrological dynamics, challenges, and solutions of the groundwater system.

3.3. Water and Soil Salinity

Salinity intrusion in the Sundarbans region is driven by changing hydrological conditions, primarily due to reduced freshwater flow from the Ganges River. This reduction, notably exacerbated by the construction of the Farakka Barrage, has significantly decreased water discharge from 3700 m³/s in 1962 to 364 m³/s in 2006 [66]. Over the past two decades, a decline in runoff from the eastern rivers has led to rising salinity levels, intensified by climate change and sea-level rise [75]. A marked increase in salinity levels was observed since the mid-1970s, with approximately 60% of the Sundarbans experiencing elevated salinity (>20 ppt) for at least 1.5 months each year [76]. The Southwestern area faces the highest salinity, with levels rising from 38.898 dS/m to between 54.025 dS/m and 69.152 dS/m [77]. Seasonal and spatial variations in salinity highlight the impacts of tidal mixing, with salinity decreasing from the west to the east along the coastline [65].

Additionally, natural geomorphological processes, such as tilting of the Bengal Basin and river migration, also play critical roles in increasing salinity [35]. The Sundarbans’ vulnerability to tidal flooding due to its low elevation and sea-level rise is widely acknowledged [40]. However, sediment deposition in some regions may mitigate these effects locally by raising land levels [78]. While the protective role of mangroves in reducing wave energy and storm surges is generally accepted, the sustainability of these services under increasing cyclonic activity remains debated [79].

Salinity fluctuations are influenced by climate, hydrology, rainfall, topography, and tidal flooding. Changes in the regional hydrology and socio-economic conditions are contributing to increased salinity levels in the Southwestern Bangladesh [75]. This salinity intrusion reduces freshwater availability, leading to shifts in species composition and biomass within the mangrove ecosystem, as well as negatively impacting agriculture due to soil salinization from climatic events like cyclones and storm surges [80,81].

Table 2. Freshwater flow and salinity dynamics in the Sundarbans region.

Aspects	Details
Cause of salinity intrusion	Reduced freshwater flow from the Ganges River, exacerbated by the Farakka Barrage.
Change in river discharge in the Ganges River	Decreased from 3700 m3/s in 1962 to 364 m3/s in 2006.
Salinity increase timeline	Significant rise observed since the mid-1970s.
Area affected	Approximately 60% of the Sundarbans, with high salinity in the Southwestern region.
Salinity levels (Southwest)	Increased from 38.898 dS/m to 54.025–69.152 dS/m.
Factors influencing salinity	Climate, hydrology, rainfall, topography, tidal flooding.
Consequences	Decreased freshwater availability, species shifts in mangrove ecosystems, soil salinization, impacts on agriculture.
Climate change impact	Rising sea levels and climate change contribute to higher salinity.

provides the summary of freshwater flow and soil salinity dynamics in the Sundarbans region.

3.4. Tidal Dynamics

The construction of earthen embankments, known as polders, in the densely populated area north of the Sundarbans National Forest has significantly altered tidal dynamics. Natural drainage in the upstream regions, aside from the primary river channels, is obstructed by extensive embankments and polders. Additionally, the creation of coastal polders has progressively reduced floodplain storage areas for tidal waters originating from the Bay of Bengal. This has led to tidal amplification and widespread reorganization of the tidal network [82,83]. The lowest recorded tidal range was 2.74 m, while the highest in the east was 5.12 m [37]. During spring tides, the maximum inundation period lasts approximately 3–4 h, with the average velocity of micro-currents ranging from 10 to 20 cm/s. Variations in water level and tidal amplitude at the coast propagate inland during each tidal cycle. The tidal range in the Sundarbans mangrove forest’s northern fringe is higher than in the southern bay.

Over the period from 1983 to 2003, the annual maximum tidal range increased by approximately 0.75 m in the eastern and central regions [76]. Meanwhile, the highest tidal high water level is rising annually, while the lowest tidal low water level is decreasing annually, at rates of 7–18 mm and 4–8 mm per year, respectively [41]. The small yearly increases in tidal water levels may have limited short-term effects, such as slightly raising the storm surges during cyclones. However, the long-term impacts of rising sea levels and the inland spread of saltwater present significant challenges. These issues must be addressed when planning infrastructure, managing resources, and preparing for emergencies. Extreme flooding and cyclonic storm surges are expected to result in higher flood levels, wider affected areas, and potentially more frequent events [84,85]. Such events could significantly affect vulnerable regions by increasing soil and water salinity [86], lowering agricultural production [87], and damaging infrastructure and aquaculture systems [88]. Feist et al. [89] categorizes risks in Southwest Bangladesh based on water level changes: low (<5 mm/y), medium (5–10 mm/y), and high (>10 mm/y).

3.5. Tropical Cyclones and Storm Surges

The Sundarbans is an ecologically significant but fragile region, historically vulnerable to frequent and severe tropical cyclones and storm surges. The Bay of Bengal, due to its geographic location and climatic conditions, experiences frequent cyclones. Its shallow waters and funnel-shaped coastline exacerbate the impacts of these storms, resulting in destructive storm surges in the Sundarbans [90,91]. The 1970 Bhola Cyclone was particularly catastrophic, with over 300,000 deaths. Other significant cyclones, such as Cyclone Sidr in 2007 and Cyclone Aila in 2009, caused extensive damage to the Sundarbans and surrounding coastal areas [92].

The Sundarbans mangrove forest serves as a natural buffer, reducing the severity of storm surges and protecting inland regions from extreme damage [93]. However, the mangrove ecosystem itself is highly susceptible to the effects of cyclones and surges. Repeated cyclonic events increase salinity, accelerate soil erosion, and damage mangrove vegetation [94]. This degradation has led to significant biodiversity loss, impacting plant and animal species. The adjoining region’s inhabitants, relying on agriculture, fishing, and forest resources, suffer from destroyed homes, farmland, and infrastructure [95,96]. Cyclones like Amphan in 2020 severely damaged livelihoods, with storm surges contaminating freshwater sources and salinizing soil, rendering it unsuitable for agriculture. These recurring disasters have intensified poverty, heightened food insecurity, and triggered migration [79].

Statistical analyses [97] indicate that, on average, 1.26 tropical cyclones affect Bangladesh annually, with peak occurrences in May and October. Severe cyclones impact the country approximately once every five years. While the frequency of cyclones has remained relatively consistent, there is evidence suggesting an increase in their intensity. This trend may be attributed to factors such as rising sea surface temperatures and climate change, which can enhance the energy available for cyclones, potentially leading to more powerful storms. The situation is expected to worsen with the rising sea levels, more intense cyclones, and an increasing frequency of storm surges, putting the Sundarbans’ ecosystem and its people at greater risk [98]. Consequently, climate change adaptation and disaster resilience are critical, as emphasized by [99], who advocated for stronger measures to safeguard the Sundarbans from future climatic threats. Historical cyclones and storm surges have profoundly impacted the ecological and socio-economic systems of the region, underscoring the urgent need for sustainable management strategies and enhanced disaster preparedness.

Table 3. Aspects of vulnerability of Sundarbans to tropical cyclones and storm surges.

Category	Key Information
Vulnerable region	Sundarbans, located in the Bay of Bengal
Ecological significance	Mangrove forest serves as a natural buffer against storm surges and cyclones
Historical cyclones	1970 Bhola Cyclone (300,000 deaths), Cyclone Sidr (2007), Cyclone Aila (2009), Cyclone Amphan (2020)
Cyclone impacts	Severe storm surges, destruction of homes, farmland, infrastructure; increased salinity, soil erosion, mangrove damage
Biodiversity loss	Cyclones damage plant and animal species; degradation of mangrove ecosystems
Socio-economic impact	Loss of livelihoods (agriculture, fishing, forestry), poverty, food insecurity, migration
Climate change effects	Rising sea levels, more intense and frequent cyclones, greater risk to both ecosystem and communities
Urgent actions needed	Climate change adaptation, disaster resilience, sustainable management, enhanced disaster preparedness

summarizes the vulnerability of Sundarbans to tropical cyclones and storm surges.

3.6. Erosion and Accretion

In the Sundarbans, the interaction of coastal landforms, climate patterns, tidal strength and duration, and the amount of freshwater creates a complex process of accretion and erosion. The extensive network of interconnected waterways influences this process. Erosion predominantly occurs along the banks of the major river channels and at the land–water interface adjoining the Bay of Bengal, which diminishes the Sundarbans mangrove forest. The reduced discharge of the Ganges River has resulted in accelerated erosion and decreased accretion along the Sundarbans coast [31,100]. In addition to the Farakka Barrage, India’s proposed National River Linking Project is anticipated to further reduce the freshwater discharge of the Ganges River by 24%, with a potential reduction in sediment load ranging from 39% to 75% [101]. The rivers are crucial for supplying sediment to the entire delta.

The rates of sediment migration and deposition are affected by the presence of forests and mangroves [102]. The mangrove forests are particularly effective at trapping sediment, with heightened trapping rates during low tide. The dense underground and aerial roots of mangrove vegetation obstruct or slow down the currents during tidal inundation, wave activity, and sediment transport, directly impacting hydrological and sediment dynamics [103]. These dense roots help bind soils, while the aerial roots promote sediment deposition and reduce erosion [104]. Over the past four decades, the flow of sediment-rich freshwater into the Bay of Bengal via the Bangladesh segment of the Sundarbans mangrove forests has dwindled, attributed to decreased water release from the Farakka Barrage [5]. The Ganges River transports 262 million tons of sediment annually, with just 7% directed towards its southern distributaries. Silt deposition in the forest threatens river flow but promotes vigorous growth of mangrove vegetation. The limited flow hampers sediment accumulation at the base of forestlands and inhibits the formation of new forest areas. The mangrove ecosystem undergoes constant changes due to large amounts of sediments carried by rivers, near shore currents, and waves [105]. The rising sea levels also impact sediment availability, directly hindering the formation of new land.

The rivers Passur, Baleshwar, and Shibsa exhibit the highest erosion rates, while the lowest rates are recorded for Arpangasia, Malancha, and Shella rivers [106] (Figure 3). Accretion rates were notably lower than erosion, with Passur (1150 ha, 29.5 ha/y) and Baleshwar (465 ha, 12 ha/y) rivers showing higher accretion than the others. Erosion predominantly affects the southern part of all rivers (3658 ha, 94 ha/y) compared to the central (1851 ha, 47 ha/y) and northern parts (1893 ha, 49 ha/y) [106]. It is anticipated that declining suspended sediment concentrations could lead to deterioration and submergence in certain regions, particularly those already depleted of sediment, such as parts of the Sundarbans National Forest [107].

Sediments from the rivers and tidal sources are dispersed across the forest floors, yet erosion predominates along the coastline, fostering the emergence of new islands (chars). Consequently, the water bodies consistently increased, while the healthy and unhealthy vegetation decreased from 1991 to 2021 [108]. However, the estimates and projections of erosion, accretion, and land subsidence in the Southwest coastal region are often debated [38]. The region is still undergoing active land formation due to the shifting courses of its rivers. Over time, the eastward meandering of the Ganges and Brahmaputra Rivers has impacted sedimentation patterns and significantly reduced freshwater inflows. Moreover, human activities such as constructing upstream dams, building embankments to protect land from tides, and over-exploiting mangrove timber have also affected water supplies, sedimentation, and the region’s topography and hydrology [38]. Additionally, during specific monsoon periods, the heightened water flow exacerbates erosion, further endangering these delicate forest ecosystems [5].

Table 4. The key factors that influence erosion and accretion in the Sundarbans region.

Factor	Description
Coastal dynamics	Interaction of landforms, climate, tides, and freshwater causes erosion and accretion.
Main erosion areas	Major riverbanks and Bay of Bengal land–water interface.
Key drivers of erosion	Reduced Ganges discharge, decreased freshwater flow.
Mangrove role	Mangroves trap sediment, reduce erosion, and promote vegetation growth.
Sediment issues	Limited sediment supply, particularly from the Ganges River, hinders forest growth and accretion.
Highest erosion rates	Passur, Baleshwar, Shibsa Rivers.
Accretion rates	Lower accretion compared to erosion, notable in Passur and Baleshwar Rivers.
Regional erosion	Southern regions suffer higher erosion (94 ha/y), while northern and central regions experience less erosion.
Human impact	Dams, embankments, and mangrove exploitation affect hydrology, sedimentation, and forest health.
Rising sea levels	Impact sediment availability, further hindering land formation.
Future threats	Declining sediment concentration could lead to submergence, particularly in already sediment-deprived areas.

summarizes the key factors related to erosion and accretion in the Sundarbans.

4. Projected Hydrological Indicators

4.1. Temperature and Rainfall

Climate change and human-induced stressors are intricately intertwined in the Sundarbans mangrove ecosystem and its surrounding mainland. Climate change threatens the Sundarbans, including increasing air and water temperatures, rising sea levels, and changes in the frequency and intensity of rainfall and storms [109]. The [110] climate projections suggest that the most extreme globally averaged surface warming could reach 2.4–6.4 °C with the best estimate of 4.0 °C by 2090–2099 relative to 1980–1999. By the end of the 21st century, the temperature increases in tropical Asia due to global warming is projected to exceed the mean expected global temperature rise [111]. This temperature rise is not uniform across Asia; the expected increases are estimated at 3.3 °C in South Asia, 3.7 °C in Central Asia, and 2.5 °C in Southeast Asia.

The Sundarbans experiences a humid tropical climate with a mean annual rainfall of approximately 1700 mm and annual temperatures ranging between 29 °C and 31 °C [112]. Throughout the year, the average monthly maximum temperature fluctuates between 34.6 °C (in April) and 27.6 °C (in November), while the average monthly minimum temperature ranges from 24.1 °C (in May–June) to 11.3 °C (in December–January). Climate change projections suggest an increase of 0.4 °C per year in Bangladesh, which could lead to more intense and frequent cyclonic storms [39]. The temperature in the Khulna region has been rising steadily, particularly in recent years [41,113]. The surface water temperature in the Indian Sundarbans’ deltaic complex has been increasing at a rate of 0.5 °C per decade from 1980 to 2007, significantly surpassing the global warming trend of 0.06 °C per decade and the rate of 0.2 °C per decade observed in the Indian Ocean from 1970 to 1999 [70,110]. In the eastern region, shifts in temperature, salinity, pH, and dissolved oxygen became more evident from the mid-1990s [70]. Rainfall intensity and frequency of rainy days have also increased, although peak annual rainfall and the frequency of heavy rainfall days have shown little change [113]. Future projections of rainfall changes show variability between models, indicating that the Sundarbans may experience wetter or drier conditions [110]. Figure 4 presents the mean annual and five-year smoothed values for precipitation and temperature [114]. Precipitation shows significant year-to-year variation without a clear trend. In contrast, temperature exhibits minor year-to-year fluctuations but consistently increases across all scenarios [115].

It is noted that, although important for predicting future changes, there are significant shortcomings with the Coupled Model Intercomparison Project (CMIP) models in representing rainfall patterns. Ref. [116] point out that global models do not accurately capture the effects of aerosols on clouds and regional circulation, which are keys to simulating rainfall correctly. Ref. [117] report that compensating errors in cloud feedback and aerosol-cloud interactions in CMIP6 models contribute to uncertainty in precipitation forecasts. Recent studies show that CMIP6 models tend to overestimate the number of rainy days across different seasons and scenarios, reducing the reliability of rainfall predictions. In specific regions like Chile, CMIP6 models struggle to represent precipitation patterns accurately, leading to biases in regional climate projections. Improving these issues is essential for making rainfall predictions more reliable in climate models.

The Sundarbans mangrove ecosystem faces significant challenges from regional climate change, requiring urgent attention. The rising air and water temperatures threaten the region’s biodiversity, affecting species adapted to the current conditions. The higher temperatures will likely intensify cyclonic storms, causing ecological damage and impacting human settlements. The rise in sea level poses risks of coastal erosion, loss of mangrove habitats, and increased saltwater intrusion, which could harm agriculture, drinking water supplies, and coastal protection. While the annual rainfall patterns may remain stable, heavier rainfall events could lead to flooding, erosion, and infrastructure damage, and rainfall variability complicates water and agricultural management. Warming waters in the Sundarbans Delta could also harm aquatic species sensitive to temperature changes. Human activities, such as deforestation, urbanization, and overexploitation of resources, compound climate impacts, weakening the ecosystem’s resilience. Addressing these issues requires strategies for climate mitigation and adaptation, including mangrove restoration, coastal defense strengthening, and disaster preparedness. Continuous research, monitoring, and coordinated efforts at all levels are essential for sustainable management and development of the region.

4.2. Extreme Events

Climate change exacerbates natural disasters, notably impacting the Sundarbans mangrove forests [28]. The bare forests increased by 220% after the super cyclone Sidr; by 2020, there were no signs of regeneration. The cyclone-induced disturbance in the mangrove ecosystem threatens environmental sustainability, leading to broader ramifications for the ecological balance. A simulation study by [21] indicated that converting mangroves to grassland could, on average, amplify surge elevation by up to 57% and notably escalate flood wave velocity by as much as 2730% during Category 3 cyclones. According to these investigators, there would be an approximate 10 km extension in inland inundation penetration for low-intensity cyclones and a corresponding 18% increase in the total flooded area. As sea levels rise, the depth and frequency of inundation are expected to increase. Additionally, transforming mangroves into grassland may result in heightened inundation distance and expanded flooded areas, especially during low-intensity cyclones.

Thus, the destruction and lack of regeneration of mangrove forests in the Sundarbans due to climate change-induced cyclones threaten environmental sustainability. Even low-intensity cyclones could cause a more extraordinary inland inundation and larger flooded areas, exacerbating the effects of sea-level rise. The transformation of mangrove forests into less protective ecosystems like grasslands could further disrupt the ecological balance, increasing vulnerability to natural disasters.

4.3. Sea-Level Rise

The Sundarbans have been categorized into three primary physiographic zones based on surface layout. These comprise the tidal flood plain of the Hooghly-Matla-Hariabhanga complex (3–5 m above mean sea level, AMSL), the Sundarbans mangrove swamps (1–3 m AMSL), and the estuarine and tidal river mouths (less than 1 m AMSL) [79]. Situated at the confluence of the sea and the upstream and downstream Ganges basin, the Sundarbans mangrove forests stand approximately 1.5 m AMSL, rendering them susceptible to sea-level rise and heightened salinity [118]. It faces risks from both human-made and environmental factors. Its physiography is presumed to undergo significant shifts as a result of climate change, primarily driven by rising sea levels. This phenomenon is expected to fundamentally alter flooding patterns and salinity levels throughout the region. The extent of dry lands in the Sundarbans is projected to diminish drastically, plummeting from 43% under baseline conditions in 2001 to a mere 7% by 2100, owing to an anticipated 88 cm rise in sea levels [119]. Essentially, approximately 84% of the current dry lands are at risk of disappearing. Furthermore, it is predicted that approximately 77% of the entire Sundarbans area will be submerged under more than a meter of water due to this sea-level increase. Alongside the heightened inundation, alterations in salinity patterns are also expected due to changes in sea levels and upstream freshwater flow. Specifically, the proportion of low saline areas (0–1 ppt) is anticipated to decline from 10.8% in 2001 to 4.0% by 2100, reflecting the profound impact of these environmental shifts on the Sundarbans ecosystem [119]. Additionally, the sea-level rise may lead to the inland migration of shorelines if sediment deposition cannot match the rate of sea-level rise [120].

Using the global sea-level rise projections from the IPCC under RCP4.5 and RCP8.5 scenarios, along with station-based measurements from Hiron Point, Fiest et al. [89] illustrates the potential inundation levels in the Sundarbans by the end of the 21st century. In the RCP8.5 scenario, the low- and mid-sensitivity projections align closely with the high- and mid-sensitivity projections of the RCP4.5 scenario as illustrated in Figure 5a,b. Furthermore, an analysis of station-based data from 1977 to 2020 reveals that sea levels in the Sundarbans are rising at an average rate of 0.36 cm per year (Figure 5c). Note that, of the four RCPs (2.6, 4.5, 6.0 and 8.5), RCP2.6 represents a scenario where global warming remains well below 2 °C above pre-industrial levels. In contrast, RCP8.5 depicts a fossil-fuel-intensive future without climate mitigation, leading to nearly 5 °C of warming by century’s end [121,122]. Originally designed to assess a high-risk but unlikely outcome [121], RCP8.5 should not serve as a baseline, as it assumes a halt or reversal of recent climate policies and technological advancements. When used, it must be clearly identified as an unlikely worst-case scenario rather than a business-as-usual trajectory. Overstating extreme climate risks can create undue pessimism, making mitigation seem unfeasible and leading to poor planning. A more realistic range of baselines would enhance climate risk assessments and policy responses. The International Energy Agency (IEA) and the UNEP’s Emissions Gap Report adopt a similar approach, comparing emissions-reduction commitments with pathways that limit warming below 2 °C, emphasizing realistic targets over worst-case projections [123].

The rise in sea level along the Bangladesh coast surpasses the global average of 1.0–2.0 mm/y [124], posing a significant threat to the Sundarbans mangrove ecosystem. Citing reports from the IPCC and existing studies on sea-level rise, the National Adaptation Program of Action (NAPA) for Bangladesh has projected sea-level rise estimates of 14, 32, and 88 cm for the years 2030, 2050, and 2100, respectively [124]. In light of the risks posed by climate change, Bangladesh’s Department of Environment (DoE) has projected potential future sea-level rise, ranging from 0.3 to 1.5 m by 2050 [125]. The sea levels are anticipated to rise by 4 mm per year, exacerbating salinity levels and reducing freshwater flow within the Sundarbans [39].

The sea-level rise poses significant threats to coastal ecosystems, potentially causing waterlogging and eventual mortality of mangroves and associated flora along landward margins [126]. The World Wildlife Fund for Nature Conservation estimates that approximately 7500 hectares of mangrove forest in the Sundarbans of Bangladesh are at risk of submergence in the near future [127]. In 2010, as the sea levels worryingly rose, New Moore Island/South Talpatti Island vanished from sight, prompting scientists to take notice. The extent of permanent inundation of the Sundarbans due to sea-level rise is uncertain, given its location in the active Ganges-Brahmaputra delta where sedimentation continues. The rise in sea level in the Sundarbans is influenced not only by eustatic changes but also potentially by ground-level changes due to subsidence [95]. Together with eustatic sea-level shifts and compaction, it has led to an effective rise in sea level of 0.7 cm/y along the southern coast, escalating to 1.7 cm/y near Khulna city. This exacerbates flood vulnerability across the entire tidal delta plain.

Thus, the rising sea levels pose a critical threat to low-lying coastal regions including the Sundarbans. This region is experiencing sea-level rise rates exceeding global averages, endangering mangrove ecosystems that protect against storm surges and erosion. Up to 84% of the Sundarbans’ land may submerge by 2100, leading to habitat loss, reduced freshwater availability, and increased salinity, which threaten biodiversity, agriculture, and livelihoods. Without sufficient sediment deposition to counter rising seas, shoreline migration and erosion could worsen. These changes heighten the risks of flooding, waterlogging, and displacement for coastal communities. Addressing these challenges requires urgent action, including flood defenses, habitat restoration, and adaptive governance to ensure ecological and human resilience.

Figure 5. (a) IPCC projection of sea-level rise (SLR) under the RCP4.5 scenario; (b) IPCC projection of SLR under the RCP8.5 scenario; (c) SLR of Sundarbans based on observed data (Hiron point); (d) SLR values from different scenarios (source: [128]).

Figure 5. (a) IPCC projection of sea-level rise (SLR) under the RCP4.5 scenario; (b) IPCC projection of SLR under the RCP8.5 scenario; (c) SLR of Sundarbans based on observed data (Hiron point); (d) SLR values from different scenarios (source: [128]).

4.4. Saline Water Intrusion

The greatest challenges for the Sundarbans in a changing climate are increased saltwater intrusion due to sea-level rise and a shortage of nutrients from reduced freshwater flows [86,129]. The primary impacts of rising sea levels on freshwater sources stem from the intrusion of seawater into surface waters and coastal aquifers, extending into estuaries and coastal river systems [130]. Thus, the climate change is projected to reduce the suitable area for dominant tree species in the Sundarbans by 2100, affecting timber stock [131]. The increase in aquatic salinization will inevitably alter the hydrological regime of the Sundarbans and transform its forest ecology [38]. A sea-level rise exceeding 1.0 m would submerge 10% of Bangladesh, leading to further inland seawater intrusion and increased salinity in coastal regions [132]. Consequently, climate change will drive progressive river salinization in the low-lying coastal areas of Bangladesh [133].

The freshwater flow in the Sundarbans, comprising river discharge and rainfall, significantly impacts both the freshwater flow and salinity levels [134]. The region already experiences a severe shortage of freshwater during the dry season (October–May) because some distributaries of the Ganges River feeding the Sundarbans are diminishing. Expected changes in the riverine flows from the Himalayas and rising sea levels will exacerbate salinity intrusion as climate change progresses. Decreased freshwater flow in the western regions of the Sundarbans has led to heightened salinity levels in river waters and the gradual shallowing of rivers over time. The core elements of the ecosystem, including soil, water, vegetation, and wildlife, are significantly impacted by freshwater shortages and human activities.

Rising sea levels are intruding seawater into freshwater systems like rivers, aquifers, and estuaries, increasing salinity and disrupting hydrology and forest ecology. Reduced freshwater from the Ganges and the lower dry-season rainfall is worsening water scarcity, heightening salinity, and shallow waterways. These changes threaten Sundarbans’ dominant tree species, forest biodiversity, timber resources, and ecosystem services like water purification and flood protection. A sea-level rise over 1 m could submerge land, increase salinity inland, and impact agriculture, habitation, and livelihoods, intensifying climate change vulnerabilities.

The salinization of coastal aquifers due to climate change and human activities is now widely recognized. The Sundarbans in India is a notable example of this issue, experiencing an alarming rate of groundwater salinization that renders drinking water unfit for consumption [135]. Contributing factors such as sea-level rise, shifting rainfall patterns, and increasing population density aggravate groundwater salinization. Current water sources, like the shallow aquifers, exhibit elevated salinity levels, while the deep aquifers exceed acceptable salinity thresholds [135].

5. Challenges Facing the Sundarbans Ecosystem and Necessary Actions

Rising sea levels and salinity intrusion: Rising sea levels and increased salinity threaten the Sundarbans’ delicate ecosystem, with risks of inundation, shrinkage, and biodiversity loss [89]. Up to 84% of the Sundarbans could be submerged by 2100 if the current trends continue [10]. Salinity intrusion disrupts freshwater ecosystems, agriculture, and livelihoods, worsening food and water insecurity [28,38]. The expansion of monitoring for sea-level rise, salinity gradients, and freshwater inflow using both satellite and ground-based systems is essential. Developing precise, region-specific models to predict salinity intrusion and ecosystem changes under various climate scenarios will be critical. Additionally, revising the transboundary water-sharing agreements to ensure the adequate flow of freshwater is necessary. Promoting mangrove reforestation can serve as an effective measure to buffer against the impacts of salinity changes.

Cyclonic activity and ecological disruptions: Increased frequency and intensity of cyclones amplify habitat destruction, soil salinization, and coastal erosion [20]. Mangroves, acting as natural storm barriers, are degraded, reducing their protective functions [55]. Investing in early warning systems and cyclone tracking technologies is a critical intervention to enhance disaster preparedness. Strengthening infrastructure with flood-resilient designs and cyclone shelters is essential for minimizing the impact. Additionally, promoting the restoration of mangrove forests will significantly bolster natural defenses, offering long-term protection against environmental hazards.

Freshwater scarcity and groundwater depletion: Reduced freshwater inflow, compounded by upstream interventions like the Farakka Barrage, has elevated salinity levels and harmed agriculture, fisheries, and biodiversity [28]. Excessive groundwater extraction further exacerbates salinity intrusion and water scarcity [38]. It is essential to conduct comprehensive groundwater assessments to understand its depletion rates and salinity thresholds. Integrated hydrological models should be developed to evaluate the interplay between the upstream activities, groundwater dynamics, and salinity. Additionally, implementing sustainable aquaculture and irrigation practices, along with promoting salinity-resistant crops, will be crucial for ensuring long-term agricultural and environmental sustainability.

Sediment flow and coastal erosion: Changes in sedimentation patterns, influenced by upstream dams and embankments, have disrupted the natural land formation and erosion processes [38]. This intensifies habitat loss and endangers both the ecosystems and human settlements. To enhance the assessment of geomorphological changes, it is essential to strengthen sediment transport monitoring. Additionally, integrating natural sedimentation processes into coastal defense models will improve their accuracy. Restoring natural sediment flows can be achieved by modifying the dams and embankments, ensuring more sustainable sediment management.

Biodiversity and habitat loss: Habitat fragmentation, salinity shifts, and rising temperatures threaten the iconic species such as the Bengal tiger and economically significant fish species [28]. The decline in mangrove cover due to human activity and environmental stressors further endangers biodiversity. It is essential to conduct ecological surveys to map biodiversity changes and assess the species vulnerability. Predicting the habitat suitability for key species under future climate scenarios will provide valuable insights. Policy actions should focus on enhancing conservation policies and supporting community-led initiatives to protect the biodiversity and promote sustainable resource use.

Socio-economic impacts and community resilience: Environmental changes have heightened poverty, food insecurity, and displacement risks among the local populations reliant on agriculture, fishing, and forest resources [10]. To address the emerging challenges, it is essential to analyze socio-economic trends and the adaptive capacities of local communities. Strengthening livelihood diversification programs, providing education on sustainable practices, and improving access to clean water and sanitation are critical interventions to enhance resilience and promote sustainable development.

Cross-cutting recommendations: To enhance cooperation between India and Bangladesh in water management and ecosystem preservation, both countries should prioritize joint efforts. This includes investing in research on climate adaptation, ecosystem services, and sedimentation models. Community engagement through participatory conservation and capacity-building initiatives is also important for sustainability [136]. Additionally, securing international support, such as global climate funds, is essential to finance projects in the Sundarbans, ensuring long-term ecological resilience [137]. By integrating these measures into policies, the resilience of the Sundarbans and the livelihoods of its dependent communities can be significantly enhanced.

6. Concluding Remarks

The Sundarbans mangrove forests, known for their high productivity and biodiversity, provide crucial ecosystem services, including acting as a protective barrier against cyclones and carbon sinks. However, these forests are vulnerable to climate change and human activities, which have significantly impacted their ecosystem, biodiversity, and local communities. From 1980 to 2007, water temperatures in the Sundarbans increased by 0.5 °C per decade, far exceeding the global average. This warming has intensified cyclonic storms and created significant challenges for aquatic life. Additional issues include reduced freshwater flow due to the Farakka Barrage, silt buildup, and waste dumping, all of which harm the mangrove ecosystem. To mitigate these impacts, sustainable management practices are essential. These include maintaining sediment-free rivers, ensuring continuous freshwater flow, reducing pollution and minimizing deforestation. Rehabilitation of the mangrove zones, alternative livelihoods for dependent communities and integrating salinity and mangrove patterns into conservation strategies are crucial. Policymakers should adopt regulations to oversee human activities, ensure freshwater availability, and enhance awareness and surveillance efforts. Global emission reductions, promoting saline-resistant plants, establishing flood relief centers, and protecting biodiversity are also imperative.

To sustainably manage the Sundarbans and reduce surface water pollution, a balanced, category-based approach is essential. First, immediate action should focus on pollution reduction by controlling industrial discharge, plastic waste, and oil spills to prevent further environmental harm. Second, long-term conservation efforts should prioritize habitat protection, afforestation, and sustainable fishing to maintain ecological stability. Third, strengthening community engagement and policy enforcement through stricter regulations, local participation, and eco-friendly livelihood options will ensure lasting impact. Addressing pollution first mitigates immediate threats, while sustainable management strategies safeguard the Sundarbans for the future. The USAID-funded Climate-Resilient Ecosystems and Livelihoods (CREL) project set up Co-Management Committees (CMCs) to involve local communities in protecting the Sundarbans. The project helped communities find alternative ways to earn a living, such as eco-tourism, crab farming, and handicrafts, reducing their reliance on illegal logging and fishing. More than 1000 community members received training in environmentally friendly income-generating activities.

The Sundarbans faces severe threats from climate change, including rising sea levels, increased salinity, and extreme weather events. Assessing these impacts is crucial for developing adaptive management strategies that enhance resilience. Adaptive management involves continuous monitoring, learning, and adjusting policies to changing conditions. A successful example is the Great Barrier Reef Marine Park in Australia, where adaptive management strategies, such as controlled fishing zones and coral restoration programs, have helped mitigate climate-induced coral bleaching. Implementing similar adaptive approaches in the Sundarbans can safeguard its biodiversity and the livelihoods of millions dependent on this fragile ecosystem. In conclusion, addressing the challenges faced by the Sundarbans requires a combination of sustainable management, comprehensive policies, and global climate initiatives. These efforts are essential to preserve the Sundarbans’ unique ecosystem, protect its biodiversity, and improve the living conditions of local communities for future generations.