# **1. Introduction**

Climate change poses several threats to the Sundarbans—the world’s largest remaining contiguous mangrove forest and one of its richest ecosystems.<sup>1</sup> These threats include rise in sea level, rise in air and water temperature, and change in the frequency and intensity of precipitation and storms, among others (Alongi 2008). Worldwide, globally important mangrove ecosystems are at increasing risk from inundation, salinization, and other potential impacts of climate‐driven sea‐level rise.<sup>2</sup> Rise in sea level may even threaten the survival of mangroves if their landward migration is obstructed by a lack of adequate and suitable space for expansion and the rate of sea‐level rise is greater than that at which mangroves can migrate (Ellison and Stoddart 1991; Semeniuk 1994; UNEP 1994; McLeod and Salm 2006; Lange et al. 2010). Historically, mangroves have shown considerable resilience to fluctuations in sea level (Alongi 2002; Gilman et al. 2006; Erwin 2009); however, their future adaptation may not keep pace.

The extent of permanent inundation of the Sundarbans from climate‐driven sea‐level rise is uncertain as the region is located in the active Ganges‐Brahmaputra Delta, where sedimentation is still occurring. For the Sundarbans, increased saltwater intrusion from sea‐level rise and shortage of nutrients from freshwater flows are the greatest challenges in a changing climate (Dasgupta et al. 2015a, b; IWM 2003; Peterson and Shireen 2001; SRDI 2000, 2010; UK DEFR 2007). Healthy mangroves require daily fluxes from both ocean and freshwater sources. The Sundarbans is already facing a serious freshwater shortage during the dry season (October– May) because some major distributaries of the Ganges that feed the region are currently moribund. Anticipated alteration of riverine flows from the Himalayas and an increase in sea level will intensify salinity intrusion as climate change continues (Dasgupta et al. 2015a, b; Dasgupta et al. 2014). The associated increase in aquatic salinization will inevitably change the hydrological regime of the Sundarbans and alter its forest ecology (Barik et al. 2018; Dasgupta, Sobhan, and Wheeler 2017).

These changes have significant implications for the present and future management of the Sundarbans, as well as the forest‐based livelihoods of tens of thousands of poor inhabitants. Therefore, understanding the physical and economic effects of salinity diffusion and planning for appropriate adaptation will be critical for management of the Sundarbans, as well as for long‐ term development and poverty alleviation in adjacent areas. This paper attempts to contribute to this understanding by assessing the impact of aquatic salinization on the spatial distribution of mangrove species. The main focus of this analysis is on the Sundarbans in India, which accounts for 40 percent (about 4,200 km<sup>2</sup> ) of the 10,200 km<sup>2</sup> mangrove wetlands.

> 1 The Sundarbans is a tidal‐wetland forest delta along the Bay of Bengal, spanning coastal segments of Bangladesh and India.

> 2 Recent research suggests that sea level may rise by 1 m or more in the 21st century, which would increase the vulnerable population to about 1 billion by 2050 (Hansen et al. 2011; Vermeer and Rahmstorf 2009; Pfeffer, Harper, and O’Neel 2008; Rahmstorf 2007; Dasgupta et al. 2009; Brecht et al. 2012).

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