Sea-Level Rise and Coastal Wetlands

Coastal wetlands comprise marshes, swamps, mangroves, and other coastal communities. However, a precise and widely agreed upon definition of wetland is not available. The RAMSAR Convention (a UNESCO-based intergovernmental treaty on wetlands adopted in 1971) defines wetlands as “areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water with the depth of which at low tide does not exceed six meters” (Article 1.1). Article 2.1 of the convention highlights that wetlands may incorporate riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six meters at low tide lying within the islands.

See Larson et al. (1989), Williams (1990), Barbier (1991), Barbier et al. (1997), Brouwer et al. (1999), Woodward and Wui (2001), McLeod et al. (2005), Brander et al. (2006), Laffoley and Grimsditch (2009), and Mcleod et al. (2011).

In addition to SLR, causes include waves, erosion, subsidence, and storms and biotic effects. Human actions include drainage for agriculture and forestry; dredging and stream channelization for navigation flood protection, conversion for aquaculture and mariculture, construction of schemes for water supply, irrigation and storm protection, discharges of pesticides, herbicides and nutrients, solid waste disposal, sediment diversion by deep channels and other structures, mining of wetland soil, groundwater abstraction, hydrological alteration by canals, roads and other structures, and mosquito control.

See Alongi (2008), Erwin (2009), and Gilman et al. (2006). McIvor et al. (2013) provide an excellent discussion of the physical processes guiding the resilience of coastal wetlands to SLR.

See Nicholls et al. (2007) for a comprehensive review.

The Fourth Assessment Report of the IPCC projected a rise in global mean sea level ranging from 18 to 59 cm by 2100. A final draft of the Fifth Assessment Report may consider likely a 26–82 cm rise in sea levels by the end of the twenty-first century. However, these ranges have been criticized as being too conservative and not sufficiently reflective of the large uncertainty pertaining to SLR (Krabill et al. 2004; Overpeck et al. 2006; Rahmsdorf 2007). Numerous studies suggest that SLR could reach 1 m or more during this century (Pfeffer et al. 2008; Vermeer and Rahmstorf 2009; Hansen and Sato 2012). The IPCC itself noted that the upper values of projected SLR presented in its reports are not to be considered upper bounds and that higher rises in sea level cannot be ruled out.

These are East Asia and Pacific, Europe and Central Asia, Middle East and North Africa, Latin America and Caribbean, South Asia, and Sub Saharan Africa.

Coastal wetlands in this analysis comprise freshwater marshes, swamp forests, GLWD Coastal Wetlands, and Brackish/saline wetlands located at elevation of 1 meter or less above sea level.

For example in the United States, Craft et al. (2009) estimate of the impacts of SLR on coastal wetlands of the state of Georgia while Day et al. (2000) reports estimates for the Mississippi Delta. Day et al. (2011) reports estimates for specific areas of the Mediterranean deltaic region. Technical limitations also impaired the inclusion of developed countries in this analysis. For example, the elevation data used in this analysis (90 m Shuttle Radar Topography Mission data) is restricted to latitude 60°N to 56°S. These data thus result in automatically excluding countries such as Canada, Iceland, Sweden, Norway, Finland, and Russia. In addition, while the wetlands data used in this analysis (GLWD-3) assemble a large number of attributes and polygon datasets to produce the most comprehensive database of lakes, reservoirs and wetlands, we have found that data for the developed and developing countries are not directly comparable. For example, the bulk of the coastal wetlands data for the USA is classified solely into two categories “50–100 % wetland” and “25–50 % wetland.” The data on coastal wetlands for the USA have not been classified into types of coastal wetlands (such as freshwater marsh, swamp forest, and Brackish/saline wetlands). Given the inclusion of wetlands migration capacity in our analysis (and not simply exposure as has been done in existing literature thus far), information on wetlands type is important as different wetland types have different migration capacity. Information on wetland types is available for developing countries, but is not systematically available for developed countries, including the USA.

The potential use of LIDAR survey (laser-based elevation measurement from low-flying aircraft) was beyond the scope of this analysis.

It is not immediately possible to assess the impact of excluding small islands countries in the analysis. However, according to the estimates of the World Resources Institute (WRI) based on the data from the World Vector Shoreline, the total length of the world coastline would reach approximately 1.6 million kilometers (WRI 2000). According to the same estimates, the total coastline length of small island countries was estimated to reach approximately 105 000 km, or 6 % of the world total. Furthermore, the largest 15 countries in terms of coastline length (none of them being small islands states) represent approximately 60 % of the world’s total coastline. As a result, we expect that the exclusion of small islands countries from this analysis does not significantly impact the results of the analysis. A limited number of country-level studies have aimed at estimating the impacts of climate change and SLR in small island states. For example, see Ellison (1993) and Schleupner (2008) and for the Caribbean islands of the Martinique and Bermuda, respectively.

GLWD Coastal Wetlands is a term used in this paper to distinguish coastal wetlands from the specific coastal wetlands type in the GLWD. GLWD Coastal Wetlands type is derived from a number of data sources and categories: “Lagoon” from ArcWorld; and “Delta,” “Lagoon,” “Mangrove,” “Estuary,” “Coastal Wetland,” and “Tidal Wetland” of WCMC wetlands map—see Lehner and Döll (2004) for a detailed description.

It should be noted that the SRTM database suffers from known limitation in urban as well as forested areas where the SRTM elevation data may capture the height of building or trees instead of ground level elevation. A similar limitation is noted by Nicholls et al. (2007).

Coastal zone with elevation derived from SRTM, which is 10 or less meters above sea level.

Latitude and longitude were specified in decimal degrees. The horizontal datum used is the World Geodetic System 1984.

Swamp Forest results are also dependent on the elevation from SRTM, which can have interference from features such as a dense tree canopy.

We have attempted to validate our estimates of country-level impacts with country-level detailed assessments available in the literature. However, an extensive search of the existing literature has revealed the rarity of such an assessment (which indeed is a key rationale for this paper). In India, Dwidedi and Sharma (2005) have reported a potential loss of 58 % of coastal wetlands in West Bengal. Our estimates are that India would lose 84 % of its GLWD Coastal Wetlands and 13 % of its freshwater marsh. Snidvongs et al. (2003) study the impacts of climate change on wetlands of the Mekong River Basin, but do not report quantified estimates of the potential impacts of SLR.

Brouwer et al. (1999), in their analysis, selected their sample exclusively from studies using contingent valuation as the means of valuation. Woodward and Wui (2001) included 39 valuation studies in their analysis with of these studies from the United States, thus focusing on temperate wetlands. Woodward and Wui (2001) reported an average value of approximately USD 2200 ha⁻¹ year⁻¹ (1995 USD).

The year 2000 was selected as the year of analysis in order to make the valuation comparable with the base year of the valuation study by Schuyt and Brander (2004) used in Table 4 in this paper. USD 150 (base year 1995) is equivalent to USD 163.4 (base year 2000) according to World Bank estimates. US GDP deflator has been used in the conversion.

In all likelihood, this is a conservative estimate as the recent studies on the dynamic implications of ice sheet stability are indicating sea-level may rise more than 1 m in the twenty-first century and opportunity cost of wetlands is likely to increase with the scarcity of coastal wetlands in future.