Potential Impacts of Sea Level Rise on Mangroves
Overview of Mangrove Ecosystem
Climatic factors such as temperature and moisture affect mangrove distribution. Mangroves are distributed latitudinally within the tropics and subtropics, reaching their maximum development between 25°N and 25°S. Temperature controls latitudinal distributions of mangrove; perennial mangroves generally cannot survive freezing temperatures. The richest mangrove communities occur in areas where the water temperature is greater than 24ºC in the warmest month. The most recent estimates suggest that mangroves presently occupy about 14,653,000 ha of tropical and subtropical coastline (McLeod & Salm, 2006) (Fig.1).
The cumulative effects of natural and anthropogenic pressures make mangrove wetlands one of the most threatened natural communities worldwide. Roughly 50% of the global area has been lost since 1900 and 35% of the global area has been lost in the past two decades, due primarily to human activities such as conversion for aquaculture. Mangroves are declining in area worldwide. The global average annual rate of mangrove loss is about 2.1%, exceeding the rate of loss of tropical rainforests (0.8%) (Gilman et al., 2006a).
Mangrove Ecosystem Values
The mangrove ecosystem provides income from the collection of the mollusks, crustaceans, and fish that live there. Mangroves are harvested for fuelwood, charcoal, timber, and wood chips. Services include the role of mangroves as nurseries for economically important fisheries, especially for shrimp. Mangroves also provide habitats for a large number of molluscs, crustaceans, birds, insects, monkeys, and reptiles. Other mangrove services include the filtering and trapping of pollutants and the stabilization of coastal land by trapping sediment and protection against storm damage. Also, mangroves provide recreational, tourism, educational, and research opportunities, such as boardwalks and boat tours, and are important for research and education.
Benefits as Measured by Market Prices
The annual economic values of mangroves, estimated by the cost of the products and services they provide, have been estimated to be USD 200,000 - 900,000 per hectare (Gilman et al., 2006a). However, the location and values of the beneficiaries can result in substantial variation in mangrove economic value. For instance, mangroves fronting a highly developed coastline or located near major tourist destinations may have a higher economic value than mangroves in less developed areas with little or no tourism sector development (Gilman et al., 2006a).
Potential Impacts of Sea Level Rise
Global and pacific Projections for Sea Level Rise
Global mean sea level is projected to rise by 0.09 to 0.88 m between 1990 and 2100 based on the Intergovernmental Panel on Climate Change’s full range of 35 climate projection scenarios. The projected short-term sea level rise from 1990 to 2100 is due primarily to thermal expansion of seawater and transfer of ice from glaciers and ice caps to water in the oceans, which both change the volume of water in the world oceans (Fig. 3).
The level of the sea at the shoreline is determined by many factors in the global environment that operate on a great range of time scales, from hours (tidal) to millions of years (ocean basin changes due to tectonics and sedimentation). On the time scale of decades to centuries, some of the largest influences on the average levels of the sea are linked to climate and climate change processes (Fig.4).
Mangrove Responses to Changing Sea Level
Sea-level rise is the greatest climate change challenge that mangrove ecosystems will face (McLeod & Salm, 2006). Mangroves can adapt to sea-level rise if it occurs slowly enough, if adequate expansion space exists, and if other environmental conditions are met .
There are three general scenarios for mangrove response to relative sea level rise, given a landscape-level scale and time period of decades or longer (Fig. 5).
No change in relative sea level: When sea-level is not changing relative to the mangrove surface, mangrove elevation; salinity; frequency, period, and depth of inundation; and other factors that determine if a mangrove community can persist at a location will remain relatively constant and the mangrove margins will remain in the same location (Fig. 5A) (Gilman et al., 2006a <ref="G"/>).
Relative sea level lowering: When sea level is dropping relative to the mangrove surface, this forces the mangrove seaward and landward boundaries to migrate seaward (Fig. 5B) and depending on the topography, the mangrove may also expand laterally; and
Relative sea level rising: If sea level is rising relative to the mangrove surface, the mangrove’s seaward and landward margins retreat landward, where unobstructed, as mangrove species zones migrate inland in order to maintain their preferred environmental conditions, such as period, frequency and depth of inundation and salinity (Fig. 5C). Depending on the ability of individual true mangrove species to colonize new habitat at a rate that keeps pace with the rate of relative sea level rise, the slope of adjacent land, and the presence of obstacles to landward migration of the landward boundary of the mangrove, such as seawalls and other shoreline protection structures, some sites will revert to a narrow mangrove fringe or experience extirpation of the mangrove community (Gilman et al., 2006a <ref="G"/>) (Fig. 5D). The sediment composition of the upland habitat where the mangrove is migrating may also influence the migration rate (Gilman et al., 2007).
Review of the stratigraphic record of mangrove ecosystems during sea-level changes of the Holocene shows that low islands such as Grand Cayman, Bermuda, and Tongatapu will be particularly vulnerable to the loss of mangrove ecosystems during the rises of relative sea-level projected for the next 50 years. Mangrove ecosystems in these locations could keep up with a sea-level rise of up to 8-9 cm/100 years, but at rates of over 12 cm/100 years could not persist. This is due to low rates of sediment accumulation, with limited sources from outside the mangrove zone, such as from rivers or soil erosion sources. Other factors contributing to mangrove persistence are the primary production rate of forests, shoreline erosion due to deeper and more turbulent water and the frequency and intensity of tropical storms .
On high islands such as Viti Levu and Lakeba, Fiji, and Kosrae, Caroline Islands sediment supply has been accelerated by anthropogenically enhanced rates of soil erosion that the dominant process affecting mangroves of high islands and continental coasts may be input of terrestrial sediment such that the effects of sea level rise are lessened. Because of the allochthonous component in these sediments, mangrove substrates are accreting at a faster rate than the peats of low limestone islands, up to 25 cm/100 years .
The nature of the problems produced by sea-level rise varies between and within regions due to a range of natural, socioeconomic, institutional and cultural factors. It is important to emphasize that there are no winners given sea-level rise, rather there are small losers and big losers. The Pacific Small Islands appear to be highly vulnerable to sea-level rise while Europe is less vulnerable than the other regions (Robert & Mimura, 1998).
To assess mangrove vulnerability to sea level rise and other climate change effects and to plan for adaptation, island countries and territories need to build their technical and institutional capacity to:
(1) Determine trends in relative mean sea level and trends in the frequency and elevations of extreme high water events, and incorporate this information into land-use planning processes.
(2) Measure trends in the change in mangrove surface elevation to determine how sea level is changing relative to the mangrove surface.
(3) Acquire and analyze historical remotely sensed imagery to observe historical trends in changes in position of mangrove margins.
(4) Produce topographic maps and maps of locations of development and roads for land parcels adjacent to and containing mangroves, and establish or augment GIS programs. The World Bank-funded Infrastructure Asset Management Project in progress in Samoa might serve as a suitable model.
(5) Develop standardized mangrove monitoring programs as part of a regional mangrove-monitoring network. Provide training opportunities for in-country personnel to manage the mangrove-monitoring program, coordinate with a regional hub, and conduct monitoring techniques. Monitoring methods would include periodic delineation of mangrove margins.
(6) Assess efficacy of mangrove management frameworks and provide assistance to manage coastal activities to prevent unsustainable effects on mangroves and other coastal habitats, in part, to increase resilience to climate change effects, and plan for any landward mangrove migration in response to relative sea level rise.
(7) Augment regional capacity to rehabilitate mangroves. Establishing a regional mangrove monitoring network may enable many of the identified capacity building priorities to be fulfilled, and should be one of the highest regional priorities. Participating countries and territories could share technical and financial resources to maximize monitoring and conservation benefits through economy of scale. Assessing the efficacy of management frameworks to avoid and minimize adverse affects on mangroves and other valuable coastal ecosystems and plan for any landward mangrove migration is also critical. Ensuring that management frameworks are capable of eliminating and minimizing stresses that degrade mangroves is necessary to provide for mangrove resilience to anticipated stresses from sea level and other climate change effects. And managers will need the institutional capacity to plan for site-specific mangrove response to climate change effects, such as instituting setbacks from mangroves for new development for appropriate sections of coastline. However, management frameworks will only be effective if local communities and management authorities recognize the value of mangrove conservation. It is therefore also a priority to continually develop and augment a mangrove conservation ethic.
The value of wetlands conservation is often underestimated, especially in less developed countries with high population growth and substantial development pressure, where short-term economic gains that result from activities that adversely affect wetlands are often preferred over the less-tangible long-term benefits that accrue from sustainably using wetlands.
Local communities and leaders must recognize the long-term benefits of mangrove conservation to reverse historical trends in loss of mangrove area, maximize mangrove resilience to climate change, and where sea level is projected to rise relative to mangrove surfaces, enable unobstructed natural landward migration wherever possible. Education and outreach programs are an investment to bring about changes in behavior and attitudes by better informing communities of the value of mangroves and other ecosystems. This increase in public knowledge of the importance of mangroves can provide the local community with information to make informed decisions about the use of their mangrove resources, and can result in grassroots support and increased political will for measures to conserve and sustainably manage mangroves (Gilman et al., 2006b ).
- ↑ 1,0 1,1 McLeod, E. and R.V. Salm. 2006. Managing Mangroves for Resilience to Climate Change. IUCN, Gland, Switzerland. 64p.
- ↑ 2,0 2,1 2,2 Gilman, E., Van Lavieren, H., Ellison, J., Jungblut, V., Wilson, L., Areki, F., Brighouse, G., Bungitak, J., Dus, E., Henry, M., Sauni, I. Jr., Kilman, M., Matthews, E., Teariki-Ruatu, N., Tukia, S. and K. Yuknavage. 2006a. Pacific Island Mangroves in a Changing Climate and Rising Sea. UNEP Regional Seas Reports and Studies No. 179. United Nations Environment Programme, Regional Seas Programme, Nairobi, KENYA.
- ↑ 3,0 3,1 3,2 Bibliography of sea-level change and mangroves. Compiled by Andrea Schwarzbach.
- ↑ Gilman, E. (ed.) 2006. Proceedings of the Symposium on Mangrove Responses to Relative Sea-Level Rise and Other Climate Change Effects, 13 July 2006. Catchments to Coast. The Society of Wetland Scientists 27th International Conference, 9-14 July 2006, Cairns Convention Centre, Cairns, Australia. Published by the Western Pacific Regional Fishery Management Council, Honolulu, USA.
- ↑ Gilman, E.H., Ellison, J., and Coleman, R. 2007. Assessment of Mangrove Response to Projected Relative Sea-Level Rise and Recent Historical Reconstruction of Shoreline Position. Environmental Monitoring and Assessment, Vol. 124 (1-3): 105-130
- ↑ Robert J. N. and N. Mimura. 1998. Regional issues raised by sea-level rise and their policy implications. Climate Research. Vol.11:5-18.
- ↑ Gilman, E.H., Ellison, J., Jungblut, V., Van Lavieren, H., Wilson, L., Areki, F., Brighouse, G., Bungitak, J., Dus, E., Henry, M., Kilman, M., Matthews, E., Sauni, I. Jr., Teariki-Ruatu, N., Tukia, S., and K. Yuknavage. 2006b. Adapting to Pacific Island mangrove responses to sea level rise and climate change. Climate Research, 32(3): 161-176.
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