11.2.4.2. Deltas, Estuarine, and Other Coastal Ecosystems
River-borne sediments have formed at least 10 deltas in the coastal zones of
Asia with an area of more than 10,000 km2 each (Coleman and Wright,
1975). Delta and estuarine ecosystems are sensitive to complex responses to
agents associated with climate change (Sanchez-Arcilla and Jimenez, 1997). Low-lying
deltas are especially vulnerable to sea-level rise and increasing shoreline
wave action (Walker, 1998). A decrease in river water discharge, as projected
under some climate change scenarios, could lead to hindrance of delta progradation
and increase the risk of irreversible change for the ecosystem in estuarine-deltaic
areas (Qian, et al., 1993; Shi, 1995). Tidal rivers and estuaries will become
more prone to saltwater intrusion as a result of projected sea-level rise (Huang,
et al., 1982; Li, 1984, 1985; Shi, 1995). Sea-level changes associated with
global warming would be exacerbated by tectonic submergence, ground subsidence
as a result of groundwater withdrawal, rise of water level created by delta
progradation, and eustatic sea-level rise.
Low-lying muddy coastlines associated with large deltas form a significant
resource and support large human populations. In China, for example, such low-lying
deltas cover about 4,000 km— 22% of the total coastline. These muddy coastal
ecosystems are basically distributed along large deltas and partly in semi-closed
bays (Ren, 1985). Erosion of muddy coastlines in Asia—as documented in
China, for example—is triggered largely by sediment starvation resulting
from human activities and delta evolution rather than sea-level rise (Ji et
al., 1993; Chen and Chen, 1998). Beach erosion is widespread in the coastal
zone of Asia and has been reported in China, Japan, Indonesia, Sri Lanka, Thailand,
Bangladesh, and Malaysia (Xia et al., 1993; Sato and Mimura 1997; Teh 1997;
Nishioka and Harasawa, 1998; Huq et al., 1999; Middleton, 1999).
Coastal wetlands frequently are associated with deltas, estuaries, lagoons,
and sheltered bays. Tidal flats of the muddy coast in Asia constitutes the main
part of the coastal wetland (Bird, 1992). Large-scale wetland reclamation in
the major deltas has taken place during the past few decades (Lang et al., 1998;
Liu et al., 1998). Lagoons, which are important wetlands, are located across
the coastal regions of India, Sri Lanka, Malaysia, Indonesia, China, and Russia.
The transitional area between uplift and subsidence belts has favored the formation
of lagoons along China's coastline (Li and Wang, 1991). These lagoons tend
to be decreasing in area as a result of silting of sediments and plant growing
(Zhu, 1991). Sea-level rise and reduction of river-borne sediments will decelerate
delta progradation and wetland renewal. The rich biodiversity of wetlands in
Asia is seriously threatened by loss of wetlands from sea-level rise (Nicholls
et al., 1999).
Mangroves are made up of salt-adapted evergreen trees; they are restricted
to the intertidal zone along the vast coastlines of tropical countries in Asia
and extend landward along tidal rivers. The Sundarbans in Bangladesh and adjacent
areas in India, covering about 6,000 km2, are the largest mangrove
forests in the world (Allison, 1998). Depletion of mangrove forests by anthropogenic
pressures has become a serious problem (Farnsworth and Ellison, 1997). Approxmately
half of mangrove forests in Thailand were reduced by 56% during 1961-1996.
In the Philippines, more than 75% of the mangrove forests have been lost in
less than 70 years. Destruction of Indonesia's estimated 44,000 km2
of mangroves has taken place mainly since 1975 (Middleton, 1999; UNEP, 1999).
Mangrove forests are highly vulnerable to climate change-induced sea-level rise
because it will change the salinity distribution and hence productivity. Large-scale
changes in species composition and zonation in mangrove forests also are expected
as a result of changes in sedimentation and organic accumulation, the nature
of the coastal profile, and species interaction (Aksornkaoe and Paphavasit,
1993).
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