10.4.3.2 Deltas, estuaries, wetland and other coastal ecosystems

Future evolution of the major deltas in monsoonal Asia depends on changes in ocean processes and river sediment flux. Coastal erosion of the major deltas will be caused by sea-level rise, intensifying extreme events (e.g., storm surge) due to climate change and excessive pumping of groundwater for irrigation and reservoir construction upstream. In the Tibetan Plateau and adjoining region, sediment starvation is generally the main cause of shrinking of deltas. Annual mean sediment discharge in the Huanghe delta during the 1990s was only 34% of that observed during the 1950s and 1970s. The Changjiang sediment discharge will also be reduced by 50% on average after construction of the Three-Gorges Dam (Li et al., 2004b). Saltwater intrusion in estuaries due to decreasing river runoff can be pushed 10 to 20 km further inland by the rising sea level (Shen et al., 2003; Yin et al., 2003; Thanh et al., 2004).

Many megacities in Asia are located on deltas formed during sea-level change in the Holocene period (Hara et al., 2005). These Asian megacities with large populations and intensified socio-economic activities are subject to threats of climate change, sea-level rise and extreme climate event. For a 1 m rise in sea level, half a million square hectares of Red River delta and from 15,000 to 20,000 km² of Mekong River delta is projected to be flooded. In addition, 2,500 km² of mangrove will be completely lost, while approximately 1,000 km² of cultivated farm land and sea product culturing area will become salt marshes (Tran et al., 2005).

Rise in water temperatures and eutrophication in the Zhujiang and Changjiang estuaries have led to the formation of the bottom oxygen-deficient horizon and an increase in the frequency and intensity of red tides (Hu et al., 2001). Projected increases in the frequency and intensity of extreme weather events will exert adverse impacts on aquatic ecosystems, and existing habitats will be redistributed, affecting estuarine flora distribution (Short and Neckles, 1999; Simas et al., 2001; Lu, 2003; Paerl et al., 2003).

Recent risk analysis of coral reefs suggests that between 24% and 30% of the reefs in Asia are projected to be lost during the next 2 to 10 years and 10 to 30 years, respectively (14% and 18% for global), unless the stresses are removed and relatively large areas are protected (Table 10.6). In other words, the loss of reefs in Asia may be as high as 88% (59% for global) in the next 30 years under IS92a emission scenario (IPCC, 1992; Sheppard, 2003; Wilkinson, 2004). If conservation measures receive increasing attention, large areas of the reefs could recover from the direct and indirect damage within the next 10 years. However, if abnormally high sea-surface temperatures (SST) continue to cause major bleaching events (see Chapter 6, Section 6.2.5, Box 6.1), and reduce the capacity of reefs to calcify due to CO₂ increase, most human efforts will be futile (Kleypas et al., 1999; Wilkinson, 2002).

A new study suggests that coral reefs, which have been severely affected by abnormally high SST in recent years, contain some coral species and their reef-associated micro-algal symbionts that show far greater tolerance to higher SST than others. Bleaching thresholds may be more realistically visualised as a broad spectrum of responses, rather than a single bleaching threshold for all coral species (Hughes et al., 2003; Baker et al., 2004). This corals’ adaptive response to climate change may protect devastated reefs from extinction or significantly prolong the extinction of surviving corals beyond previous assumption.

Net growth rates of coral reef, which can reach up to 8 to 10 mm/year, may exceed the projected rates of future sea-level rise in the South China Sea, so that coral reefs could not be at risk due merely to sea-level rise. Water depth increased by sea-level rise would lead to storminess and destruction of coral reefs (Knowlton, 2001; Wang, 2005).