The global mean sea-level rise scenarios (Table 6.3) are based on thermal expansion and ice melt; the best estimate shows an acceleration of up to 2.4 times compared to the 20th century. These projections are smaller than those of Church et al. (2001), reflecting improved understanding, especially of estimates of ocean heat uptake. If recently observed increases in ice discharge rates from the Greenland and Antarctic ice sheets were to increase linearly with global mean temperature change, this would add a 0.05 to 0.11 m rise for the A1FI scenario over the 21st century (Meehl et al., 2007). (Large and long-term sea-level rise beyond 2100 is considered in Box 6.6.)

Importantly, local (or relative) changes in sea level depart from the global mean trend due to regional variations in oceanic level change and geological uplift/subsidence; it is relative sea-level change that drives impacts and is of concern to coastal managers (Nicholls and Klein, 2005; Harvey, 2006a). Meehl et al. (2007) found that regional sea-level change will depart significantly from the global mean trends in Table 6.3: for the A1B scenario the spatial standard deviation by the 2080s is 0.08 m, with a larger rise than average in the Arctic. While there is currently insufficient understanding to develop detailed scenarios, Hulme et al. (2002) suggested that impact analysis should explore additional sea-level rise scenarios of +50% the amount of global mean rise, plus uplift/subsidence, to assess the full range of possible change. Although this approach has been followed in the UK (Pearson et al., 2005; Thorne et al., 2006), its application elsewhere is limited to date.

Furthermore, coasts subsiding due to natural or human-induced causes will experience larger relative rises in sea level (Bird, 2000). In some locations, such as deltas and coastal cities, this effect can be significant (Dixon et al., 2006; Ericson et al., 2006).

Increases of extreme sea levels due to rises in mean sea level and/or changes in storm characteristics (Table 6.2) are of widespread concern (Box 6.2). Meehl et al. (2007) found that models suggest both tropical and extra-tropical storm intensity will increase. This implies additional coastal impacts than attributable to sea-level rise alone, especially for tropical and mid-latitude coastal systems. Increases in tropical cyclone intensity over the past three decades are consistent with the observed changes in SST (Emanuel, 2005; Webster et al., 2005). Changes in other storm characteristics are less certain and the number of tropical and extra-tropical storms might even reduce (Meehl et al., 2007). Similarly, future wave climate is uncertain, although extreme wave heights will likely increase with more intense storms (Meehl et al., 2007). Changes in runoff driven by changes to the hydrological cycle appear likely, but the uncertainties are large. Milly et al. (2005) showed increased discharges to coastal waters in the Arctic, in northern Argentina and southern Brazil, parts of the Indian sub-continent, China and Australia, while reduced discharges to coastal waters are suggested in southern Argentina and Chile, Western and Southern Africa, and in the Mediterranean Basin. The additional effects of catchment management also need to be considered (Table 6.1).

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