6.4.2.3 Human settlements, infrastructure and migration

Climate change and sea-level rise affect coastal settlements and infrastructure in several ways (Table 6.4). Sea-level rise raises extreme water levels with possible increases in storm intensity portending additional climate impacts on many coastal areas (Box 6.2), while saltwater intrusion may threaten water supplies. The degradation of natural coastal systems due to climate change, such as wetlands, beaches and barrier islands (Section 6.4.1.1), removes the natural defences of coastal communities against extreme water levels during storms (Box 6.5). Rapid population growth, urban sprawl, growing demand for waterfront properties, and coastal resort development have additional deleterious effects on protective coastal ecosystems.

Much of the coast of many European and East Asian countries have defences against flooding and erosion, e.g., the Netherlands (Jonkman et al., 2005) and Japan (Chapter 10, Section 10.5.3), reflecting a strong tradition of coastal defence. In particular, many coastal cities are heavily dependent upon artificial coastal defences, e.g., Tokyo, Shanghai, Hamburg, Rotterdam and London. These urban systems are vulnerable to low-probability extreme events above defence standards and to systemic failures (domino effects), e.g., the ports, roads and railways along the US Gulf and Atlantic coasts are especially vulnerable to coastal flooding (see Chapter 14, Section 14.2.6). Where these cities are subsiding, there are additional risks of extreme water levels overtopping flood defences, e.g., New Orleans during Hurricane Katrina (Box 6.4). Climate change and sea-level rise will exacerbate flood risk. Hence, many coastal cities require upgraded design criteria for flood embankments and barrages (e.g., the Thames barrier in London, the Delta works in the Netherlands, Shanghai’s defences, and planned protection for Venice) (Fletcher and Spencer, 2005) (see Box 6.2 and Section 6.6).

There is now a better understanding of flooding as a natural hazard, and how climate change and other factors are likely to influence coastal flooding in the future (Hunt, 2002). However, the prediction of precise locations for increased flood risk resulting from climate change is difficult, as flood risk dynamics have multiple social, technical and environmental drivers (Few et al., 2004b). The population exposed to flooding by storm surges will increase over the 21st century (Table 6.5). Asia dominates the global exposure with its large coastal population: Bangladesh, China, Japan, Vietnam and Thailand having serious coastal flooding problems (see Section 6.6.2; Chapter 10, Section 10.4.3.1; Mimura, 2001). Africa is also likely to see a substantially increased exposure, with East Africa (e.g., Mozambique) having particular problems due to the combination of tropical storm landfalls and large projected population growth in addition to sea-level rise (Nicholls, 2006). Table 6.6 shows estimates of coastal flooding due to storm surge, taking into account one adaptation assumption. Asia and Africa experience the largest impacts: without sea-level rise, coastal flooding is projected to diminish as a problem under the SRES scenarios while, with sea-level rise, the coastal flood problem is growing by the 2080s, most especially under the A2 scenario. Increased storm intensity would exacerbate these impacts, as would larger rises in sea level, including due to human-induced subsidence (Nicholls, 2004). Figure 6.8 shows the numbers of people flooded in the 2080s as a function of sea-level rise, and variable assumptions on adaptation. Flood impacts vary with sea-level rise scenario, socio-economic situation and adaptation assumptions. Assuming that there will be no defence upgrade has a dramatic impact on the result, with more than 100 million people flooded per year above a 40 cm rise for all SRES scenarios. Upgraded defences reduce the impacts substantially: the greater the upgrade the lower the impacts. This stresses the importance of understanding the effectiveness and timing of adaptation (Section 6.6).

Figure 6.8. Estimates of people flooded in coastal areas due to sea-level rise, SRES socio-economic scenario and protection response in the 2080s (following Nicholls and Lowe, 2006; Nicholls and Tol, 2006)