Coastal flooding risks

2.2 Coastal flooding risks

Cities are highly concentrated in coastal zones, which puts a large portion of the urban population at risk from rising sea levels and intensifying storm surges. 11 Mean sea level has risen 10-20 centimetres in

the 20 th century, and while specific predictions are difficult, the IPCC expects sea levels to rise 30-50

10. Citing Hallegatte, 2006 and Hallegatte et al. 2007a

11. The US National Oceanic and Atmospheric Administration estimates that 53% of United States’ population live in coastal regions (Crosset et al. 2004).

centimetres by 2100 (IPCC, 2001a). 12 Peak sea levels, which are most relevant for coastal planning as they characterise storm surges, may be rising even faster. Rising sea levels are a critical issue for major cities,

particularly in developing countries (Table 2.1). Even in Europe, 70% of the largest cities have areas that are less than 10 metres above sea level (McGranahan et al., 2007). Projected sea level rise is also associated with significant loss of land in coastal regions. For example, a 0.3 metre sea level rise in the United States, which is on the low end of IPCC projections (0.2 to 0.6 meters), would erode approximately

15 to 30 metres of shoreline in New Jersey and 60 to 120 metres in California. 13 Adaptation measures to combat sea level rise will be necessary because of the lag time between warming and its effects on glaciers.

Even under scenarios where emissions are eliminated, sea level rise continues well after temperature has stabilised (Ruth & Gasper in OECD, 2008a).

Table 2.1. Cities are highly concentrated in coastal zones

Share of urban settlements whose land area intersects the Low Elevation Coastal Zone (LECZ), by urban settlement size, 2000 Region <100K (%) 100–500K 500K–1M (%) (%)

1–5M (%) 5M+ (%) Africa

9 23 39 50 40 Asia

12 24 37 45 70 Europe

17 22 37 41 58 Latin America

11 25 43 38 50 Australia and New Zealand

N/A North America

9 19 29 25 80 Small island states

N/A World

Source: McGranahan, G., D. Balk, and B. Anderson (2007), “The Rising Tide: Assessing the Risks of Climate Change and Human Settlements in Low Elevation Coastal Zones”, Environment and Urbanization, Vol. 19, International Institute for Environment and Development, London.

Recent OECD work demonstrates that a 50-cm sea level rise, combined with predicted socio- economic development patterns, could result by 2070 in a tripling of the population at risk of coastal flooding and a tenfold increase in the amount of assets exposed, or from 5% of 2008 GDP to 9% of 2070’s GDP (Figure 2.1). About two-thirds of the increase in population exposed to coastal flooding is due to the socio-economic factors that drive coastal settlement, while the remaining third is expected to result from climate change and land subsidence. Port cities most at risk for coastal flooding are located both in rapidly growing developing countries such as India and China (e.g. Kolkata, Shanghai, Guangzhou) and in wealthy countries such as the United States (e.g. Miami, New York City), the Netherlands (e.g. Rotterdam, Amsterdam) and Japan (e.g. Tokyo, Osaka) (Nicholls et al., 2008). Adaptation efforts can also incur costs in the form of negative side effects. For example, coastal infrastructure to protect the city against storm surge, such as sea walls, can damage local landscapes, ecosystems and beaches, which may result in a reduction in tourism. Fisheries industries may also suffer as infrastructure to reduce coastal flooding can damage coastal ecosystems, on which 90% of fish species depend during at least one stage of their life cycle (Hallegatte et al., 2008).

12. This is the variation between emission scenarios; the range including variation between climate models is

9 to 88 centimetres (IPCC, 2001c).

13. Ruth & Rong (2006) cited in Ruth & Gasper in OECD (2008a).

Figure 2.1. Top 20 port cities’ exposed assets (a) and exposed population (b)

Future scenario with socio-economic development, subsidence and climate change

b.

Note: Note the different scales in the key. Source: Nicholls, R. et al. (2008), “Ranking Port Cities with High Exposure and Vulnerability to Climate Extremes”, OECD

Environment Working Papers No. 1, OECD Publishing, Paris.