Coastal Floods and Climate Change

In the previous post, we looked at the impacts of climate change on river floods. So in this post, we will turn our attention towards another category of floods – coastal floods. Across the globe, numerous coastal cities and towns are susceptible to such floods that create massive destruction and havoc each time they hit. For example, when Hurrican Sandy made landfall last year, large portions of Lower Manhattan was flooded with seawater as a record storm surge of 4.15m swept into New York City. A year after the floods, New York City is still trying to rebuild homes, buildings and transport systems that have been damaged, while many business and building owners are still battling to make insurance claims or gain compensation from the government. Meanwhile, other low-lying regions like the Maldives and Ganges Delta are under the threat of being inundated as sea levels continue to rise in the 21^st century.

Seaside Heights, New Jersey flooded after Hurricane Sandy made landfall. Source: AP

Although these coastal cities and towns are vulnerable to these devastating floods, they are ironically home to large concentrations of population. 13 out of 20 of the most populated cities in the world are coastal cities, including Shanghai, Guangzhou and Jakarta (Hanson et al. 2011). Moreover, many of these cities are also major national and regional economic hubs as they house key ports responsible for handling a large proportion of global seaborne trade. Therefore, the impacts of climate change on the risk coastal flooding have major social and economic consequences on these societies.

Sea level rise

One of the main causes of coastal inundation is the rise in sea levels. Sea level rise is caused by the combined effects of (1) the thermal expansion of seawater due to ocean warming; and (2) input of water from land ice melt and land water reservoirs.

From 1880 to 2009, it is estimated that sea level rise is about 210mm (Church and White 2011). Tide gauge measurements since the 19^th century has shown that sea level rose by an average of 1.7 ± 0.3mm/year since the 1950s. As seen from Fig.1, however, from the early 1990s onwards, the mean rate of sea level rise increased to 3.3 ± 0.4mm/year (Nicholls and Cazenave 2010). Evidently, sea level rise has accelerated in recent years. Moreover, it is critical to note that since the early 1990s, global average sea level rose at a rate near the upper end of the sea level projections for both the Intergovernmental Panel on Climate Change’s 3^rd and 4^th Assessment Reports (Church and White 2011).

Fig.1 Global mean sea level trend from the late 18th to early 21st century. The red curve is based on the tide gauge measurements while the black curve is based on the high-precision altimetry record from 1993 to 2009. As can be seen, there is a close match between tide gauge and altimetry records during that period suggesting that tide gauge records are reliable. Source: Nicholls and Cazenave (2010). 

However, it is important to note that sea level is not rising uniformly across the world as seen in Fig.2. Local factors can either amplify or increase the effects of eustatic sea level rise. These factors include the isostatic rebound of the Earth’s crustal due to the unloading of ice (as evident over Scotland), the subsidence of land due to the withdrawal of groundwater as well as the subsidence of deltaic regions due to the shortage of sediment supply caused by upstream damming. Nonetheless, based on Fig.1, it still seems that there is still a larger proportion of regions that have experienced sea level rise between 1992 and 2009, compared to regions that have experienced a decline in sea level for that period.

Fig.2 Regional sea level trends based on satellite altimetry records from 1992 to 2009. Source: Nicholls and Cazenave (2010).

Modelling studies carried out by Jevrejeva et al. (2012) have shown that sea level rise ranges from 0.57 to 1.10m by 2100, with the maximum rate of sea level rise reaching 17mm/year by 2100 under the RCP8.5 pathway (Fig.3). Moreover, rising sea levels will also elevate storm surges brought about by cyclonic activity, which is another cause of coastal floods (Brecht et al. 2012). Therefore, the impacts of climate change on the risk of coastal floods cannot be underestimated.

Fig.3 Sea level projections till 2100 with RCP scenarios: red - RCP3PD, blue - RCP4.5, green - RCP6.0 and black - RCP8.5. The shaded regions of similar color around the projections represent the upper (95%) and lower (5%) confidence levels. Source: Jevrejeva et al. 2012.  

Coastal flood risks

Two recent reports have attempted to quantify the losses associated with the exposure of people and assets to coastal flood risks under future sea level rise, socio-economic changes and subsidence of land. 

-----

Population exposure

Currently, the total number of people currently exposed to coastal flood risks is about 38.5 million, which is about 0.6% of the global population. Hanson et al. (2011) estimates that this figure will increase by more than threefold to around 150 million people for the period 2070-2080 due to population growth, sea level rise and subsidence of the coastal areas. Asia will be most vulnerable to coastal floods due to the high population density along the coast. The Asian cities that are within the top 20 cities with the largest population exposure to coastal flood risks in the period 2070-2080 include Mumbia, Guangzhou, Shanghai, Ho Ching Minh City, Bangkok and Tokyo (Fig.4). 

Fig.4 Map showing the top 20 cities for exposed populations under the FAC scenario for the period 2070-2080. The FAC scenario takes into account population and economic growth, natural subsidence/uplift, global sea level rise and potential human-induced subsidence. As can be seen, there is a disproportional number of Asian cities within the top 20 cities. Source: Hanson et al. 2011.  

Assets exposure

A recent report by Hallegatte et al. (2013) assessed the average annual flood losses in 136 coastal port cities and estimated that at present, the aggregated average annual flood losses (AAL) in these cities is about US$6 billion per year. Guangzhou tops the list with an AAL of US$687million, which is about 1.32% of the city’s GDP (Fig.5). In fact, most of the losses are concentrated within a small number of cities, with Guangzhou, Miami, New York and New Orleans accounting for 43% of the global losses.

Fig.5 The top 20 cities where the economic average annual losses with respect to local GDP are the largest in 2005. Source: Hallegatte et al. (2013).

The report suggested that future losses associated with coastal floods will increase dramatically under sea level rise, socio-economic changes and subsidence of land. They projected that if no adaptations were made to maintain or reduce the flood risks, the projected increase in average losses will rise to more than US$1 trillion per year. On the other hand, if adaptations were carried out, the losses will be lower, reaching between US$60 and 63 billion per year.

-----

Given that coastal flood risks will inevitably increase in future due to the combined effects of sea level rise, population growth in coastal areas as well other isostatic effects such as subsidence of land, coastal cities must attempt to adapt to the increase in coastal flood risks. Failure to adapt will result in massive losses both in terms of deaths and economic losses. Moreover these studies have not factored in other causes of coastal flooding including storm surges and tsunamis. Hence, future coastal risks might be even larger than what has been projected. However, Hallegatte et al. (2013) highlighted a grim reality that even though improvements in flood defence infrastructure can help to reduce the risk levels and reduce the number of floods, but when an extremely large flood overwhelms these infrastructure, the resultant losses will still increase. Therefore, there is a limit to these flood defence infrastructure and effort must be channelled to other forms of adaptations as well. This includes effective land use planning, early warning systems, flood evacuation plans and post-disaster relief response, which would help communities be more resilient towards coastal floods.