Future flood losses in the world’s largest coastal cities

 

Over the next few decades climate change induced sea level rise and subsidence due to ground water pumping is expected to affect a greater proportion of people living in low lying  regions. Coupled with economic growth and increasing populations in coastal cities this trend will result in higher annual losses from flooding.

A recent Nature Climate Change article estimates that the average annual losses from flooding in the world’s largest coastal cities could rise from about $6 billion per year in 2005 to over $1 trillion per year by 2050. Even if investments are made to maintain flood probabilities at current levels, subsidence and sea level rise alone will increase annual losses to around $63 billion by 2050.

The top 20 coastal cities with the most Average Annual Loss increase in 2050. Hallegatte et al. 2013

The above figure, from the article, shows the 20 cities where Average Annual Losses (AAL) increase the most (in relative terms in 2050 compared with 2005) if adaptive measures are taken to only maintain present defence standards or flood probability. Note that this assumes a relatively optimistic sea level rise of about 20cm. Many coastal cities are expected to have much higher sea levels of ~60cm by 2050.

Another way to look at flood risks is to rank them in order of how much much damage is expected from a 100 year repeat flooding event with respect their annual GDP. The table below lists the top 10 cities ranked by 2005 losses as a percentage of its GDP.

	City	Country	AAL (% of GDP)
1	Guangzhou	China	1.32
2	New Orleans	USA	1.21
3	Guayaquil	Ecuador	0.95
4	Ho Chi Minh City	Vietnam	0.74
5	Abidjan	Ivory Coast	0.72
6	Zhanjiang	China	0.50
7	Mumbai	India	0.47
8	Khulna	Bangladesh	0.43
9	Palembang	Indonesia	0.39
10	Shenzen	China	0.38

China currently occupies three of the top 10 positions in this table. Which is why the Chinese government is actively investing in flood defence technologies and climate change research. Many of the countries where the annual losses are a significant proportion of the annual GDP are developing nations. These countries will continue to suffer with greater losses in future years unless something drastic is done to improve their flood defence systems.

It is worth noting that Hallegatte et al. (2013)  do not consider the other issues a rising sea level will have on coastal populations. Higher sea levels coupled with rising temperatures will result in a greater frequency and/or magnitude of coastal storms and cyclones. Historically these single events have been the cause of the greatest loss of lives and damage to infrastructure.

It is clear that flood risk and exposure needs to be reduced to below current levels if we are to avoid large financial and human losses in the future.

CGS academic Professor Nigel Wright explores the links between flood vulnerability and climate change and is currently helping to develop a Coastal City Flood Vulnerability Index (CCFVI) based on exposure, susceptibility and resilience to coastal flooding.

Read more about his recent work here: http://link.springer.com/article/10.1007/s11069-012-0234-1

Ekbal