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Floods May Cost Coastal Cities $60 Billion a Year by 2050

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A little bit of sea level rise could go a long way to increasing flood risk for the globe’s coastal cities. New research finds it’s possible to reduce that risk, but not make it disappear completely. It also finds that cities not typically thought of as hot spots for sea level rise and flooding will actually be some of the biggest losers in a soggier future.

Global sea levels have risen 8 inches over the past century. Some areas have experienced even greater amounts due to land subsiding due to natural and human causes. At the same time, populations along coasts have experienced major growth. In the U.S., an estimated 87 million people live along the coast, up from 47 million in 1960. Globally, six of the world’s 10 largest cities are on the coast.

Flooded streets in Ho Chi Minh, Vietnam. 
Credit: Rock Portrait Photography/Flickr

The confluence of these factors has caused major economic losses from flooding. Hurricane Sandy caused up to $70 billion in damages while Katrina caused $125 billion in losses, much of it due to rising water.

With these environmental and social trends set to continue and possibly accelerate over the next 40 years, losses due to flooding represents a key challenge for coastal cities to deal with. A Nature Climate Change study published Sunday found that costs could rise dramatically if nothing is done to account for rising seas and subsiding land. Assuming sea levels rise 15.75 inches above today’s heights by 2050 and subsidence from human development continues, the study found that losses from flooding in 136 of the world's coastal port cities could near $1 trillion annually by 2050.

The estimate of 15.75 inches is within the range of probable sea level rise scenarios according to a 2012 National Oceanic and Atmospheric Administration report.

However, if $1 trillion in annual losses sounds implausible, that’s because it basically is. “To get to $1 trillion, we have to basically have places which are destroyed every two years and are rebuilt again and again,” Stéphane Hallegatte, a senior economist at the World Bank and lead author of the study, said. “It’s not a realistic scenario; it just shows it’s impossible to have a scenario that we do nothing.”

Implementing an arsenal of adaptation measures could cut those costs to between $60 billion and $63 billion annually by 2050. In comparison, the study estimated annual average losses from coastal flooding to be $6 billion in 2005.

Of the projected losses in 2050, $52 billion is due to economic and population growth. The other $8 to $11 billion is due to sea level rise and land use change. The reason for the range is the study also factored in a slightly more optimistic sea level rise scenario of only 7.9 inches by 2050.

Adaptation would come at a financial cost. Hallegatte puts a price tag of $50 billion annually on helping the coastal port cities included in the study to maintain a “relative risk level.” That level assumes that as city economies grow, the ratio of flooding losses to gross domestic product stays constant with 2005 estimates.

Rotterdam's Maeslantkering, a barrier that can open and close to protect the city's harbor from storm surges. 
Credit: Lymantria/Wikimedia Commons

On the ground, that means cities would need to invest in a number of protective measures. Levees offer the main form of defense. However, they also lock in rainwater meaning pumping infrastructure would be needed to make sure cities don’t flood from the inside out. Movable storm barriers that help ensure harbors remain operational similar to Rotterdam’s Maeslantkering and flood monitoring and early warning systems would also on the shopping list of cities looking to adapt to rising seas.

All these efforts don’t completely mitigate the risk, though. “You can make floods rarer and rarer,” Hallegatte said, but even the best adaptation measures can’t ensure they’ll never occur again. This creates a paradox where floods are less likely but when they do occur, they wreak more havoc. That paradox poses a particularly big challenge for countries such as Haiti, which have coastal cities that are big players in a relatively small economy.

The costs aren’t the only factor to shift by 2050. Miami, New Orleans, and other cities typically thought of as at-risk to sea level rise will experience increased losses. However, the cities with the biggest percentage increase in losses are cities in regions rarely talked about when it comes to sea level rise.

Hallegatte points to the Mediterranean basin as one of those regions. “It’s a region with very little tide so the water level right now is very stable, which means all protection is is designed for a very precisely known water level,” he said. “If it changes a little, you’re completely out of the range you’ve planned for.”

Even in a more optimistic scenario that assumes only 7.9 inches of sea level rise by 2050, the study finds that cities such as Beirut, Istanbul, Athens, and Tel Aviv will see their average annual losses increase by more than 50 percent compared to 2005 even with appropriate adaptation measures. Another city in the basin, Alexandria, Egypt, will see losses increase by 154 percent, the highest in the world. Using the 15.75 inch sea level rise scenario, the increase is 192 percent. Cities in the Gulf of Mexico and East Asia also face similar concerns.

Planning for future flooding is certainly the domain of city planners and local government, but the issue ultimately extends beyond the coasts. “It’s also a challenge for insurers who will have to insure floods that are larger than today. It’s a challenge for countries because we’re talking about big cities and their hubs. All the countries rely on the activity in these cities. It’s really a national issue.”

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By Lewis Cleverdon
on August 18th, 2013

“Assuming sea levels rise 15.75 inches above today’s heights by 2050 and subsidence from human development continues, the study found that losses from flooding in 136 of the world’s coastal port cities could near $1 trillion annually by 2050.”

This leaves some central factors undeclared, and thus doesn’t give the reader any means of assessing its plausibility. For instance, what rate of warming is occurring, either under BAU or under the accelerated warming of global Emissions Control and the resulting loss of the fossil sulphate parasol, or under Emissions Control plus the Albedo Restoration that is indispensable for halting the global destabilization of agriculture that is now on track for intensifying global crop failures starting by the 2020s ? Presumably one of these three is operative, but which and to what resulting warming is unknown.

There is also a need to describe what assumptions are made regarding the increase and intensification of storms both hitting the coasts and deluging the land under whatever degree of warming is assumed, as well as describing the assumed degree of disruption of the NH Jetstream contributing to those changes, which in turn reflects an assumed degree of annual Arctic sea-ice loss.

Without describing these factors, the study appears to lack credible foundations. As the link goes to a different article’s abstract, and even the right abstract is not likely to define these missing assumptions, if they were described in the text but not reported here, perhaps it would be worth adding an update ?



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By Brian Kahn
on August 19th, 2013

Hi Lewis,

My understanding from the study/speaking with the lead author is that adjusting for changes in storm landfall, intensity, and subsequent flooding in each of the 136 cities is basically an impossible calculation, hence the lack of a mention. The study instead gives a general senses of the losses that would occur based on the variables of SLR and subsidence, which researchers have a much better handle on. Thank you for pointing out the busted link (fixed now if you want to take a look).


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By Lewis Cleverdon
on August 20th, 2013

Brian - thanks for your response.

I would of course agree that predicting impacts for individual port cities is impracticable, but given that the report focuses on the aggregates of potential damage costs and adapted damage costs, this explanation by Dr Hallegatte seems less than satisfactory. As a senior economist at the World Bank he would need only to request the relevant data from Munich Re, with its 40yr database of intensifying impacts worldwide, to have chapter and verse on the relevant rates of change.

Aggregate measures of their data are freely available on the web, and show a rising trend in the number of catastrophic meteorological, hydrological, and climatological events of ~3.75% /yr from 1980 to 2011. If that trend were projected neither to increase nor decline out to 2050 (the default option) it would indicate a rise of such catastrophes hitting port cities by 2050 of around 290% over the present frequency - almost a 4-fold increase. No doubt Munich Re could have provided more detailed information specifically on coastal storm and flooding events’ trends, as well as the related trend of damage costs, which I understand are rising globally at over 6% /yr overall.

The upshot is that this report evidently underestimates the coming damages that the lack of a commensurate climate treaty would impose by a factor of somewhere between 3 to 9 fold, and thus greatly overestimates the feasibility of adaptions to withstand their impacts, and yet is being presented here and around the world’s media, and in government and UN Agency offices, as a reliable scientific estimate of the threat faced by the port cities crucial to a global trading society.  This problematic optimism bias is not limited to economists at the World Bank but is also prevalent among scientists, with Lord Stern’s scathing critique of the IPCC’s ongoing failure to put a value on Permafrost GHG outputs being a major case in point. His view (even as an economist) was that when data is uncertain, it is entirely improper to insert a zero value.

The fact that the climate treaty’s negotiation by the UNFCCC must by mandate base its deliberations on the IPCC’s advice, is one measure of just how damaging the scientific community’s practice of tolerating optimism bias is to our prospects of commensurate mitigation. In this light it would seem very helpful if Climate Central, rather than simply reporting the latest scientific papers, were to actively critique those clearly understating the threat due to a discernible optimism bias.



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By Dave (Basking Ridge, NJ 07920)
on August 21st, 2013

Lewis, I also appreciate your comment here. It is an important point. I think the “optimism bias” you refer to and identify so clearly in this specific case is active more broadly in all sorts of change topics where there is inertia between fact and acceptance. With respect to climate change, this is so clearly often the case and psychological and political causes and conflicts also cannot be ignored at various levels. It also does not ‘help’ that scientists are careful in the manner a professional conclusion is expressed. That style is also sometimes misinterpreted by others in too much of a hurry. This can also lead to erroneous popular and cited versions of those conclusions.

I would like to see Climate Central take an initiative to place greater emphasis on additional contextual evaluation versus the current balance towards more mainly serial plain reporting on this latest data, this latest paper and so on. I have no doubt though that that may be easier said than done!

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