We owe much of our prosperity to oceans. As UK readers know best, easy access to the sea allows economies to trade and to grow. Shanghai, New York, Hong Kong, London, Sydney, Mumbai, the Hansa, Tokyo, San Francisco and Cape Town demonstrate the economic advantages of a good harbour. Conversely, landlocked countries (such as Kyrgyzstan and the Central African Republic) are poor and falling behind. Cities with deep water ports are the most fertile soil on which we can cultivate businesses, the incubators of innovation and economic growth.

We have long been told that these cities, our strongest economic and political units, are at risk from rising sea levels. Until fairly recently, hydrogeologists and climate scientists have forecast increases in sea levels by one metre by 2050. Such estimates have worried leaders in the Maldives, but the rest of us have largely ignored sea-level changes as an important economic issue in Europe or North America. The Dutch can build bigger dams, and the Thames can be curtained off. Bloomberg has been busy redesigning the coast of New York City since Hurricane Sandy. We have assumed we could adapt.

So it is with some surprise that we read recently that these estimates have been too conservative, and that scores of cities are at far greater risk than we may have believed.

Ben Strauss from Climate Central has published a paper (PDF) in which he looks at rising sea-level predictions and matches them to the elevation of US cities. His paper is unique in that he has combined city-level data (population and elevation) with research by Anders Levermann, a professor at the Potsdam Institute for Climate Impact Research, on the rate at which sea levels are rising (2.3 metres per degree Celsius).

He predicts that unless we significantly change our projected carbon emissions, 315 US cities are under threat. "By the end of this century, if global climate emissions continue to increase, that may lock in 23 feet of sea-level rise."

The critical term here is, of course, "lock in". Forecast levels of CO2 emissions will generate enough warming to raise the oceans seven metres. The greenhouse gases already emitted will eventually result in a rise of 1.3 metres.

Strauss' analysis only looks at the likelihood coastal cities will be under water. Strauss forecasts the impact of rising seas without storms. He doesn't forecast the likelihood that calamitous weather events like Hurricane Sandy will cause far greater damage when the oceans have risen closer to the level of the cities, overwhelming roads, sewers, underground trains, and water systems. We can now imagine a day when storms do not merely damage coastal cities but destroy them.

He contends that by 2100, more than 25% of Boston, Miami, New Orleans, and Atlantic City could be under water. The same forecast (23 ft or 7m by 2100) can be plugged into a global map of elevations and sea levels here. Such a calculation is even more alarming. Most of the globe's economic activity is funneled through cities that will be fighting to stay above water. Imagine a world without Shanghai, Mumbai and Boston, a world in which London and New York are risky settings for markets.

A very thorough set of estimates of the impact of rising sea levels on the economies of coastal cities, Future flood losses in major coastal cities, has appeared as an advance online publication in Nature Climate Change. This study uses 20cm and 40cm increases to generate scenarios for anticipated flood losses in large coastal cities. Here's the central message:

Average global flood losses in 2005 are estimated to be approximately US$6 billion per year [in 136 coastal cities], increasing to US$52 billion by 2050 with projected socio-economic change alone. With climate change and subsidence, present protection will need to be upgraded to avoid unacceptable losses of US$1 trillion or more per year.

The enormously cautious IPCC has forecast of a best case of 20cm and a worst case of 100cm of sea level rise by 2100. If alarmists are estimating seven metres of sea level rise, and the most cautious scientists are estimating 20-100 cm, there is a considerable likelihood that the US$1 trillion estimate of average annual losses due to flooding in coastal cities is not too high.

Given the enormous populations migrating to coastal cities, their oversize economic impact, and their anchoring of global prosperity in the businesses, markets and universities, the eventual drowning of coastal cities is an economic, social and even moral issue of the highest importance. We need the business acumen of cities like Mumbai, Guangzhou, Miami and Ho Chi Minh City to generate the innovations and the capital necessary to resolve the world's critical problems. We cannot afford to lose their economies and societies.

Last year, we saw Wall Street under water. Climate change risks turning the globe's centres of economic growth into Atlantis. If we want prosperity, we must save our coastal cities.

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