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New Research on Possible Sea Level Increases by 2100

S Jevrejeva, A Grinsted and J C Moore have published new research in Environmental Research Letters (10 Oct. 2014) which estimates that there is a 95% chance that the level of global sea level increase by 2100 will not exceed 1.8 metres, although there remains a possibility (less than 5% probability) that sea levels may increase by 1.9 metres.

This is a higher end projection and is important for engineers and planners working on “worse case” scenarios, the actual increases will depend on a number of factors and hopefully will be less. However, as the authors point out rising sea levels will not stop in 2100, but will continue to rise long after, for hundreds of years – even if ALL CO2 emissions were stopped tomorrow, as we have already put enough heat into the global system to ensure that there will continue to be extensive melting of water held in ice caps and glaciers. What is important is that governments start the process of relocating their vulnerable urban centres to higher ground, because for some cities, like Miami, it will be difficult, if not impossible to protect existing centres with sea walls and other defensive barriers – the impact of Hurricane Katrina on New Orleans also showed the consequences of failure of existing sea defences.

It is also the case that even if the actual rise in sea levels is only half the maximum predicted (and the authors’ median (50%)  is 0.8 metres by 2100), it will still overwhelm existing sea defences, inundate coastal plains which are important sources of food (the UK is pulling back sea defences in certain low-lying areas, allowing the sea to reclaim the land, this is a pattern that we will see followed elsewhere as the waters rise). London is already planning to replace its existing Thames Barrier in order to cope with higher sea levels and increased tidal surges.

At present it is estimated that 150 million people live on land within 1 metre of high tide, and more than 600 million people live on coastal areas which are less than 10 metres above sea level. Hurricane Sandy caused most of its damage in New York by a tidal surge, which exposed the lack of effective sea defences in the City, and large areas of the Eastern seaboard of the United States are vulnerable.  Thirteen of the world’s fifteen largest cities are on coastal plains. Smaller cities, like Alexandria, in Egypt, are also extremely vulnerable, as is the Nile Delta area.  Bangladesh, Vietnam, India, and China have very large populations living in river deltas, and other low-lying land. The threat to the survival of small island states in the Pacific and Indian Oceans is also well-known, particularly Kiribati, the Seychelles, The Torres Strait Islands, The Solomon Islands, Palau, The Carteret Islands, Tuvalu, the Maldives and the Marshall Islands.

The ten most vulnerable cities are estimated to be: Mumbai, Guangzhou, Shanghai, Miami, Ho Chi Minh City, Calcutta (Kolkata), Greater New York, Osaka-Kobe, Alexandria and New Orleans.  But other large cities are also vulnerable, including Lagos (Nigeria), Jakarta, Chennai (Tamil Nadu, India), and Nagoya (Japan).  In Egypt, a 25 cm rise in sea level would overwhelm 60% of Alexandria’s population (affecting 2.4 million people), and over 56% of its industry.

The Nile Delta, Egypt

We have to assume that over the next ninety years we will see around a metre of sea level rise and that the actual levels could be nearly double that. Given that earlier predictions envisaged a rise of 2 to 3 metres by 2300, it is not too early to start the process of relocating coastal urban centres to higher ground, and removing housing and business from low-lying areas – which could be used as flood-plains, with public parks. If a programme of urban relocation were to be put in hand now the process would be straightforward for many cities, although in some instances, like Miami and New Orleans, it may be worth planning to abandon these settlements in the long-term, halting new conventional construction and allowing for the development of “water parks” and floating houses (the Dutch have developed this concept).

In some cases, notably for the small Pacific and Indian Ocean islands, it is clear that there can be no long term future, and arrangements need to be put in hand now for the relocation of vulnerable populations. There are some countries that will be particularly hard hit, Bangladesh and Egypt, for example, because so much of their agricultural land is near to existing sea levels. This will require international coordination and careful planning by their national authorities. It is to be hoped that such planning is well in hand.

Impact of 1 metre sea level rise on Florida

S Jevrejeva et al 2014 Environ. Res. Lett. 9 104008


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