Low-lying islands

Storm surges

If the oceans remained calm and tranquil, sea level rise, being generally in the range of 2 to 3 mm per year, would not pose much of an immediate threat to small islands. There would be time to take the necessary measures to protect coastal communities and infrastructure. However, the oceans are in constant motion, and when powerful storms drive waves to heights of several meters above normal levels, they can cause huge amounts of damage.

Storm surge is an abnormal rise of water generated by a storm, over and above the normal height of the regular tides. The term ‘storm tide’ is defined as the sea water level rise due to the combination of storm surge and normal tide, as depicted in the figure below.

This rise in sea water level can cause extreme flooding in coastal areas. Storm surges can easily reach 8 meters.

Reports on tropical cyclones rarely mention the height of the storm surge, but the extensive coastal and inland flooding caused by many of these cyclones is testimony to the high storm surges often generated by these extreme events.

 

Hurricane Matthew, which pummeled Jamaica, Haiti, Cuba and the Bahamas in October 2016 generated a storm surge of between 3 to 5 meters approaching the Bahamas [1].

In January 2016, a hurricane named Alex formed in the northern Atlantic—an unusual event for that time of the year. Although not an especially fierce storm, it reportedly produced a storm surge of 18 meters. A red alert was issued for five of the Azores’ nine islands. It was noted at the time that sea water temperatures were about 2°C higher than normal.

Sea level islands

Recent advances in satellite imagery have enabled scientists to more accurately estimate the number of people, and the extent of the coastal area, at different elevations above mean sea level. People that live only a few meters above mean sea level in areas of the world where cyclones and hurricanes are frequent obviously face an extremely high risk of being hurt or killed by storm surges associated with these extreme events.

The small island states where populations are most at risk are shown in the table below. The population threshold in this table is cut off at 50,000 to highlight the islands where the greatest number of people are most at risk [2]

Coastal populations (thousands) at different elevations above mean sea level in meters

These are huge numbers. Singapore has more than a third of a million people living within one meter of mean sea level.

Due to the diminishing Coriolis force towards the equator, the equatorial zone is generally cyclone-free. However, on 27th December 2001, tropical cyclone Vamei formed near latitude 1.4 °N, and made landfall about 60 km northeast of Singapore. Although this event did not cause any major destruction to Singapore, the neighboring states of Johor and Pahang in Malaysia suffered inland flooding and landslides Low Elevations Coastal lZones

caused by the heavy precipitation of the storm.

Vamei was unusual in that it formed so close to the equator. But climate change is affecting weather patterns worldwide, and clearly Singapore is at extreme risk if tropical cyclones at low latitudes becomes more frequent. Even without the advent of tropical storms, Singapore is vulnerable: sea level rise alone will eventually inundate coastal areas where hundreds of thousands of people live.

Guinea Bissau is not in a region where tropical cyclones occur. However, all the other countries on the list above are frequently impacted by hurricanes in the Caribbean or typhoons in the Pacific. Cuba and Puerto Rico look particularly exposed. Both countries have almost a third of a million people living in low-lying area less than 5 meters above mean sea level.

Low Elevation Coastal Zones

In terms of the percentage of populations at risk, the picture is different—since the islands with smaller populations do not show up in the table above. The table below shows islands where more than 20 % of the population is within the indicated LECZ.

Percentage of small island coastal populations close to sea level

What is striking about this data is the large percentage of the populations within 5 meters of mean sea level—and therefore well within the reach of storm surges associated with hurricanes and typhoons. In Tuvalu and the Maldives, almost half the population is living less than 5 meters above mean sea level. In Suriname, a relatively large country, over the half the population is living on the coast. If an island is large enough, people can be quickly moved inland and out of harm’s way, but infrastructure cannot be moved so easily.

If the data are arranged to show the area of land that is likely to be flooded by storm tides, the picture looks a little different. A significant fraction of these low-elevation coastal zones is likely to be urban areas which, if flooded, will result in substantial loss of economically essential and productive infrastructure, and well as the probable shutdown of local government services and communication systems.

All land on islands within 3 meters of present mean sea levels is highly vulnerable to catastrophic flooding from storm surges. Six island states: the Bahamas, Cabo Verde, Kiribati, Maldives, Marshall Islands and Tuvalu, have more than 20% of their land area exposed to this threat. Kiribati once again looks hugely exposed.

It is certain that the majority of tourism infrastructure is constructed on these coastal lands and, if flooded, the economic impact of this loss of infrastructure will be catastrophic. For the Bahamas and the Maldives, tourism receipts as a percentage of GDP are estimated at 28% and 77% respectively.

The exposure to sea level rise and extreme weather of the Maldives is well captured by the photograph of the capital Malé on North Malé atoll in the Maldives.

The island of Male, capital of the Maldives

The value of the coastal infrastructure at risk is of course substantial, as many islands have invested considerable amounts of money in beach hotels and associated infrastructure to support tourism.

A study in 2015 of 12 Pacific Island States found that 57% of the infrastructure identified in the study was within 500 meters of the coast. [3]

The total replacement value of all the infrastructure was estimated at $21.9 billion.

 

Kiribati, Marshall Islands and Tuvalu have 95%, 98%, and 99% respectively of their built infrastructure by value within 500 m of the ocean, indicating that almost all infrastructure by value is located along the coast.  For Vanuatu, although 52% of built infrastructure is beyond the 500-m band, this represents only 10% by value, indicating that the most valuable infrastructure, which includes ports and in some cases oil refineries, is on the coast.

Although sea evel rise and storm surges are major threats, ocean swell caused by weather events thousands of kilometres away can cause substantial damange to coastal communities in the tropics. In 1987, long-period swells originating in the Southern Ocean 6000 km from the Maldives caused flooding, property damage, destruction of sea defense structures, and serious coastal erosion [4] .

In 2008, swells generated in the North Pacific Ocean overwashed several lowlying islands in the Pacific causing severe damage to housing and infrastructure that affected about 100,000 people across the region. [5]

 

For more information check out the following :

[1] See the National Hurricane Centre reports at  https://www.nhc.noaa.gov/outreach/history/

[2] Population data on coastal zones are from Coumbia University’s Center for International Earth Science Information Network (CIESIN).  See:   http://sedac.ciesin.columbia.edu/data/sets/browse

[3] See : Billions of dollars of Pacific infrastructue at risk from climate, at http://www.abc.net.au/science/articles/2015/07/07/4266340.htm

[4] IPCC 5th Assessment report Chapter 29 Small islands.Avaliable at:  http://www.ipcc.ch/pdf/assessment-report/ar5/wg2/WGIIAR5-Chap29_FINAL.pdf

[5] See the articles : Widespread inundation of Pacific islands triggered by distant-source wind-waves, at  https://www.sciencedirect.com/science/article/pii/S0921818113001483