Our Increasingly urbanized coasts and sea level

Currently, 10% of the world's population (approximately 600 million and growing) live on land that is within 10 meters in elevation from sea level. Some of the world's most populated regions lie within this elevation range. In many regions, migration to these population centers is occurring at a rapid pace. These low-lying cities include London, New York, Miami, and Tokyo, as well as Mumbai in India, and the rapidly growing megalopolis of Guangzhou in China, and other large cities we may rarely think about. Numerous smaller, little-known communities are equally precariously located, but come into sudden focus when a disaster hits. Tacloban in the Philippines, which was devastated by Super Typhoon Haiyan in 2013, is a good example.

We know that while populations have increased in cities close to coastlines, the sea level around the earth has also been rising at an increased pace. These two factors have contributed to an increased probability of inundation of coastal communities by storm surges, high tides, and other events. Measured global sea level rise has been well documented using worldwide tide gauge and satellite altimeter data. This shows that during the 20th century, global sea level rose at an increasing rate. Between 1880 and 2011, this rate was 0.07 inches (0.18 cm) per year. Between 1993 and 2011, the rate increased to 0.11 inches (0.28 cm) per year. This represents a 64% increase (US EPA).

×

Mean Sea Level Trend, The Battery, New York.
Credit: NOAA: Tides and Currents, SL Trends, The Battery, New York

These rates are global averages and, in fact, sea level change varies greatly from place to place around the globe. Above is data from the tide gauge at The Battery in southern Manhattan, NY, with mean sea level in meters plotted over time from 1850 to 2010 (and average rates in millimeters rise per year).

If subsidence (sinking of the land level) is factored in, a location's relative sea level rise can be much higher that the global average. This is illustrated by comparing the tide gauge data for The Battery with that of Grand Isle, LA. (below), where subsidence of Mississippi River Delta sediment exacerbates sea level rise rates considerably – a difference in sea level rise of 6.47 mm/year between these two locations!

×

Mean Sea Level Trend, Grand Isle, Louisiana.
Credit: NOAA: Tides and Currents, SL Trends, Grand Isle Louisiana

Projections into the future of sea level rise indicate that the global sea level will rise an additional 0.6 meters (about 2 ft.) by 2100. (Intergovernmental Panel on Climate Change - IPCC, but note that this may be an underestimate. See How Far Will Sea Levels Rise). This may not seem like a large amount, but communities that are currently at an elevation close to sea level will see (and are already seeing) increased flooding during that time period. There is a great deal of uncertainty in predictions of future sea level rise, so this also must be taken into account when considering the future of our coastal cities.

Sea Level Rise: The Uncertainty of Predicting Future Sea Levels

The following modules will explore in detail sea level changes in relation to coastal processes and hazards and their impacts on coastal communities. Here in Module 1, we will just touch briefly on the topic of sea level rise so that we can think globally about coastal cities at risk from the hazards presented by sea level rise. You will learn about some of the controversy surrounding the prediction of future sea levels. This topic has become politicized, but from a science point of view, the issue is the uncertainty involved in these types of predictions. The links we have provided will help to explain the ways in which the predictions are made. The sea level rise predictions used – now given to be approximately 1 meter by the year 2100 – were made using computer modeling. The models run by different groups have provided a range of predictions, underlining the complexity of predicting future sea levels. The Environment 360 article (How Far Will Sea Levels Rise) explains how quantifying the contribution of the melting of the polar (Greenland and Antarctic) ice sheets to sea level rise has been a source of uncertainty as scientists struggled to fully understand the mechanisms at work. This aspect of sea level rise prediction is now better understood and scientists are now more confident in determining the contribution of the water flowing from the ice sheets, hence the 2013 projections of 28-98 cm by 2100. But still, there is a disclaimer that this figure could be raised as the fate of the ice sheets becomes clearer. This has given coastal managers a firm number of 1 meter to use in their planning for protection of coastal communities in this century.

Storm Frequency and Intensity

Another factor that impacts the coastal hazard risks experienced by coastal communities is storm frequency and intensity. The International Panel on Climate Change reports that predictive modeling suggests that storms could become more intense in the future. However, the frequency of storms may actually decrease. This quote from IPCC Climate Change, 2007 sums up the evidence for these predictions from modeling results:

"There is evidence from modelling studies that future tropical cyclones could become more severe, with greater wind speeds and more intense precipitation. Studies suggest that such changes may already be underway; there are indications that the average number of Category 4 and 5 hurricanes per year has increased over the past 30 years. Some modelling studies have projected a decrease in the number of tropical cyclones globally due to the increased stability of the tropical troposphere in a warmer climate, characterized by fewer weak storms and greater numbers of intense storms. A number of modelling studies have also projected a general tendency for more intense but fewer storms outside the tropics, with a tendency towards more extreme wind events and higher ocean waves in several regions in association with those deepened cyclones. Models also project a poleward shift of storm tracks in both hemispheres by several degrees of latitude".

Mandatory Readings:

Before you begin working through this module, please read the following articles to build some required background knowledge on sea level rise predictions and future changes in storm intensity. After you have read these readings, you will have the knowledge to move forward.

Background reading on sea level rise predictions:

• How Far Will Sea Levels Rise? Environment 360, Oct 21, 2013.
• Which Coastal Cities Are at Highest Risk of Damaging Floods? The World Bank, August 19, 2013.
• Sea-level rise: Where we stand at the start of 2013. RealClimate Jan 9, 2013 (Note: If the link doesn't bring up the correct article immediately, use the search window to find the article title.)