5. What would be the consequences of sea level rise?

Sea level rise will be felt both through changes in mean sea level, and, perhaps more importantly, through changes in extreme sea level events.

Even if there are no changes in extreme weather conditions (for example, increases in tropical cyclone intensity), sea level rise will result in extreme sea levels of a given value being exceeded more frequently. This change in the frequency of extreme events has already been observed at many locations. The increase in frequency of extreme events will depend on local conditions, but events that currently occur once every 100 years could occur as frequently as once every few years by 2100. Even without the increase in the frequency of extreme events, millions of people will be impacted by the direct effects of sea level rise over the next century. The large changes in sea level over the last few ice ages are sometimes used to dismiss the importance of the relatively small current changes in sea level, on the grounds (apparently) that mankind adapted to these changes in the past, so it should be able to adapt again. The obvious fallacy in this argument is that, previously, the inhabitants of a region would have picked up all of their worldy goods and gone somewhere more suitable when changes of sea level made their current habitat untenable. With the massive infrastructure (e.g. London and New York) that are located near sea level and the many millions of people living in threatened regions (e.g. Bangladesh and southern Vietnam), such a solution is not available.

The physical effects of sea level rise can be classified as land loss due to inundation of low-lying areas and erosion of beaches and bluffs, salt intrusion into aquifers and surface waters, higher water tables, and increased flooding and storm damage.

Figure 6 Estimates of people flooded in coastal areas in the 2080s as a result of sea level rise and for given socio-economic scenarios and protection responses Source: UNEP: Global outlook for Ice & Snow

A rise of a few mm per year by the sea, although not threatening spectacular inundation is still extremely important. Direct land loss of low lying areas can rapidly (decadal to centennial periods) damage or destroy coastal ecosystems. Rising sea levels inundate wetlands and other low-lying lands, erode beaches, intensify flooding, and increase the salinity of rivers, bays, and groundwater tables. Some of these effects may be further compounded by other effects of a changing climate.

Coastal wetland ecosystems, such as salt marshes and mangroves are particularly vulnerable to rising sea level because they are generally within a few feet of sea level (IPCC, 2007). Wetlands provide habitat for many species, play a key role in nutrient uptake, serve as the basis for many communities’ economic livelihoods, provide recreational opportunities, and protect local areas from flooding.

As the sea rises, the outer boundary of these wetlands will erode, and new wetlands will form inland as previously dry areas are flooded by the higher water levels. The amount of newly created wetlands, however, could be much smaller than the lost area of wetlands – especially in developed areas protected with bulkheads, dikes, and other structures that keep new wetlands from forming inland. The IPCC suggests that by 2080, sea level rise could convert as much as 33 percent of the world’s coastal wetlands to open water. (IPCC, 2007). Tidal wetlands are generally found between sea level and the highest tide over the monthly lunar cycle. As a result, areas with small tide ranges are the most vulnerable.

In addition to inundation, long-term sea level rise can cause erosion and shoreline retreat by creating a sediment budget deficit. Coastal erosion is caused by a variety of factors, which broadly fall into two categories. First, sand often migrates along the shore, causing some areas to erode and others to accrete. Second, rising sea level causes virtually all shores to erode. Using a model first developed by Danish coastal engineer Per Bruun, coastal geologists have estimated that a one-meter rise in sea level will cause beaches to erode 50 to 100 meters from New England to Maryland, 200 meters along the Carolinas, 100 to 1000 meters along the Florida coast, and 200 to 400 meters along the California coast. These model calculations are roughly consistent with the observed rate of erosion. (http://www.cmar.csiro.au/sealevel)

The effect of rising mean sea levels will be felt most profoundly during extreme storm conditions when strong winds and falling pressure bring about a temporary and localised increase in sea level known as a storm surge. Storm surges occurring on higher mean sea levels will enable inundation and damaging waves to penetrate further inland increasing flooding, erosion and the subsequent detrimental impacts on built infrastructure and natural ecosystems. In the tropics storm surges are caused by tropical cyclones while elsewhere mid-latitude storms and their associated cold fronts are the main cause of storm surges.

Sea level rise also increases the vulnerability of coastal areas to flooding during storms for several reasons. First, a given storm surge from a hurricane or northeaster builds on top of a higher base of water. Considering only this effect, it is estimated that existing development in the U.S. Coastal Zone would experience a 36-58 percent increase in annual damages for a 30 cm rise in sea level, and a 102-200 percent increase for a 1 m rise. Shore erosion also increases vulnerability to storms, by removing the beaches and dunes that would otherwise protect coastal property from storm waves (FEMA 2000). Sea level rise also increases coastal flooding from rainstorms, because low areas drain more slowly as sea level rises.

Other impacts of climate change may further enhance or mitigate coastal flooding. Flooding from rainstorms may become worse if higher temperatures lead to increasing rainfall intensity during severe storms. An increase in the intensity of tropical storms would increase flood and wind damages.

Rising sea level increases the salinity of both surface water and ground water through salt water intrusion. If sea level rise pushes salty water upstream, then the existing water intakes might draw on salty water during dry periods. Salinity increases in estuaries also can harm aquatic plants and animals that do not tolerate high salinity. Shallow coastal aquifers are also at risk (IPCC, 2007). As rising water levels submerge low-lying portions of the coastal areas, portions of the aquifers would become saline.

All the mentioned impacts of sea level rise can cause human loss due to increased level of storm surges, socio-economic losses of coastal communities as well as impact on national economies, environmental problems such as loss of wetlands, dunes and associated ecosystem habitats and loss of cultural heritage due to inundation of these sites (http://epa.gov/climatechange/effects/coastal/index.html).