SEA LEVEL RISE is caused by thermal expansion of the oceans, melting of glaciers and ice caps, melting of the Greenland and Antarctic ice sheets, and changes in terrestrial storage. Changes in sea level will be felt through increases in the intensity and frequency of storm surges and coastal flooding; increased salinity of rivers, bays, and coastal aquifers resulting from saline intrusion; increased coastal erosion; loss of important mangroves and other wetlands (the exact response will depend on the balance between sedimentation and sea level change) and its effect on marine ecosystems (i.e., coral reefs).
Global sea level rose by about 394 ft. (120 m.) during the several millennia that followed the end of the last Ice Age (approximately 21,000 years ago) before stabilizing between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century, when the instrumental record of modern sea level change shows evidence for onset of sea level rise. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 mm. per year.
Satellite observations available since the early 1990s provide more accurate sea level data with nearly global coverage. This decade-long satellite altimetry data set shows that since 1993, sea level has been rising at a rate of around 3 mm. per year—significantly higher than the average during the previous half century. Coastal tide gauge measurements confirm this observation and indicate that similar rates have occurred in some earlier decades.
Sea level rise is currently determined by the employment of two techniques: the use of tide gauges and satellite altimetry. Tide gauges provide sea level variations with respect to the land on which they lie. To extract the signal of sea level change resulting from ocean
water volume and other oceanographic change, land motions need to be removed from the tide gauge measurement. Sea-level change based on satellite altimetry is measured with respect to the Earth's center of mass and thus is not distorted by land motions, except for a small component resulting from large-scale deformation of ocean basins. The total 20th-century rise is estimated to be around 0.5 ft. (0.17 m.).
Sea-level rise is accelerating worldwide. Globally, 100 million people live within about 3 mi. (1 m.) of sea level. Eight to 10 million people live within 3 mi. (1 m.) of high tide in each of the unprotected river deltas of Bangladesh, Egypt, and Vietnam. Intergovernmental Panel on Climate Change (IPCC) reports estimate that the global average sea-level rose at an average rate of 1.8 (1.3-2.3) mm. per year between 1961 and 2003, and within that period, the rate of rise was faster between 1993 and 2003—about 3.1 (2.4-3.8) mm. per year. Overall, the IPCC concludes that there is high confidence that the rate of observed sea-level rise has risen from the 19th to the 20th century. The total 20th-century rise is estimated to be 0.17 (0.12-0.22) m. In 2001, IPCC projections were for a sea-level rise of between 9 and 88 cm. between 1990 and 2100 and a global average surface temperature rise of between 2.5-10.4 degrees F (1.4-5.8 degrees C). In 2007, IPCC projections based on different scenarios predict seal level rise from 0.18 to up to 0.59 mm. by 2099.
Toward the end of the 21st century, projected sea-level rise will affect low-lying coastal areas with large populations. The cost of adaptation could amount to at least 5 to 10 percent of gross domestic product. Mangroves and coral reefs are projected to be further degraded, with additional consequences for fisheries and tourism. Snowmelt runoff as a result of sea-level rise will have major consequences. For example, one change will be a change from spring peak flows to late winter peaks in snowmelt-dominated regions. Many species, both aquatic and riparian (i.e., riverine), have evolved to take opportunity of the spring flows as a result of snowmelt. For example, some fish time their reproduction strategies specifically to avoid the stress of springtime flows. Changes in springtime flow regimes, or high winter flows associated with rain or snow events, can scour streambeds and destroy eggs. Trees that provide riparian habitat along rivers may find it harder to reproduce, as they are dependent on high spring flows. Many species, such as salmon, that are already under pressure from other environmental effects will be significantly affected by climate change. For example, higher temperatures and a reduced stream flow in the Columbia River Basin may be increasing the mortality of juvenile coho salmon, or in some cases, increased temperatures may be creating thermal barriers for the migration of adult salmon.
There are a number of associated events that are the result of climate change and that will also have affects on sea-level rise. For example, the Kangerdlugssuaq Glacier in Greenland is moving much faster, now melting at a rate of 8.7 mi. (14 km.) a year in comparison to just 3 m. (5 km.) a year in 1988. This loss will also have serious implications for sea-level rise, with some scientists predicting that within the next 100 years, ice cover in this region will completely disappear over summer, and hence species living within it, such as polar bears, will be threatened. The complete melting of the Greenland Ice Sheet and the West Antarctic Ice Sheet would lead to a contribution to sea-level rise of up to 23 ft. and about 16 ft. (7m. and 5 m.), respectively.
The potential socioeconomic impacts of sea-level rise are as follows: direct loss of economic, ecological, cultural, and subsistence values through loss of land, infrastructure, and coastal habitats. For example, it is estimated that the total of people at risk of sea-level rise in Bangladesh could be 26 million, in Egypt 12 million, in China 73 million, in India 20 million, and elsewhere 31 million. This makes an aggregate total of 162 million people affected by sea-level rise. There will be an increased flood risk for people, land, and infrastructure. There will be other affects related to changes in water management, salinity, and biological activities. A rise in sea level would inundate wetlands and lowlands, accelerate coastal erosion, exacerbate coastal flooding, threaten coastal structures, raise water tables, and increase the salinity of rivers, bays, and aquifers. Similarly, the areas vulnerable to erosion and flooding are also predominantly located in the southeast, whereas potential salinity problems are spread more evenly throughout the coast. Such a loss would reduce available habitat for birds and juvenile fish and would reduce the production of organic materials on which estuarine fish rely.
Some of the most important vulnerable areas are recreational barrier islands and spits such as found within the Atlantic and Gulf coasts. Coastal barriers are generally long narrow islands and spits (peninsulas) with the ocean on one side and a bay on the other.
Typically, the ocean-front block of an island ranges from 6.5 ft. to 13 ft. (2 to 4 m.) above high tide, whereas the bay side is less than a meter above high water. Thus, even a 1-m. rise in sea level would threaten much of this valuable land with inundation. Erosion, moreover, threatens the high parts of these islands and is generally viewed as a more immediate problem than the inundation of their bay sides. Although inundation alone is determined by the slope of the land just above the water, coastal engineer Per Bruun showed that the total shoreline retreat from a rise in sea level depends on the average slope of the entire beach profile. For example, most U.S. recreational beaches are less than 30 m. (100 ft.) wide at high tide, thus even a 30-cm. (1-foot) rise in sea level would require a response.
Finally, a rise in sea level would enable saltwater to penetrate farther inland and upstream in rivers, bays, wetlands, and aquifers, which would be harmful to some aquatic plants and animals and would threaten human uses of water. In Delaware in the United States, for example, salinity is seen as a factor resulting in reduced oyster harvests.
Coastal areas worldwide will become more vulnerable to flooding as a result of sea-level rise because higher sea levels provide higher bases for storm surges to build on. In this context, a 1-meter rise in sea level would mean that a 15-year storm will flood many areas that today are only flooded by a 100-year storm. Beach erosion will make land more vulnerable to storm waves, and higher water levels will increase the effects of flooding caused by rainstorms by reducing coastal drainage. Sea level will also raise water tables in various systems.
There are many examples of nations vulnerable to sea level rise. Japan, for instance, is particularly vulnerable to the effects of sea-level rise; a 1-meter rise in sea level would increase the area situated below mean high water from 332 sq. mi. to 903 sq. mi. (861 sq. km. to 2340 sq. km.). Future estimates show this would affect up to 4.1 million people and cost 109 trillion Yen ($1,300 billion). Over 57 and 90 percent of the existing sandy beaches would be eroded by sea-level rises of 0.3 and 1.0 m., respectively. In response, Japan has initiated a new coastal policy that combines disaster prevention, human resource use, and nature conservation. This policy includes an increase in monitoring of changes in mean sea level and the frequency of extreme events, the consideration of climate-change scenarios when developing plans for ports and landfills, and the preparation of a set of technological countermeasures to prevent effects on port facilities and maintain coastal protection.
Egypt's Nile Delta is one of the world's areas most vulnerable to sea-level rise. It is estimated that about 30 percent of the area will be lost because of inundation, almost 2 million people will lose their homes, and approximately 195,000 jobs will be lost, with a predicted economic impact of over US$3.5 billion over the next century.
In the chapter on coasts, the United Nations Environment Programme Handbook on Adaptation and Mitigation Methodologies specifically outlines a suite of strategic responses to sea-level rise. It cautions, however, that before applying these strategies, policymakers must decide whether or not their adaptation is to be autonomous or planned, reactive or proactive.
There are three management responses to sea-level rise: retreat, accommodation, and protection. Retreat involves no effort to protect the land from the sea. The coastal zone is abandoned, and ecosystems shift landward. This choice can be motivated by excessive economic or environmental effects of protection. In the extreme case, an entire area may be abandoned.
Accommodation implies that people continue to use the land at risk but do not attempt to prevent the land from being flooded. This option includes erecting emergency flood shelters, elevating buildings on piles, converting agriculture to fish farming, or growing flood- or salt-tolerant crops.
Protection involves hard structures such as sea walls and dikes, as well as soft solutions such as dunes and vegetation, to protect the land from the sea so that existing land uses can continue.
SEE ALSO: Climate Change, Effects; Floods; Refugees, Environmental.
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Melissa Nursey-Bray Australian Maritime College Rob Palmer Research Strategy Training
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