As human activities—such as building along coastlines, mining of beach sand, cutting of mangrove forests, and damming of rivers—place stresses on coastal environments and sea levels rise due to climate change, the ecosystems and outlines of many coasts around the world are changing. Such changes will probably accelerate in coming decades, affecting human populations of many coastal areas. In many areas, there is no option for reducing impacts to ecosystems by sea-level rise except mitigating climate change itself by emitting fewer greenhouse gases.
In developed or wealthier nations, some of the coastal effects of climate change and other human activities may be prevented or adapted to. This will cost money—but then, so will doing nothing—and experts have warned that the costs of inaction will greatly outweigh the costs of action. In poorer areas, such as many parts of Latin America, Southeast Asia, and Africa, and the small island countries of the Pacific Ocean, there will not be enough money to adapt effectively to coastal changes. In these areas, as well as in parts of the developed world, there may be no choice but for populations to retreat inland from rising seas and more dangerous coastal storms.
Historical Background and Scientific Foundations
Global climate change places several stresses on coastal ecosystems and human populations. Increased glacial melting, as in the Himalayas and elsewhere, is increasing river flow (until the glaciers are gone); hurricanes and monsoons are increasing in severity; permafrost is melting; and sea levels are rising. All of these changes affect coastlines and therefore the people who live near them. Sea-level rise is perhaps the most important single aspect of climate change as far as coastal populations are concerned, because it affects almost all coastlines at the same time and does so in several ways at once. Rising sea level moves coastlines inland, bringing waves and therefore erosion to new areas. Flat coastal lands and wetlands may be covered with rising water, or rendered vulnerable to exceptional high-water events such as storm surges (brief increases in local sea level caused by large storms such as hurricanes). They may also be eroded at their edges, forcing natural and human communities to move inland—if they can.
Sea levels have changed little for the last 7,000 years. They have been particularly stable for the last 2,000 to 3,000 years. In the late nineteenth century, however, at which time anthropogenic (human-caused) climate change began to swell the upper layer of the oceans by warming them and to deepen the oceans by melting glaciers, the oceans began to rise at an average global rate of about 0.07 in per year (1.7 mm per year). Sea level rose at this rate for most of the twentieth century. From 1993 to 2007, it has been rising at 0.12 in per year (3 mm per year), almost twice the average rate of the first 90 years of the twentieth century.
The affects of climate change on coastlines threaten property and lives in many locations around the world. More than 100 million people live no more than 3 ft (about 1 m) above sea level, while about 400 million live no more than 66 ft (20 m) above sea level and within 12 mi (20 km) of a coast. The number of people per square mile in coastal areas is, on global average, about three times greater than elsewhere. All these populations are at increased risk from erosion or storm flooding due to ongoing climate change.
WORDS TO KNOW
ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.
DELTA: Triangular–shaped area where a river flows into an ocean or lake, depositing sand, mud, and other sediment it has carried along its flow.
EROSION: Processes (mechanical and chemical) responsible for the wearing away, loosening, and dissolving of materials of Earth's crust.
ESTUARY: Lower end of a river where ocean tides meet the river's current.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
MANGROVE FOREST: Coastal ecosystem type based on mangrove trees standing in shallow ocean water: also termed mangrove swamp. Mangrove forests support shrimp fisheries and are threatened by rising sea levels due to climate change.
PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F (0°C) for several years.
RIVER DELTA: Flat area of fine-grained sediments that forms where a river meets a larger, stiller body of water such as the ocean. Rivers carry particles in their turbulent waters that settle out (sink) when the water mixes with quieter water and slows down; these particles build the delta. Deltas are named after the Greek letter delta, which looks like a triangle. Very large deltas are termed megadeltas and are often thickly settled by human beings. Rising sea levels threaten settlements on megadeltas.
STORM SURGE: Local, temporary rise in sea level (above what would be expected due to tidal variation alone) as the result of winds and low pressures associated with a large storm system. Storm surges can cause coastal flooding, if severe.
WETLANDS: Areas that are wet or covered with water for at least part of the year.
Eleven of the 15 largest cities in the world are located on coasts or estuaries. (An estuary is a partly enclosed body of water connected to the ocean on one side and a river on the other.) For example, New York, London, and Jakarta, the capital of Indonesia, are all located on estuaries. About 53% of the population of the United States lives on a coast, and further development of coastal regions is continuing rapidly. In developed nations, people are increasingly attracted to coastline developments because such locations are often thought to be more pleasant or desirable. Waterfront property tends to bring a high price on the real-estate market. All these developments will eventually be at risk from rising sea levels.
In poorer countries, populations are settled by the ocean because they draw their living from it or are attracted to the job markets in cities that were originally sited by the ocean for trade and fishing. The possible impacts on densely settled populations in the deltas—the very large, flat areas of land built up at the outflows of large rivers, such as the Ganges, Nile, Indus, and Mekong—are catastrophic. As of 2006, about 300 million people lived on some 40 large river deltas around the world. These deltas are large, flat deposits of sediment fanning out from river outlets. They are often fertile, easily built on, and attractive to settlement because they are in contact with both a major river and the ocean.
Deltas are experiencing faster effective sea-level rise than actual sea-level rise, because human activities are causing many of them to subside (become lower). Diverting rivers upstream for irrigation or damming them for hydroelectric generation reduces the amount of sediment—fine-grained mineral material, such as clay and sand—that is carried to the delta system. When the decrease is great enough, erosion can cause the delta to subside and shrink rather than to grow. Alternatively, as with the Sundarbans delta in India and Pakistan, at the outflow of the Ganges River, increased flow from accelerated melting of glaciers upstream in the Himalayas can raise water levels in the delta, increasing effective sea-level rise by raising river levels rather than by lowering the delta by starving it of sediment, as is more commonly the case.
Even some large settlements in the developed world are profoundly vulnerable to rising seas and increasingly severe storms. For example, much of the city of New Orleans, Louisiana, in the United States is below sea level and is protected from instant flooding only by an elaborate system of artificial barriers called levees. The devastation in this city brought by Hurricane Katrina in 2005 exemplifies the impact of a severe storm on such a heavily populated coastal area.
Impacts and Issues
The Arctic Climate Impact Assessment, written by an international scientific group, reported in 2004 that a
1.5-ft (50-cm) rise in sea level by 2100, which is within the range pronounced likely in 2007 by the Intergovernmental Panel on Climate Change (IPCC), would flood low-lying parts of the Florida and Louisiana coasts, pushing the coastline inland by about 150 ft (45 m).
The IPCC, although careful to note that no single hurricane event can ever be attributed to climate change, says that such events illustrate the consequences of climate change as storms become more intense and possibly more common. These consequences include the loss of what are called ecosystem services, that is, features of natural systems like coastal wetlands upon which human communities depend. Hurricane Katrina, which resulted in more than 1,800 deaths along the U.S. Gulf Coast and caused over $100 billion of damage in 2005, also destroyed 150 sq mi (388 sq km) of coastal wetlands, islands, and levees that previously protected New Orleans from storm surge. Thus, storm damage can render a coastal settlement more vulnerable to future storm damage. Sea-level rise combined with other human activities, such as damming of rivers, is rendering many large cities built on river deltas, including New Orleans, increasingly vulnerable to such effects.
IN CONTEXT: PLANNING FOR SEA LEVEL RISE
Within the United States, Maine is one of the only states whose laws and building codes specifically acknowledge the possible impacts of global climate change. Collating data from observed and recorded tidal changes on Maine's coastlines since 1912, the Intergovernmental Panel on Climate Change 2001 report, and recommendations from the U.S. Geological Survey, the State of Maine adopted an official policy of planning for a 2-3 ft (0.61-0.91 m) rise in sea level over the next century.
Maine's state and municipal laws try to accommodate the diverse interests of coastal preservation, long-term planning, and current development. Existing coastal structures are permitted to remain at the water's edge. However, the state prohibits rebuilding of some storm-damaged structures, especially those closest to the water. For example, if a structure is at least 50% damaged, it cannot be rebuilt on the same site unless the owner can demonstrate with “clear and convincing evidence” that the site will remain stable and intact after a 3-ft (0.91-m) rise in sea level.
The long-term effects of climate change on coastal populations will depend largely on how much the sea level rises and how rapidly. There is scientific disagreement about how much sea-level rise is likely to occur in this century and beyond. The IPCC predicted in 2001 that sea-level rise in the twenty-first century would probably be between 0.3 and 2.9 ft (0.09–0.9 m); in 2007, it narrowed the likely range to 0.6–1.9 ft (0.18–0.6 m), an apparently reassuring change. However, of all the statements in the IPCC's 2007 Assessment Report on climate change, this has probably been more vigorously criticized by scientists than any other. Many experts have argued that the IPCC underestimates future sea-level rise by leaving out of account possible accelerated melting of the Greenland and West Antarctic ice sheets. Accelerated melting in Greenland has been confirmed by satellite observations since December 2005, the cutoff date for research to be included in the IPCC's 2007 report. Greenland's ice sheet contains enough water to raise sea level by about 23 ft (7 m), although complete melting could not take place within a single century.
Primary Source Connection
Much of the world's population and economic development are located along coastlines. Thus, anticipated sea-level rise has sparked intensive study of vulnerable coastal populations. Here, Vivien Gornitz, a lead scientist at Columbia University's Center for Climate Systems Research, describes methods and uncertainties involved in estimating coastal risk from climate change.
COASTAL POPULATIONS, TOPOGRAPHY, AND SEA LEVEL RISE
A frequently cited consequence of global climate change is the potential impact of sea level rise (SLR) on coastal populations. Eleven of the world's 15 largest cities lie along the coast or on estuaries. In the United States, around 53% of the population lives near the coast. In spite of this widespread concern over sea level rise, no accurate worldwide estimate of the number of people likely to be affected by coastal inundation or flooding has been published.
In a recent study with Christopher Small (Columbia Univ. Lamont-Doherty Earth Observatory) and Joel E. Cohen (Rockefeller Univ.), I attempted to provide an improved assessment of human vulnerability to sea level rise by integrating the best currently available information about global population distributions, elevation, and sea level. Population data are compiled from a 1997 study by Tobler and others; topography comes from the EROS Data Center (Sioux Fall[s], SD) 30 arc second gridded elevations, and the sea level records from the Permanent Service for Mean Sea Level, Bidston Observatory…. We considered the following scenarios:
- Extrapolation of current trends. No climate change occurs and current rates of sea level rise are extrapolated from 1990 to 2100.
- Goddard Institute for Space Studies (GISS) coupled ocean-atmosphere General Circulation Model (GCM) (Russell et al. 1995) at 4° by 5° horizontal resolution. In the GS run, CO2 increases by 1%/yr and sulfate aerosols increase annually up to 2050, with a slight decrease thereafter.
- The IPCC IS92a “best estimate” includes effects of sulfate aerosols.
The two model scenarios show increases of around 40–45 cm in sea level over the next 100 years, which is about three times that of extrapolating the current trend….
In addition to permanent inundation due to sea level rise, the coast is at risk to flooding by storms. Average country-wide surge levels for given return periods are taken from a 1993 Delft Hydraulics study. Total flood levels comprise storm surges, tides, and local sea level change, including vertical land motions…. Most sea level stations show significantly less than 1m subsidence over the next 100 years. Areas experiencing uplift include Scandinavia and Alaska, due to ongoing glacial rebound, and Japan due to tectonic uplift.
Analysis of the spatial distribution of population with respect to topography reveals that the number of people and population density diminishes rapidly with increasing elevation and increasing distance from the shoreline. Approximately 400 million people live within 20 m of sea level and within 20 km of a coast, worldwide. However, this figure is not very precise—the spatial distribution of coastal populations is not known to better than 20–30 km. In fact, a major conclusion of our study is that the available data are still inadequate to permit quantitatively precise global estimates of the number of people likely to be affected by plausible levels of sea level rise or storm surges in the coastal zone. In the near future, airborne and satellite-based radar and laser altimeters can map coastal topography and its changes at much higher resolutions than those used in this study. Satellite monitoring of coastal land cover transformations will provide means of quantifying habitation patterns, and thus indirectly, population trends.
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See Also Coastlines, Changing; Hurricanes; Sea Level Rise.
Committee on Mitigating Shore Erosion Along Sheltered Coasts, National Research Council. Mitigating Shore Erosion Along Sheltered Coasts. Washington, DC: National Academies Press, 2007.
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