Coastlines, Changing

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Coastlines, Changing

Introduction

Coasts are places where land meets ocean; a coast's shape, as seen from above, is its coastline. Coastlines change when either the land or the ocean changes. Land changes include erosion, deposition (increase of land by the arrival of solid material, often small particles brought to the coast by rivers), or rising or falling of the land itself due to geological forces. The ocean may change by shifting its current and wave patterns or by rising or falling in level. Rising or falling sea level affects coastlines simultaneously all over the world and so is the most important determinant of changes to coastlines.

Sea level has risen and fallen hundreds of times over the 4.5-billion-year history of Earth, and has been rising from about 22,000 years ago to the present, though not at a steady rate. Recently, sea-level rise has been accelerated by anthropogenic (human-caused) climate change, which has caused the top layer of the ocean to warm (and so expand) and ice to begin melting more quickly in Greenland and Antarctica. As a result, coastlines are changing worldwide, and are projected to change much more in coming decades and centuries as global warming continues.

Historical Background and Scientific Foundations

Coastlines change greatly over geological time as continents drift together and apart, sea levels fall and rise during ice ages and interglacial periods, and other geological processes rework the world. For example, the whole interior portion of North America from what is now the Arctic Ocean to the Gulf of Mexico was a shallow sea during the Cretaceous period, about 100 million years ago. The coastlines of that sea, the Western Interior Seaway, no longer exist. And as sea levels rise and fall many meters over geological time, entire islands appear and disappear or are greatly altered in coastal outline.

Coastlines have changed even in recent human pre-history. For example, about 12,000 years ago, during the withdrawal of the glaciers of the most recent Ice Age, the removal of the ice's weight on what is now northern Germany and the Baltic Sea allowed the underlying rock to tip like a see-saw. Inhabited coastal areas sank beneath the sea as the coastline of northern Europe shifted southward, while inland areas gained altitude.

The amount of water locked up in ice sitting on land in Greenland, Antarctica, and elsewhere has a large effect on sea levels and thus coastlines worldwide. Since the end of the last major Ice Age about 22,000 years ago, the melting of glaciers has liberated enough water to raise sea level about 400 ft (120 m). All but a few feet of that increase took place before 7,000 years ago. Sea level stabilized about 2,000–3,000 years ago and did not rise significantly again until the late nineteenth century, when it began rising steadily again. During the twentieth century, it rose at an average rate of 0.07 in/year (1.8 mm/year). From 1993 to the present, it has been rising at 0.12 in/year (3 mm/year), almost double the average rate from 1900 to 1990.

Steep coastlines are least affected by sea-level rise, because each inch of increased water depth only climbs a little way up the shore, affecting a relatively narrow band of ground. The effect is much greater for gently sloping or nearly flat shores, such as river deltas (flat areas of fine particles carried to the sea by rivers and dumped at their outlets), swamps, and beaches, where each inch of sea-level rise may cause the sea to advance several inches— sometimes many yards—inland. Rising seas also shrink small, flat islands and seep into their underground freshwater supplies.

The effects of sea-level rise on coastal lands can be destructive. Along the East Coast of the United States, for example, three quarters of the coast that is not protected from the ocean by natural or artificial barriers is experiencing erosion because of sea-level rise over the last 100 to 150 years. The impacts of storms have also increased along the East Coast because of sea level rise.

However, rising sea level is not the only force causing shores to erode around the world. Other climate-related coastal changes, including the melting of perma-frost, have contributed to the fast retreat of some Arctic coastlines. Reduced winter sea-ice cover along some shores, such as those of the gulf of the St. Lawrence River in Canada, allows waves to attack open shore during more of the year and thus speed erosion. Increasingly severe storms, a consequence of global climate change, also speed coastal erosion.

Human activities not directly related to global climate change that affect coastlines include: the damming of rivers, which starves river deltas of silt (small particles carried downstream); the removal of mangrove forests whose roots anchor mud and sand along some shallow tropical coasts; the mining of beaches for sand; and the pumping of gas, oil, and water from beneath the land, which causes the land to subside (sink). Hurricane Katrina, which inundated the U.S. city of New Orleans Louisiana, in August 2005, was particularly destructive because land subsidence placed much of the city below sea level. Rising sea level can have more impact on coastal wetlands, which include the flattest types of coastline, when combined with other human pressures on the environment.

Sea-level rise is not uniform all over the world. Although the world's oceans are in effect a single body of water, they are connected by a complex system of deep and shallow circulations, with regional differences in saltiness and temperature that affect sea level. Sea level is actually falling in a few places. In most places, however, the trend is toward steadily increasing sea levels and retreating coastlines

Impacts and Issues

Erosion, changes to wetlands, and changes to coastal vegetation have already been observed in many parts of the world due to climate change and other forces. A few of these changes are as follows:

Shoreline erosion is affecting 75% of the unprotected coastline of the eastern United States. Louisiana's shoreline retreated by 3.3 ft/year (1 m/year) from 1988 to 2002. Nineteen percent of the shoreline recently examined in the Manitounuk Strait in Canada is retreating due to melting of permafrost.
Wetland changes include the loss and degradation of marshes in Chesapeake Bay in the United States; loss of land area in Venice, Italy; wetland losses in the Thames estuary in England; decreases in salt-marsh area in Long Island and Connecticut in the United States; and changes in other parts of the world. In some places, where rates of sediment accumulation have increased, salt marshes can keep up with sea-level rise, as along the coast of Normandy, France.
Coastal vegetation changes include the replacement of grassy marshes by mangroves in Florida in the United States and similar changes in southeastern Australia.

WORDS TO KNOW

CRETACEOUS PERIOD: Geological period from 145 million years ago to 65 million years ago. During the Cretaceous, the supercontinent that geologists call Gondwana broke up, radically altering regional climates. The Cretaceous ended in the Cretaceous-Tertiary (K-T) extinction event, probably caused at least partly by an asteroid impact, which included the extinction of dinosaurs and about 85% of all other species.

DEPOSITION: Process by which water changes phase directly from vapor into a solid without first becoming a liquid.

EROSION: Processes (mechanical and chemical) responsible for the wearing away, loosening, and dissolving of materials of Earth's crust.

ICE AGE: Period of glacial advance.

MANGROVE FOREST: Coastal ecosystem type based on man-grove trees standing in shallow ocean water; also termed mangrove swamp. Mangrove forests support shrimp fish-eries 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.

WETLANDS: Areas that are wet or covered with water for at least part of the year.

In the large delta at the outflow of the Ganges river in India, rising sea level combined with increased river flow from melting glaciers in the Himalayas (an effect of global warming) have recently destroyed some of the Sundarbans islands. About 31 square mi (80 square km) of land have vanished over the last 30 years due to rising water, forcing about 600 families to relocate, often with an increase in poverty. Flooding and erosion are expected to continue in such areas, and will probably worsen in coming decades.

Sea level may rise anywhere from about a foot to more than a yard (0.3-1 m) in the coming century. Forecasts are rendered uncertain by many variables, especially the future melting rate of some of the Greenland ice cap and the West Antarctic ice sheet (the rest of the Antarctic ice sheet is thought to be very stable). Coastal wetland ecosystems, particularly where there is no opportunity to migrate inland, will be degraded.

As with other effects of climate change, developed countries with more monetary resources and better technology will find it easier to adapt to some coastal changes than will less developed nations. The processes causing sea-level rise are difficult to reverse and will continue to operate for centuries, even if human beings stabilize the amount of greenhouse gases they are putting into the atmosphere.

Impacts on natural systems are likely to be severe even in the next century. By the year 2080, scientists estimate that up to 20% of the world's coastal wetlands could vanish due to sea-level rise.

Primary Source Connection

An overwhelming majority of researchers agree that climate change will alter coastlines worldwide. However, there remains some uncertainty about the possible extent of sea-level rise and how it will reshape coastlines. The following article discusses the threat of rising sea levels to coastal property development in the United States and the emerging technology researchers are using to predict and map coastline change.

R. L. Evans is a marine geophysicist and an associate scientist at Woods Hole Oceanographic Institution, the world's largest nonprofit marine research organization.

This text has been suppressed due to author restrictions.

BIBLIOGRAPHY

Books

Committee on Mitigating Shore Erosion Along Sheltered Coasts, National Research Council. Mitigating Shore Erosion Along Sheltered Coasts. Washington, DC: National Academies Press, 2007.

Parry, Martin, et al, editors. Climate Change 2007: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Dean, Cornelia. “Expert Federal Panel Urges New Look at Land Use Along Coasts in Effort to Reduce Erosion.” The New York Times (October 13, 2006).

Web Sites

Gornitz, Vivien. “Coastal Populations, Topography, and Sea Level Rise.” NASA, Goddard Institute for Space Studies, March 2000. < http://www.giss.nasa.gov/research/briefs/gornitz_04/> (accessed October 25, 2007).

Park, Sung Bin. “A Sea of People to Show How Climate Change Will Alter New York City.” NYC Indymedia, April 4, 2007. < http://www.transalt.org/press/media/2007/890.html> (accessed October 25, 2007).

Larry Gilman

Climate Change: In Context