Coastline changes
Coastline changes refer to the dynamic alterations occurring at the boundary where land meets ocean, influenced by natural forces like tides, waves, and sea-level fluctuations, as well as human activities. Coastlines can be classified as erosional or depositional; erosional coastlines experience the removal of sediment and rock due to wave action and past geological processes, resulting in features like cliffs and sea arches. In contrast, depositional coastlines grow outward from sediment accumulation, characterized by beaches, sandbars, and coral reefs.
Climate change, particularly rising sea levels due to global warming, poses significant threats to coastlines around the world. Many coastal regions, especially those with low-lying urban areas, face increased flooding risks, potentially displacing millions of residents. The effects of climate change may also lead to the migration of coastal ecosystems, such as wetlands and mangroves, due to salinity changes and habitat loss. Additionally, infrastructure like roads and power plants may be endangered by coastal erosion and flooding. Monitoring and understanding these changes are crucial for developing strategies to mitigate risks and protect vulnerable communities. Remote-sensing technologies play a key role in tracking these coastline transformations, providing critical data to inform conservation and management efforts.
Subject Terms
Coastline changes
Definition
The term “coastline” is used to describe those places where the land and the water of the world’s oceans meet. These are dynamic parts of the globe that are daily shaped by tides, winds, waves, changing sea levels, and human activities. Large river discharges may dominate them entirely, or they may be ice covered much of the time; cities may border them with their polluting activities, or they may be remote with no human presence at all. Oceanographers generally classify coastlines as being in one of two categories: either erosional or depositional.
Erosional coastlines are found where wave action is actively removing bedrock or sediment exposed along a coast, or where rivers or glaciers eroded the coast in past times when sea levels were lower than they are today. Some of the distinctive features found along erosional shorelines include rocky headlands separated by bays and coves, sea cliffs, sea caves, sea arches, sea stacks, and uplifted wave-cut terraces. Additional features such as fjords, old glacial moraines, drowned river valley systems, faults, and volcanoes may also be present.
Depositional coastlines are places where the shoreline grows outward because of the accumulation of sediment or the action of organisms such as mangroves and corals. Important features found along these coasts include beaches, sandbars, cusps, spits, sand hooks, and offshore barrier islands with long, narrow lagoons behind them. In addition, coral reefs, deltas, mangrove swamps, and salt marshes may be present. The term “coastal zone” is used in legal and legislative documents to describe the shoreline area. The landward boundary of the coastal zone is defined as the distance inland from a chosen reference point, usually the high watermark. This distance is frequently 60 meters. The seaward boundary of the coastal zone is variously defined by local, state, or federal laws.
Significance for Climate Change
As sea levels rise as a result of the of seawater and the addition of new water from ice melting on land, a variety of coastline changes can be anticipated to take place. Areas that were previously above wave action will be subject to vigorous wave attack, and storm surges should intensify and extend farther toward the poles as a warming ocean generates more lethal storms. While cliffs made of resistant rock may remain unaffected, those coastlines built of loose sedimentary materials, such as sand or gravel, may be eroded back as much as a few meters in a single storm. Coastal areas that were never flooded before may be covered with seawater during severe storms and high tides; finally, they will be flooded by the ocean on a permanent basis. Low-lying urban areas would be particularly affected. The flooding of New Orleans during Hurricane Katrina in 2005 is an example, and already the city of Venice experiences periodic floods when tides are extremely high.
Ultimately, the people who live in high-risk areas such as these will be forced to move, and millions of people will be displaced. The Nile and Mississippi deltas are examples of high-risk places with large urban populations, as are the low-lying coastal areas in Bangladesh and the adjoining countries of Southeast Asia. As seas rise because of global warming, beaches will be forced to migrate inland or drown in place. Those in important tourist areas, such as Miami Beach and the Gulf Coast of the United States, will require expensive nourishment programs. Estuaries, which are critical for commercial fish stocks during spawning and early life stages, will gradually change from to saline or, in time, may become part of the sea itself. The coastal wetlands behind them, which filter out pollution from the land and act as sponges for storm surges, will become saline too. These, along with the mangroves that protect the shoreline from wave attack and are also part of the nutrient system, will eventually have to migrate inland, provided the areas behind them are not already developed.
Even the offshore coral reefs that provide protection to the shoreline from wave erosion may be affected, because corals have a very low tolerance to elevated water temperatures. It can also be anticipated that salt water will gradually intrude upon coastal aquifers, making their supplies of drinking water unfit for humans, and that expensive structures such as dikes and seawalls will have to be built to protect most of the urban areas. Infrastructure, such as highways, bridges, waterways, ports, mass transit systems, airports, water supply facilities, and waste storage systems, may be threatened as well, and some of the nuclear power plants that have been built next to the ocean to make use of seawater for their cooling towers will have to be moved. Even lowly storm drains, built to carry rainwater off the land, will play a significant role in coastal flooding by providing conduits to send the seawater onshore.
Efforts have been undertaken to establish the vulnerability of coastal areas in the United States to the damaging effects of erosion. This is particularly important as rising sea levels, and an increase in both the frequency and severity of storms, are accelerating erosion rates. Scientists estimated in 2024 that by the year 2100 more than 410 million people could be placed at risk by rising sea levels.
Those involved with monitoring changing coastlines use various tools, some of which became accessible for the first time in the twenty-first century. Many are referred to as remote-sensing technologies for their ability to provide desired data on an object from a long distance. These include imagery from aircraft or satellites, photogrammetry, and Light Detection and Ranging, or LIDAR. These sensors can provide highly accurate information on physical changes incurred by shorelines as a result of natural or human-made events.
Bibliography
Bird, Eric. Coastal Geomorphology: An Introduction. Hoboken, N.J.: Wiley, 2008.
"Coastal Erosion" U.S. Climate Resilience Toolkit, 1 Apr. 2021, toolkit.climate.gov/topics/coastal-flood-risk/coastal-erosion#footnote1‗mp5focg. Accessed 13 Dec. 2024.
"Coastal Shoreline Change" National Park Service, 14 Dec. 2022, www.nps.gov/im/secn/shoreline-change.htm. Accessed 13 Dec. 2024.
Davis, Richard A., and Duncan M. Fitzgerald. Beaches and Coasts. Malden, Mass.: Blackwell Science, 2004.
Glick, Daniel. “The Big Thaw.” In Global Climate Change, edited by Paul McCaffrey. New York: H. W. Press, 2006. A sampling of firsthand reports illustrating the ways in which rising sea levels are affecting shorelines in many parts of the world.
Masterson, Victoria, et al. "Sea Level Rise: Everything You Need to Know." World Economic Forum, 20 Sept. 2024, www.weforum.org/stories/2024/09/rising-sea-levels-global-threat/. Accessed 13 Dec. 2024.
"Remote Sensing Coastal Change." United States Geological Survey, 31 Aug. 2022, www.usgs.gov/centers/pcmsc/science/remote-sensing-coastal-change. Accessed 13 Dec. 2024.