Sea Level

Sea level is the average position of the oceans' surface relative to the land, providing a frame of reference for land elevations and ocean depths. Sea levels change over time and are affected by various factors, and these changes have a major influence on Earth's geology.

Changing Sea Level

Sea level is a major aspect of the modern world. The distinction between land and sea depends on the level of the sea. A rise in sea level means the flooding of adjacent low-lying land. A drop in sea level means exposure of some of the sea floor. Much of the world’s commerce, food supply, and recreation is associated with shallow coastal waters. The position and rate of sea level change is important to these activities, especially as construction has encroached right up to the water’s edge in many places. Sea level is most accurately defined as mean (average) sea level, which is the average height of the sea surface measured over an extended period for all conditions of tides, seasons, and storms. Land elevations are measured by reference to sea level. Nautical charts, however, use mean low water, the average position of low tide, to measure ocean depths, while mean high water, the average position of high tide, is used for marking the exposure of adjacent land areas. The reason for the use of low- and high-tide values, for depth and land, respectively, is purely practical: Successful navigation requires accurate knowledge of water depths and land exposures. By referring depths to mean low tide, ship captains can be more assured that, regardless of tide position, they know the minimum water depth available for their vessels. The same principle applies to land areas: Using a high-tide value to show land allows land users to be sure that the low-lying areas will not flood with the high-tide cycle.

The most challenging aspect of sea level is that it is not constant and can change over time. Change can be hours, days, years, or centuries. When the sea level rises and floods the land, it is called a marine transgression. The flooded land becomes part of the ocean environment, and marine sediments are deposited on what was once dry land. Marine regression, by contrast, occurs when sea level drops and the shallow sea floor is exposed. The exposed sea floor becomes part of the land environment and is subject to the same type of erosion processes that function on land. Almost every continental coastal area includes a continental shelf, or a flat-lying or gently sloping portion of the continent that extends under the sea to depths up to 150 meters over a width of several tens of kilometers to over 100 kilometers. The gentle slope of the continental shelf means that a minor rise in sea level results in a major amount of the shelf being drowned. A minor drop in sea level produces a major exposure of the continental shelf sea floor. The rocks of the continents show that over Earth’s history, sea level has transgressed onto, and regressed from, the continents many times and to many different levels.

Sea level can be changed in six basic ways: by oscillating the ocean surface; by changing the force of gravity; by moving the land up or down; by changing the characteristics of the ocean water; by changing the amount of water in the oceans; and by changing the volume of the ocean basins. The change in sea level brought about by these six ways results in two major types of sea-level change: local and eustatic. Local sea-level change means that only a specific area of coastline is involved and that coastlines relatively far away or on other continents are not changed. Oscillation of the water surface, changing gravity, and land moving up and down also produce local sea-level change. Eustatic sea-level change means that coastlines around the planet all experience a sea-level change of the same magnitude at the same time. Changing water characteristics, changing the amount of water on the planet, or changing the volume of the ocean basins produces eustatic sea-level change.

Local Sea-Level Change

Sea-level change produced by oscillation of the ocean surface refers to waves, storm surges, and tides. A wave breaking on the beach runs up the beach, then slides back down. Each wave event can be considered a short-term, low-magnitude sea-level change. In extreme cases of large waves, erosion and coastline modification can occur during each sea-level “micro-event.” Storm surges are changes in sea level brought about by the movement of surface waters under the influence of strong winds and low atmospheric pressures, such as occurs with hurricanes. Low atmospheric pressure appears to pull sea level up a small amount, though in actuality this is the result of higher air pressure surrounding the system that pushes down on the surrounding water while lower air pressure inside the system allows the interior water to rise. As for the direct effect of wind on water levels, high onshore winds can pile water up on coastal areas by 3 or 4 meters, while offshore winds can lower sea level in the same way but by lesser amounts.

Tides are produced in the oceans by the force of lunar gravity, solar gravity, and by the rotation of the Earth-moon system about its common center of mass. Coastline configuration, latitude, time of the lunar month, and many other factors control the timing and magnitude of ocean tides. Tide characteristics are extremely variable from place to place, but in a given area, the changes in sea level are quite predictable, with magnitudes from a fraction of a meter to more than 10 meters possible. Because ocean surface oscillations are common, low in magnitude, and regular, they are often not recognized as sea-level changes. They are different for every coastline, producing local sea-level change.

The force of gravity is not perfectly uniform over Earth. It depends on the amount of mass beneath the surface and thus, because of tectonics and other activities, may vary by a small amount from location to location. This variation is most marked between the continental landmasses, where there is a great deal more matter to exert gravitational force, and the ocean floors, where there is much less crustal mass. The ocean, as a fluid, responds to the force of gravity: If gravity is slightly weaker in one area than in another, the sea will rise slightly higher; if it is slightly stronger, the sea will sink slightly lower. This sea-level variation occurs only in time frames of millions of years, but it is used to explain local sea levels that are different in certain areas from predicted values.

In many areas, the land itself is moving up or down. This movement has three sources: tectonics, isostasy, and subsidence. Tectonics is the movement and distortion of Earth’s crust by convective forces generated within the magma of the mantle, influenced by the thermal activity in the planetary core. Tectonic action is responsible for the formation of volcanoes, mountains, and earthquakes. In an area where tectonic processes are active, the land may be forced up or down, causing it to rise or sink with respect to sea level. In tectonically active areas such as Southern California or in the Mediterranean basin, this type of land movement is common, and sea-level changes of many meters up and down have been historically documented. Isostasy is the vertical movement of the crust downward when a load is applied—as when a glacial mass of ice forms over the land—or its rebound upward when the load is removed. When isostasy occurs in a coastal area, the land will rise or sink, with a subsequent change in sea level. Glaciers are examples of how a load can be applied to the crust, causing isostatic subsidence. When the glacier melts, the load is removed, and the crust can isostatically rebound. This is the process responsible for the gradual rise of the land west of the Niagara Escarpment in southwestern Ontario, Canada, and of the rising east coast of Britain such that thousand-year-old coastal installations now stand as much as 400 meters above sea level. Coral atolls and volcanic islands are other examples of a load being placed on the crust, which then isostatically sinks. Land can sink beneath sea level by a process called subsidence. Subsidence is often caused by compaction of the land material, which commonly happens when oil or water is withdrawn from the ground. The loss of the fluid allows the rock to compact, and the overlying land sinks. All three of these methods of moving land up or down occur in localized areas and so result in local sea-level change.

Eustatic Sea-Level Change

Changing sea level eustatically, or worldwide, requires changes that affect the oceans as opposed to the land. The ocean basins generally have a fixed volume. If the nature of the water in it is changed, if the amount of water is changed, or if the shape of the container (ocean basin) is changed, sea level will change. The two characteristics of water that control sea level are its temperature and its salinity. For each degree Celsius that the oceans warm, thermal expansion will raise sea level 2 meters due to the lesser density (mass per unit volume) of the water. Increasing the salinity makes the water solution denser and causes sea level to fall. Sea-level variations caused by changes in salinity measure approximately 1 to 2 meters.

The amount of water available to the oceans is changed in three ways: by the growth and melting of glaciers, by steam released from volcanoes, and by water lost to the formation of hydrated crystals in sediments. Glaciation is one of the most important sea-level controls. As ice sheets grow, they are fed by evaporated seawater falling as snow; this seawater is then “trapped” as ice on the continents, and sea level falls by the corresponding amount. This is a negligibly small amount per snowflake, but over thousands and millions of years it has produced glacial coverage up to 4 kilometers in thickness in some places such as Greenland and Antarctica. If the ice melts and the water flows back to the ocean, sea level rises. During the last two million years, glaciations have come and gone at least four times, and sea level has risen and fallen over a range of approximately 125 meters. This range is enough to almost totally expose continental shelves during maximum ice advance on the continents. Eighteen thousand years ago, sea level was 125 meters below its present level, yet by three thousand years ago, it was essentially at today’s level. If the remaining ice on Antarctica were to melt entirely, sea level would rise an additional 60 or more meters, drowning coastal cities worldwide. As a result, global warming and its effect on rising sea levels became an area of critical focus in the twenty-first century.

The oceans are thought to have originated by the outgassing and release of water as steam from early volcanic activity. Volcanoes are still actively adding water to the oceans. When sediments are deposited in the ocean, they normally contain some entrapped water. Tectonic activity can return these water-bearing sediments back into the crust by the process of subduction. There appears to be a rough balance or equilibrium between water escaping the crust from volcanoes and water returning to the crust through sediment deposition, with no overall sea-level change caused by these processes in the modern world.

Sea level is also affected by changes in the volume of the ocean basins. Continents are continually losing sediments to the sea through processes of erosion. This addition of sediment is slowly filling in the ocean basins, pushing sea level up. At the same time, tectonic activity is distorting the edges of continents, often folding them, which increases the volume of the ocean basins. Tectonic activity in the ocean floor can force the sea floor upward, limiting ocean basin volume. Tectonic activities tend to occur episodically, in fits and starts, rather than as a smooth, continuous process. When tectonics is active, it may increase or decrease ocean volume, causing a drop or rise in sea level. Sediments deposited on the ocean floor may be plastered back onto the continents by tectonic activity, first decreasing and then increasing ocean basin volume. Sea-level changes, in response to sediment balance and tectonic location and magnitude, occur slowly over millions of years.

Study of Sea Level

When sea level changes, it leaves an imprint on the land. Transgressing ocean waters erode the land surface with waves and then, as waters deepen again, deposit marine sediments. When regression occurs, the shallowing water leads to wave erosion of the sea floor. Further regression exposes the sea floor to the air and to erosion by wind, rain, and running water from precipitation. A casual examination of the continents reveals clear evidence of past marine transgression and regression. Limestones, marine shales, and marine sandstones are common around the world on the dry land of the continents. They range from billions of years old to only a few hundred years old. The sea has transgressed and regressed many times in Earth’s history. The timing and exact nature of the sea-level change may be more difficult to determine. In the Bahamas, fossil corals exist several meters above sea level. By sampling the corals and dating them using trace amounts of radioactive elements in the samples, scientists can determine their age. In this case, their age is about 125,000 years. Therefore, more than 100,000 years ago, the land to which the fossil coral is attached was underwater. Did the land rise, or did the water drop? Measurement of tectonic activity shows that the islands of the Bahamas are not rising or sinking. Examination of the history of glaciation shows that 125,000 years ago, ice sheets had melted back a little more than they are presently. From this information, scientists concluded that the coral grew when sea level was eustatically higher than it is today.

Further evidence of sea-level change can be obtained by examining the tectonic, deposition, and erosion history of the planet. Examination of major episodes of tectonic activity, sediment deposition, and erosion can allow the construction of graphs showing how sea levels rose and fell, in general terms, over much of Earth’s history. Most of the sea-level changes preserved in the rock record were tectonically generated. Glaciation has occurred only a few times in the past, and the sea-level changes caused by glaciation have left a unique and distinctive record in rock structures.

If sea level never changed, the geology of all surface rocks would be very simple. There would be no exposed marine rocks at all. If there were no tectonics to help drive sea-level change, the continents would eventually erode to sea level. Earth would eventually become a flat, relatively featureless place.

The study of changing sea levels has become critical. Worldwide industrialization has greatly escalated climate change and global warming. As such, polar ice caps and other glaciers have been melting at an alarming rate, causing global sea levels to rise. Studies show that they are rising faster than they have for 2,800 years, and indeed, the rise may be accelerating over time. The National Oceanic and Atmospheric Administration (NOAA) reports that the rate of sea level rise held steady at about 1.7 millimeters annually for much of the twentieth century, but from 1993 to 2017, it sped up to 3.4 millimeters per year.

In 2013, the Intergovernmental Panel on Climate Change projected that global sea levels could rise 26 to 82 centimeters (0.85 to 2.7 feet) by 2100; however, several studies and reports published in 2016 and 2017 suggested those figures might be too low. An October 2017 Environmental Research Letters article suggested sea levels could reach as much as 200 centimeters (6.6 feet) by 2100 under a high fossil fuel–burning scenario but 55 centimeters (1.8 feet) if emissions are drastically reduced.

A 2019 report from the Organisation for Economic Co-operation and Development (OCED) concluded that annual worldwide flood damage from a 1.3-meter rise in sea levels would result in the equivalent of 4 percent of the global gross domestic product (GDP), or $50 trillion each year globally. Then, in 2021, the National Oceanic and Atmospheric Administration released a report that revealed 2021 was the sixth-warmest year on record in terms of both land and ocean temperatures. (2019 and 2020 were among the top three warmest.) The continual warming of the Earth throughout the twentieth and twenty-first centuries has caused an increased global focus on the effects that global warming will have on a steadily rising ocean.

In 2022, a study published in Nature Climate Change examined the Greenland Ice Sheet and found that climate change had caused the sheet to begin melting faster than nature could replenish. As such, the study asserted that humanity had passed a point of no return where the melting Greenland Ice Sheet would contribute to more than ten inches of sea level rise regardless of remedial action. A previous study by the Intergovernmental Panel on Climate Change (IPCC) estimated that the Greenland Ice Sheet's summer melting has increased since the 1990s to record high levels that are somewhere between two and five times higher than pre-industrial levels.

Between 1993 and 2024, the sea rose over ten centimeters. As the Greenland Ice Sheet began melting at a rate of thirty million tonnes of ice per hour, scientists predicted a total collapse was imminent. In short, coastal cities across the globe are under threat, and small island countries, such as those in Micronesia, could disappear completely under the ocean unless decisive action is taken.

Principal Terms

continental shelf: the extension of a continental mass beneath the sea; a flat or gently sloping platform usually 10 to 100 kilometers wide, extending to a depth of 100 to 150 meters

eustatic sea-level change: a change in sea level worldwide, observed on all coastlines on all continents

glaciation: commonly known as an “ice age,” the cyclic widespread growth and advance of ice sheets over the polar and high-latitude to midlatitude regions of the world

greenhouse effect: the warming of the atmosphere caused by absorption and re-emission of infrared energy by carbon dioxide and other gases in the atmosphere

isostasy: the passive, vertical rise or fall of the crust caused, respectively, by the removal or addition of a load on the crust

local sea-level change: a change in sea level only in one area of the world, usually by land rising or sinking in that specific area

mean sea level: the average height of the sea surface over a multiyear period, taking into account storms, tides, and seasons

regression: the retreat of the sea from the land, allowing land erosion processes to occur on material previously below the sea surface

tectonics: the process of origin, movement, and deformation of large-scale structures of the crust

transgression: the advance of the sea over the land, allowing marine sediments to be deposited on what had previously been dry land

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