Sea Level

I

Introduction

Sea Level, or mean sea level, the average height, or elevation, of the sea surface (see Ocean and Oceanography). Scientists measure sea level in different locations over a long period of time to eliminate temporary changes in water level due to tides, waves, wind, and air pressure.

Geographers use sea level as the starting level for measuring the height, or elevation, of Earth’s surface. However, over long periods of time and great distances, the level of the sea changes. The science of measuring land features in relation to one another and in relation to sea level is called surveying.

Surveyors have established that the highest location above sea level is 8,850 m (29,035 ft) on the summit of Mount Everest in Nepal, and the lowest depth is 11,033 m (36,198 ft) beneath the Pacific Ocean at the bottom of the Mariana Trench. The lowest elevation on the continents is 408 m (1,340 ft) below sea level at the Dead Sea between Israel and Jordan.

II

Measuring Sea Level

The level of the sea is continually changing. In a matter of seconds, a wave can raise and then drop the level of the sea by several meters. In a matter of hours, the tides can also raise or lower the level of the sea, as can changes in air pressure or wind. In order to average out the fluctuations caused by waves, tides, weather, and wind, periodic measurements of sea level over at least 19 years are averaged to determine the mean sea level.

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The sea level of a particular place is measured using tide gauges. A tide gauge is a device built to measure water-level variations due to tides and weather and to eliminate effects due to waves. A tide gauge can be as simple as a long ruler nailed to a post on a dock. More sophisticated instruments are usually placed in a stilling well, or pipe, that protects a float connected to a recording device from waves. As tides rise and fall, the float’s motion is recorded.

Sea level can also be measured from space. The TOPEX/Poseidon satellite, launched in 1992, measured sea level and was also used to map the ocean floor. As TOPEX/Poseidon orbited Earth, an altimeter bounced radar signals off the ocean’s surface. The altimeter recorded the time it took for the radar signal to return to the satellite and that gave a precise measurement of the distance between the satellite and the sea surface. Measurement from orbit is the only way to assess sea level independently of land-level changes. TOPEX/Poseidon served as the primary means for monitoring the oceans until it was supplanted by the more accurate sea level monitoring satellite Jason-1, launched in 2001. NASA launched the more advanced Ocean Surface Topography Mission/Jason-2 (OSTM) satellite in 2008.

III

Historic Changes in Sea Level

The existence of sunken ruins of Roman ports in the Mediterranean Sea appears to indicate that the sea can rise up and flood coastal areas. However, it could also be claimed that sea level stayed constant and the ports sank. Since both the sinking of land and the rising of the sea result in coastal flooding and the appearance of a rise in sea level, scientists separate global trends, called eustatic changes, from regional trends. The changes in sea level at any particular location are the sum of the eustatic and regional effects. Consequently, local sea level can be rising in some places and falling in other places at the same time.

Sometimes, the change in a local sea level can be abrupt. Earthquakes can cause land to rise or fall suddenly with respect to the sea. For example, an earthquake in 1899 in Alaska raised parts of the coast by as much as 14 m (46 ft).

Sometimes, the change in sea level can be slow and gradual. The pumping of water or oil from the ground can lead to the gradual sinking of the ground. For example, parts of the Imperial Valley in California have dropped more than 8 m (26 ft) as groundwater has been withdrawn for irrigation. The withdrawal of drinking water and oil along the coast of Texas caused some of the land around Galveston Bay to drop by as much as 2.5 m (8 ft) over several decades. This drop in land led to the flooding of entire neighborhoods.

Some changes in sea level, called isostatic changes, occur due to large amounts of weight being loaded or unloaded onto a region, which causes the land to sink or rise. For example, when a river delta forms, the weight of the accumulating sediments can lead to subsidence, or a sinking of the land. The Louisiana coast is sinking up to 10 mm (0.4 in) per year due to the growing weight of the Mississippi River Delta. Similarly, the removal of a large weight can cause the land to rebound. The Hudson Bay shoreline in Canada is rising out of the sea at about 10 mm (0.4 in) per year because of the recent melting of glaciers that were weighing it down.

Eustatic sea-level changes affect all the shorelines of the world simultaneously. During ice ages, glaciers grow, transferring water from the ocean to snow and ice on land. Such a transfer caused the eustatic sea level to fall 120 m (390 ft) 21,000 years ago, when the glaciers of that ice age were at their greatest extent, and to rise back to the present elevation when most of the ice melted.

Sea-level changes following an ice age cause profound changes in the shape of shorelines. At times of lower sea level, land bridges formed between Alaska and Asia and between England and France. These land bridges permitted plants and animals to migrate. The shorelines of the world have retreated from the edge of the continental shelf to their present location as the sea level has risen. In broad coastal plains, for every meter that sea level rises, the shoreline typically retreats about one kilometer.

IV

Current Concerns

Many scientists are concerned that human activities could be causing sea levels to rise. Effects would include damage to property, increased threats from storms and flooding along the coastlines, and possible large-scale evacuation of some populations living on islands or in low-lying land areas. As cars and factories burn fossil fuels, they emit carbon dioxide and other gases that may lead to an increase in the world’s average temperature. The temperature increase, called global warming, is caused by what scientists call the greenhouse effect. The greenhouse effect permits sunlight to penetrate to Earth’s surface but restricts the escape of heat. As the planet warms, glaciers, the Greenland ice cap, and parts of the Antarctic ice sheet may melt, causing a eustatic rise in sea level, or a rise in sea level around the world, in addition to a rise in sea level from the thermal expansion of the oceans. On the other hand, warming may cause greater ocean evaporation and snowfall, possibly offsetting some of the rise in sea level.

Because of the many factors that affect sea level and global temperature, and the scarcity of long-term measurements from all parts of the world, scientists are uncertain about past changes. They are even more uncertain about future changes in climate and sea level. Scientists use computer models linking greenhouse gas concentration in the atmosphere with global temperature and sea level to project future conditions.

In 1988 the United Nations and the World Meteorological Organization created the Intergovernmental Panel on Climate Change (IPCC), a panel of more than 200 renowned earth scientists to study climate change. In 2001 it reported that the planet warmed by between 0.4° and 0.8°C (0.7° and 1.4°F) over the course of the 20th century, and that global sea level increased 1 to 2 mm (0.04 to 0.08 in) per year. An IPCC report issued in 2007 revised the figures for recent decades, indicating that the global average sea level had risen since 1961 at a rate of 1.3 to 2.3 mm (0.05 to 0.09 in) per year and since 1993 at a rate of 2.4 to 3.8 mm (0.09 to 0.15 in) per year. The eustatic increase in sea level was caused by the melting of glaciers, ice caps, and ice sheets; the thermal expansion of the ocean as surface water warmed; and the withdrawal of groundwater from deep aquifers on land.

The 2007 IPCC report predicted a maximum likely global temperature rise between 2.4° and 6.4°C (between 4.3° and 11.5°F) by the year 2100. If this maximum increase occurs, the IPCC predicted the sea level could rise by between 26 and 59 cm (between 10 and 23 in) by the year 2100. However, a meeting of climate change experts in Copenhagen in 2009 said the IPCC estimate did not factor in all the effects of melting ice and predicted a sea level rise of more than 1 m (40 in) by the end of the 21st century. See also Meteorology: Human Induced Global Warming.

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