Since at least the start of the 20th century, the average global sea level has been rising.
Mean sea level is an average level of the surface of one or more of Earth's oceans from which heights such as elevations may be measured.
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Between 1900 and 2016, the sea level rose by 16–21 cm.
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More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm from 1993 to 2017, which is a trend of roughly 30 cm per century.
In the context of spaceflight, a satellite is an artificial object which has been intentionally placed into orbit.
Radar is an object-detection system that uses radio waves to determine the range, angle, or velocity of objects.
This acceleration is due mostly to human-caused global warming, which is driving thermal expansion of seawater and the melting of land-based ice sheets and glaciers.
Global warming and climate change are terms for the observed century-scale rise in the average temperature of the Earth's climate system and its related effects.
An ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2.
Thermal expansion is the tendency of matter to change its shape, area, and volume in response to a change in temperature.
Between 1993 and 2018, thermal expansion of the oceans contributed 42% to sea level rise; the melting of temperate glaciers, 21%; Greenland, 15%; and Antarctica, 8%.
Greenland is an autonomous constituent country within the Kingdom of Denmark between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago.
Climate scientists expect the rate to further accelerate during the 21st century.
Projecting future sea level is challenging, due to the complexity of many aspects of the climate system.
Climate is the statistics of weather, usually over a 30-year interval.
As climate research into past and present sea levels leads to improved computer models, projections have consistently increased.
For example, in 2007 the Intergovernmental Panel on Climate Change projected a high end estimate of 60 cm through 2099, but their 2014 report raised the high-end estimate to about 90 cm.
The Intergovernmental Panel on Climate Change is a scientific and intergovernmental body under the auspices of the United Nations, set up at the request of member governments, dedicated to the task of providing the world with an objective, scientific view of climate change and its political and economic impacts.
A number of later studies have concluded that a global sea level rise of 200 to 270 cm this century is "physically plausible".
A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of approximately 2.3 meters over a period of two millennia: an example of climate inertia.
Climate inertia is the phenomenon by which climate systems show resistance or slowness to changes in significant factors, such as greenhouse gas levels.
The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units.
The sea level will not rise uniformly everywhere on Earth, and it will even drop in some locations.
Local factors include tectonic effects and subsidence of the land, tides, currents and storms.
Subsidence is the sudden sinking or gradual downward settling of the ground's surface with little or no horizontal motion.
Tectonics is the process that controls the structure and properties of the Earth's crust and its evolution through time.
Sea level rises can influence human populations considerably in coastal and island regions.
The coast, also known as the coastline or seashore, is the area where land meets the sea or ocean, or a line that forms the boundary between the land and the ocean or a lake.
Widespread coastal flooding is expected with several degrees of warming sustained for millennia.
Coastal flooding occurs when normally dry, low-lying land is flooded by seawater.
Further effects are higher storm-surges and more dangerous tsunamis, displacement of populations, loss and degradation of agricultural land and damage in cities.
A tsunami, also known as a seismic sea wave, is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake.
A dam is a barrier that impounds water or underground streams.
Natural environments like marine ecosystems are also affected, with fish, birds and plants losing parts of their habitat.
An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system.
Marine ecosystems are the largest of Earth's aquatic ecosystems and are distinguished by waters that have a high salt content.
Societies can respond to sea level rise in three different ways: to retreat, to accommodate and to protect.
Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made among different strategies.
Ecosystems that adapt to rising sea levels by moving inland might not always be able to do so, due to natural or artificial barriers.