Rising Sea Levels is an increase in sea level.
Multiple complex factors may influence such changes.
Sea level has risen around 130 metres (400 feet) since the peak of the last ice age about 18,000 years ago. Most of the rise occurred before 6,000 years ago. From 3,000 years ago to the start of the 19th century sea level was almost constant, rising at 0.1 to 0.2 mm/yr.
Sea level rise can be a product of global warming through two main processes: expansion of sea water as the oceans warm, and melting of ice over land. Global warming is predicted to cause significant rises in sea level over the course of the twenty-first century.
Local and eustatic sea level
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
Eustatic change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
Short term and periodic changes
There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level.
Longer term changes
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the volume of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice.
Over much longer (geological) timescales, changes in the shape of the ocean basins and in land/sea distribution will affect sea level.
Glaciers and ice caps
Each year about 8 mm (0.3 inches) of water from the entire surface of the oceans goes into the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 8 mm every year. Although approximately the same amount of water returns to the ocean in icebergs and from ice melting at the edges, scientists do not know which is greater the ice going in or the ice coming out. The difference between the ice input and output is called the mass balance and is important because it causes changes in global sea level.
Ice shelves float on the surface of the sea and, if they melt, to first order they do not change sea level.
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