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- For the effects of global warming on sea levels, see global warming
The sea level is the average height of the ocean (informally called the sea). The word 'average' must be used because the height of the sea changes with the tides. The height of mountains, countries, and so on, is almost always given as "above sea level".
MSL is a type of standardised geodetic reference point. It is used, for example, as a geodetic datum in cartography and marine navigation. In aviation it is the standard sea level at which atmospheric pressure is measured. This is used to calibrate altitude, which influences aircraft flight levels. A common mean sea-level standard is the midpoint between a mean low and mean high tide at a particular place.
Sea levels can be affected by many factors. They have varied greatly over geological time scales. The careful measurement of variations in MSL offers insights into ongoing climate change. The present slight rise in sea levels is offered as proof of ongoing global warming.
In Earth's long history, the continents and sea floor have changed due to plate tectonics. This affects global sea level because it alters the depths of various ocean basins, and also changes the distribution of glaciers.
Over most of geologic time, the long-term mean sea level has been higher than today (see graph). Only at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today. This included the Permian–Triassic extinction event, though this may not be relevant to the concept of sea level.
Rapid changes may happen by huge lakes breaking through into seas. This has happened on a number of occasions. When the latest ice age was ending, melting caused huge lakes in central North America. This eventually broke through into the Atlantic. Melting ice in the North Sea are also broke through into the English Channel. The largest known example of marine flooding was when the Atlantic breached the Strait of Gibraltar about 5.2 million years ago. This restored Mediterranean sea levels, which had dried up.
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