Sea level

Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised geodetic datumthat is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is instead a long-term average of tide gauge readings at a particular reference location.

Sea levels can be affected by many factors and are known to have varied greatly over geological time scales. Current sea level rise is mainly caused by human-induced climate change. When temperatures rise, mountain glaciers and polar ice sheets melt, increasing the amount of water in the oceans, while the existing seawater also expands with heat. Because most of human settlement and infrastructure was built in response to a more-normalized sea level with limited expected change, populations affected by sea level rise will need to invest in climate adaptation to mitigate the worst effects or, when populations are at extreme risk, a process of managed retreat.

The term ''above sea level'' generally refers to the height above mean sea level (AMSL). The term APSL means above present sea level, comparing sea levels in the past with the level today.

Earth's radius at sea level is 6,378.137 km (3,963.191 mi) at the equator. It is 6,356.752 km (3,949.903 mi) at the poles and 6,371.001 km (3,958.756 mi) on average. This flattened spheroid, combined with local gravity anomalies, defines the geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant depression in the Indian Ocean, whose surface dips as much as below the global mean sea level (excluding minor effects such as tides and currents). Provided by Wikipedia