Height above sea level is the elevation of land above the local mean sea level. Sea level is used as the base level for measuring depth and elevation on Earth. Temperature, gravity, winds, currents and river discharges affect sea levels.
Local mean sea level is determined by taking hourly measurements of sea levels over a 19-year period and then averaging the measurements. Global warming is melting glaciers and ice sheets, causing an elevation in sea levels, explains National Geographic. Sea level rise may threaten low-lying nations, such as Comoros and Maldives. New York City and Mumbai, which are coastal cities, also must prepare for high sea levels.