Sea level the warmer water expands. For at least the last 100 years, sea level has been rising at an average rate of about per year. Most of this rise can be attributed to the increase in temperature of the sea and the resulting slight thermal expansion of the upper of sea water. Additional contributions, as much as one-quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human uses. Pilots can estimate height above sea level with an altimeter set to a defined barometric pressure.
Sea level rise satellites have been recording the change in sea level. Those satellites can measure the hills and valleys in the sea caused by currents and detect trends in their height. To measure the distance to the sea surface, the satellite sends a microwave pulse to the ocean's surface and records the time it takes to return. A microwave radiometer corrects any delay that may be caused by water vapor in the atmosphere. Combining this data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimeters (about one inch). Current rates of sea