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 studying the state of frozen water and the ocean on our planet factor in ongoing deformations of the solid Earth, in particular due to landmasses still rising from past ice masses retreating. Additionally, Earth gravitation and rotation have to be accounted for. These factors are dependent on the different layers which make up the Earth (lithosphere, asthenosphere), and the order in which land-based ice melts. Because the involved processes, which are collectively known as the "Sea-level equation", change very slowly, on time scales of thousands of years, they are considered to be constant. Since the 1992 launch of TOPEX/Poseidon, altimetric