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 level rise from satellite altimetry have been estimated to be per year for the period 1993–2017. Earlier satellite measurements were previously at odds with tide gauge measurements. A small calibration error for the Topex/Poseidon satellite discovered in 2015 was identified as the cause of this mismatch. It had caused a slight overestimation of the 1992–2005 sea levels, which masked the ongoing sea level rise acceleration. With satellites it is possible to capture regional variations in sea level rise well. In the 1993–2012 period for instance, sea level rose substantially in the western tropical Pacific. The sharp rise in this area