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 challenging, due to our imperfect understanding of many aspects of the climate system. As climate research leads to improved computer models, projections have consistently increased. For example, in 2007 the Intergovernmental Panel on Climate Change (IPCC) projected a maximal rise of through 2099, but their 2014 report raised the high-end estimate to about . A number of later studies have concluded that a global sea level rise of this century is "physically plausible". The contributing factors to sea level rise between 1993 and 2018 are thermal expansion of the oceans (42%), melting of temperate glaciers (21%), Greenland (15%) and Antarctica