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 if several degrees of warming are sustained for millennia. For example, sustained global warming of more than 2 C° relative to pre-industrial levels could lead to an eventual sea level rise of . Societies can respond to sea level rise in three different ways: retreat, accommodate and protect. Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made between different strategies. Ecosystems that adapt to rising sea levels by moving inland might not always be able to do so, due to natural or man made barriers. Understanding past sea level is important for