The boundary of where ocean meets the land is used to determine sea level. Sea level can vary across the globe some depending on factors such as the air pressure, ocean/wind currents, ocean density, minor gravity variations and tides. These factors are averaged out to develop sea level. Once this is determined, then a change in this value can be used to determine if global sea level is changing. This writing goes over two important influences that can change sea level.

When the temperature of water increases, the molecular motions increase. These more rapid collisions cause the water to expand. Water does not expand nearly as much as air, but given the enormous amount of water in the ocean, even a small density change produces a large change in the volume of the water. Water expands when warmed and contracts when cooled. One consequence of global warming is that liquid water will expand. This expansion will contribute to raising sea level. Other factors such as glacier melting is much more significant to sea level rise, but thermal expansion does increase sea level by several inches and a greater warming would contribute to an even greater increase.

Ice within water displaces the water but since most of the ice is underwater, the melting of this ice will not contribute much to sea level rise. Melting ice over land though will aide significantly in sea level rise since this water all adds to the amount of water in the ocean. The melting of continental ice sheets and glaciers such as in Greenland, Alaska and Antarctica will contribute to sea level rise. Significant melting of these ice sheets and glaciers can raise sea level by several meters. It is difficult though to melt large portions of the Antarctic sheet since much of it is right over the South Pole. Even with warming global temperatures, snow will continue to add to the Antarctic ice sheet. Portions of the ice sheet that are further from the pole though are more susceptible to breaking off and floating out to sea to melt. The Antarctic ice sheet is an area of ongoing research.