The level of the sea is constantly changing everywhere on Earth as tides rise and fall in predictable patterns. Both the timing and height of high tides are forecast accurately because they depend on the relative positions of the Earth, the Moon, and the Sun. However, tides are just variations on average water levels – they are not changes in average water levels.
Since about 1900, sea levels have risen by about 20 centimetres. There is a strong consensus among scientists that rising sea levels are largely a consequence of increasing concentrations of carbon dioxide and other greenhouse gases in the atmosphere raising global temperatures.
Three processes drive this rise in sea level:
- expanding sea water
- melting glaciers
- shrinking ice sheets.
Although expanding seawater and melting glaciers can be modelled with high confidence, there are still big questions around how the massive ice sheets that cover Antarctica and Greenland will react.