sea level rise
Sea level rise is a relative or absolute rise in sea level. Absolute sea level rise is actual rise. Relative rise is the sea level rise in relation to the bottom height. Absolute sea level rise is therefore a global phenomenon, while relative sea level rise is mainly a local/regional phenomenon.
Land subsidence is occurring in (the west of) the Netherlands. If the sea level rises while the bottom is falling, the relative rise is greater than the absolute rise. In some places in Scandinavia the bottom is rising faster than the sea level. In those places the relative sea level falls while the absolute sea level rises.
Determination of sea level rise
Before the advent of satellites, sea level changes were measured using gauges. This allows only relative changes to be determined. In order to also measure absolute changes, data on land movements are needed. Since about 1990, changes in sea level elevation can be accurately determined using satellites such as Envisat. Based on these measurements and physical and geological principles, computer simulations can be set up to predict future changes.
Sea level through the ages
Excavations in which old coastal areas can be found, among other things, have shown that the height of the sea level has always been subject to changes. Until 25 thousand years ago, the height of the sea level is known with reasonable certainty. Until 20 thousand years ago, it was about 120 m lower than today. Between 20 thousand years ago and the end of the last ice age (which ushered in the Holocene), a strong increase of up to 10 mm per year is visible. After that, the increase is noticeably slower with an average of 0.1 to 0.2 mm per year. It is difficult to determine the magnitude of the global fluctuations in this period, but level changes of up to 30 cm cannot be ruled out. From about 1850, however, there is a noticeable rise in temperature again, causing the sea level to rise about 20 cm. In the 20th century, the sea level rose by about 1.74 mm/yr, although it should be noted that the rise in the second half of the century (1.45 mm/yr) was lower than in the first half (2.03 mm/yr). )
. Sea level rise estimates using satellite altimeter measurement since 1993 are in the range of 2.9-3.4 mm/year.
Global warming causes sea level rise through two mechanisms:
Water expands when heated and as a result the sea level rises automatically; given the average ocean depth of 3790 meters, this means less than a meter rise in level per degree Celsius;
The melting of land ice means more water is entering the oceans; for example, all the ice in Greenland distributed over the oceans would account for seven meters of water; a theoretical situation where all the ice in Antarctica would melt would give a rise of 61 meters. However, the actual annual contribution of the melting of ice on Greenland to sea level rise is small: about 0.1 mm. A side effect of the abundant pumping of groundwater that should not be underestimated also contributes to the rise of the sea level. Water that has been locked up for a very long time in deep underground aquifers is used to irrigate fields. This water partly evaporates, another part runs off. This water enters the above-ground water cycle and also increases the water volume in the oceans.
On the other hand, the massive construction of reservoirs has the opposite effect. Rainwater that would otherwise disappear into the oceans is retained by dams on land, reducing the volume of water in the oceans. If all reservoirs on Earth were emptied, the sea level would rise by 3 cm.
About the height of the sea level rise in