Sea level rise

The sea level is defined by the height of the calm sea surface in relation to a horizontal standard level. The Normal Amsterdam Water Level (NAP in Dutch) is used as this standard level. However, as you may have read in the article on the Normal Amsterdam Water Level, the sea level is not exactly equal to the Normal Amsterdam Water Level. However, since the difference is so small, we assume them to be equal.

Not horizontal

The sea level itself is also often used as a standard surface. We say that something lies ‘above or below sea level’. However, the use of the sea level as a standard surface for determining height has a major disadvantage, since the sea level is not completely flat: it has a small inclination. This means that the sea level is higher in some places than others. The inclination can have different causes, such as the tide, the carioles force and atmospheric influences. The tides influence the sea level because the ebb and flood tides do not necessarily take place at the same time along the Dutch coast.

Another cause of the small inclination of the sea level is the so-called carioles force. The carioles force is, simply put, a force which originates at the earth’s surface due to the turning of the earth. Because of this, a moving subject on the Northern Hemisphere may turn to the right, whereas a moving subject on the Southern Hemisphere may turn to the left. With flowing water, the carioles force causes a small inclination in clockwise direction, resulting in an uneven sea surface.

The atmospheric influences may also influence the sea level. Atmospheric influences include air pressure and wind. During a heavy storm for example, the sea level is not flat.

Rise of the sea level

The sea level has risen more than 120 metres since the peak of the last ice age 18,000 years ago. The bulk of that occurred earlier than 6000 years ago. From 3000 years ago to the start of the 19th century, the sea level has almost remained constant, rising at only 0.1-0.2 mm per year. Since 1900 the level has risen at 1-3 mm per year, and since 1992 satellite altimetry indicates a rate of about 3 mm per year.

Read more:

An Overview of Coastal Land Loss: With Emphasis on the South Eastern United States. Accessed on February 14, 2005.
Changes in the Earth's shorelines during the past 20,000 years caused by the deglaciation of the Late Pleistocene ice sheets (, from the Permanent Service for Mean Sea Level
Includes pictures of sea level for past 20,000 years based on Barbado's coral records (
Global sea level change: Determination and interpretation (
Sea level rise FAQ (1997)
The Global Sea Level Observing System (GLOSS) (