According to measurements in coastal areas and with sea-based buoys, the sea level rose by 1 to 2 mm per year since 1900. Especially between 1930 and 1960 and in the past decade the increase was relatively large. Satellite measurements since 1993 show a global average sea level rise of approximately 3 mm per year, with strong regional variations between -20 up to +20 mm per year (Figure 14). These local variations are due to changes in circulation patterns in the air and in the oceans. It is currently not clear to what degree the observed acceleration of sea level rise in the past 13 years has been caused by the enhanced greenhouse effect.

Between 1993 and 2004, the sea level in the northeastern part of the Atlantic Ocean (including the North Sea) also increased approximately 3 mm per year. From 1900 on, the sea level rise in the North Sea was on average 2 mm per year, or 20 cm in the 20th century (Figure 16). The water level in the North Sea is affected by the sea level rise in the Atlantic Ocean, but also by changes in the wind and current driven pressure exerted on the water column.

The total impact along the Dutch coast (= relative sea level rise) is larger, since subsidence of the ground level also has to be taken into account (Figure 15). During the 20th century this subsidence in the Netherlands varied on average between o and 4 mm per year, depending on the exact location.