Why is the sea level rising in our regions while it is falling on the coasts of America?

Answer

Dear Edouard

By the coasts of America you probably mean the coasts of North America.

The Permanent Service for Mean Sea Level (PSMSL) organization collects local sea level data (data from the 19th century) through a global network of tide gauges.^e century) and on this basis calculates local trends of relative increase or decrease. This site offers a very interesting interactive map. On the coasts of Alaska and Canada, a trend of sea level fall is indeed observable. However, that is not the case for the rest of the United States. There a general increase of about 2 to 6 mm per year is observed. That’s strange, so how can that be?

It is best to set the interactive map for the period from eg 1980-present. Then there are more data points than before 1900-present.

The many red arrows on the map show that the global sea level is rising (about 3-5 mm per year). However, around the northernmost coastlines of the Northern Hemisphere, the (relative) sea level is falling in most places (blue arrows). The reason is that the continental crust there has been rising for more than 10,000 years, causing the sea level to fall relatively despite the global rise.

This recoil of the continental crust (or better, the lithosphere) is a result of the last ice ages, or the periods of about 100,000 years in which large ice sheets were located on Canada, Scandinavia and Siberia. The Greenland Ice Sheet is a last remnant of the last Ice Age. Due to the enormous mass of those ice caps, the earth below has collapsed to more than 1 kilometer. From the moment the ice sheets started to melt (between 20,000 and 8,000 years ago), the Earth’s crust began to spring back. This is called post-glacial rebound, or post-glacial uplift. As a result, uplift up to about 1 cm per year is still taking place around Alaska, Canada and the Baltic Sea. There the amount of post-glacial uplift is therefore greater than the general sea level rise, which is mainly the result of expansion of the oceans (due to warming of the sea water) and the melting of glaciers and thus the sea level there falls. There are also other local or regional factors involved in the sea level trends (gravity differences, oceanic currents, etc.), but this is the general picture.

Whoever owns an island on the Baltic coast of Sweden, for example, will not regret this at all, because the island gets a little bigger every year – unless it grows attached to the island of an unsympathetic neighbor of course…