In the mainstream media the problem of ocean level rise is presented rather... over-simplistically - ice melts -> more water flows into the oceans -> water level rises -> continents sink.

But it is in reality a complex system, it is not like pouring water in a glass... or a lake that resides on a continental plate.

Both continental and oceanic plates float on top of the mantle as both are lighter and lower density, and both exert weight and compress the mantle.

Therefore, if water is removed from continental plates by means of melting ice, the continental plate gets lighter. And when that water migrates to the ocean, the oceanic plate gets heavier. And since for every action there is equal in force and opposite in direction reaction, it seems logical that the now heavier oceanic plate will actually push the now lighter continental plate up, to some extent compensating for or possibly even reversing the change in ocean level relative to the continental plate.

So, do the estimates account for the dynamic properties of that complex system?

  • $\begingroup$ They are relative sea level rise. $\endgroup$ – arkaia Jun 12 '18 at 13:49
  • $\begingroup$ @arkaia well obviously they will be relative to the continental plates in any case, that doesn't in any way postulate that they factor in the continental plate rising as well. $\endgroup$ – dtech Jun 12 '18 at 13:53
  • $\begingroup$ They do. That is the point of it being relative. $\endgroup$ – arkaia Jun 12 '18 at 15:51
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    $\begingroup$ What you've overlooked are the different timescales involved. The ice melting happens over a few hundred years or so, the isostatic rebound from the melting ice happens over ~10,000 years. Many northern hemisphere areas that were glaciated during the last Ice Age are still rebounding: en.wikipedia.org/wiki/Post-glacial_rebound $\endgroup$ – jamesqf Jun 12 '18 at 19:14

Sea level rise estimates use changes in relative mean sea level. The definition being:

Relative sea level is the sea level related to the level of the continental crust. Relative sea level changes can thus be caused by absolute changes of the sea level and/or by absolute movements of the continental crust.

In general, Mean Sea Level means the 'water level when it is still' (without waves). MSL is averaged over a determined period of time, such as a month or a year and it is measured relative to a fixed mark on the land (benchmark).

As MSL is measured relative to land, it can change due to changes in water volume or due to land movements. That is why we usually refer to it as 'relative sea level'.

Long-term changes of sea level are called "secular" changes and include isostatic changes. The main reason for isostatic changes at a global scale is the Glacial Isostatic Adjustment (GIA).

The body of scientific work related to the contributions by different factors is extensive. For instance, I like Cazenave & Nerem, 2004. Also, there is a lot of information about the relative contributions of each.

contributions Source: JPL-NASA

There is a lot more information about the regional distribution of sea level rise in: How will sea level rise be distributed across the globe?

Also, here is a good simple explanation of the factors affecting sea level.


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