Are the oceans rising or the continents going down? How can we know?

Question

The century old sustained rising of oceans, at a rate of 2-4 mm/y, remains a unexplained phenomenon; there is no correlation with temperature variations, so it is not due to the thermal variation of the volume of the oceans or to a decrease of Antarctic ice (which in fact is increasing).

A possibility, given that this raise of the oceans is not absolute but relative to coast, is that it is not the oceans’ absolute level that is raising but the continents that are sinking. A possible cause is global oscillations of the mantle surface level, and tectonic plates move up and down besides drifting.  Another cause is the solidification of the mantle under continental plates due to the decrease of Earth internal temperature. What do you think? Are the oceans rising or the continents going down (in average)? Only satellite measurements can solve the riddle? 

Answer 1

To the best of our knowledge, sea-level is rising because the volume of water is increasing.

There is substantial local variation in sea-level change; it's falling in some parts of Canada. But of the dozens of controls on local and global sea-level, the net effect is currently an average global rise of about 3 mm/y. It's not really 'a riddle'. The phenomenon is well documented and there isn't too much mystery about the various causes. One could certainly debate their relative contributions.

Satellites already contribute much of the data that reveals the sea-level rise (and the relative stasis of the continents), and that substantiates some of the causes. For example, the TOPEX and Jason-2 satellites, constrained by gauges on earth, are important sources of data.

Two prominent causes of sea-level rise are thermal expansion and ice loss (for example in Greenland), but there are lots of others. Tectonic plates do definitely move up and down — it's an important phenomenon in geology and geomorphology, and it does affect sea-level in all sorts of ways (see that link).

Answer 2

While there are specific cases of continental sinking, I think this needs to answered on a global scale in which continental lithosphere is significantly more buoyant than 1) the mantle and 2) oceanic lithosphere.

For a continent to sink you either need to add mass to the continent (pushing down on the mantle causing a diverging mantle flow underneath) or you need to push mantle from underneath the continent away from internal forces. The latter case is difficult to see happening unless there is a heating gradient caused by mantle heterogeneity, there is certainly no evidence of that.

Furthermore, in regards to the effects of mantle flow on sea-level rise, I encourage you to read the seminal piece of this field, Muller et al. 2008

Mantle flow can have just as great (if not greater) effect on the sea level relative to the continent as it can on the height of continents. But because Isostasy and erosion largely governs the height of the continent, its hard to see some complex dynamic where you get global continental sinking (meaning flow is completely divergent under all continents) seems rather extraordinary and not physically consistent with our geophysical models. Subduction zones along the ring of fire certainly do their best to cycle the mantle underneath the continental lithosphere, for example.

Answer 3

Satellite measurements have been done and the sea floor is sinking relative to the continental crust so sea level SHOULD be falling. In some places the continental rise is quite extreme, the places were loss of ice has changed the isostatic balance of the continent.But sea level is not falling, its rising, so it can;t just be crustal movment.

sciencedirect.com/science/article/abs/pii/S027737910200080X

https://climate.nasa.gov/blog/3002/sea-level-101-part-two-all-sea-level-is-local/

Answer 4

Even if we disregard sinkholes and erosion, we must still consider the water underground. Massive aquifers are being drained from Iran to Kansas, USA. I see no reason why the continents might not be sinking into the sea, the deep and ancient ocean trenches filling up with silt from the continents above them. The land is certainly increasing in desertification (http://article.sapub.org/10.5923.j.ijaf.20130302.03.html) that causes more soil to flow into the seas, so the excess water from the aquifers is neither returning to the aquifers nor staying on the surface of the land. So, it might be a bit tedious, but if we could calculate how much land we are losing vs. how much water is not returning to the underground aquifers, we could know whether the continents are sinking or the seas are actually rising. I suppose we could do a census of heights above sea level and see if the average went down but we could not include places altered by construction - and that would be most places.

Answer 5

I am considering a problem that was not raised yet properly, although some has studied how can Earth size and shape change with the many phenomena that occur inside Earth. We tend to believe that continental plates are able of rising up but not the contrary, but not because of scientific reasons, and so we must be suspicious of ourselves. I think that the problem can only be solved by measuring, not by theoretical considerations. I see four possible types of measurements: surface gravity, Earth rotation, Moon-Earth distance and satellites altimetry in free orbiting. All very difficult and influenced by other phenomena. I think that VLBI measurements of Earth rotation present the clearest evidence because the already four decades of increasing rotation speed is difficult to explain without considering a decrease of Earth's equator size. Anyhow, I think we should start considering this possibility, which is not being done by now.