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

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?

• You first sentence is incorrect. It is not an unexplained phenomenon. – mankoff May 18 '15 at 18:34
• Solids are essentially incompressible. If the continents were going down everywhere (that is, not a local sinking balanced by an isostatic rebound elsewhere), the excess rock would have to go somewhere. Only way I can think of to do this is with a small black hole in the Earth's core :-) – jamesqf Jul 4 '15 at 17:22
• Another cause is the solidification of the mantle under continental plates due to the decrease of Earth internal temperature This line shows that you clearly have no idea what you are talking about. The mantle is already solid and has been for billions of years. – bon May 19 '17 at 16:45

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).

• @alf regarding ice, arctic losses are greater than antarctic gains and there is a global net loss of ice. For satellite data are you looking at oceanic heat content or just SST? – casey May 17 '15 at 16:53
• @Alf What's your source for increasing Antarctic land ice? Shepherd et al. (2012) report 20-year declines in West Antarctic, Antarctic Peninsula, and Greenland ice sheets, equivalent to 0.59 ± 0.20 mm/yr sea level. Possible slight increase in East Antarctic, but it's more than cancelled out by declines in the western and peninsular ice sheets -- and the Greenland decline dwarfs them all. – Pont May 17 '15 at 19:46
• @Alf Antarctic sea ice is increasing. Like the arctic, this has minimal impact on sea level. Antarctic land ice is drastically decreasing and adding to sea level. – mankoff May 18 '15 at 18:36
• I don't get why you think a long-term increase rules out a climatic cause. "Correlations" with temperature records aren't really useful here without filtering; day-to-day temperature fluctuations don't drive day-to-day sea surface fluctuations, but year-to-year fluctuations certainly do. If both the long-term (decadal) temperature is trending up and the sea level is trending up, then they're well correlated. Can you explain why you think this is ruled out? – Jareth Holt Jul 4 '15 at 20:40
• @Alf: Theories were made predicting a cooling of the earth due to sulfate particles and a minimum in solar output. Those predictions didn't come true; what we predict has no influence over what actually happens. And yes, sea level rise is a multiple-decade process (I hesitate to say that it's century-scale only). But human emissions of CO2 started accelerating more well over a century ago. That's still consistent with CO2 driving temperature and temperature driving sea level rise through thermal expansion, even with decades of lag between forcing and response. – Jareth Holt Jul 6 '15 at 14:39

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.

• in my question I already present sound possible explanations for continents being sinking. There are more possible causes, but those are the ones I see as more important. Note one thing: the relative level of oceans is not rising everywhere - the "rising of oceans" is just the average. Furthermore, the distribution of continents is far from uniform; is case of mantle oscillations, the relative level of oceans will vary in average and with a long period - centuries or more. And as the mantle is fluid, it necessary has oscillations, as it has currents. – Alf Jul 7 '15 at 22:13

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.

• I'm wondering if you can calculate it, to be honest. The effect would be negligible. The oceans are not rising because the trenches are being clogged up and water is flowing there from the aquifers. – Gimelist Jan 30 '16 at 7:32
• The effect of changing groundwater storage has been determined gravimetrically (by satellite). It is NOT negligible, but it is certainly much less than either glacial melt or thermal expansion. – Gordon Stanger Oct 30 '16 at 8:53

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.

• Your argument sounds reasonable enough from a but each of your examples should probably be separate questions, not an answer to your question. The Earth-Moon system for example, while there may be some fluctuation effect, the Earth-Moon system hasn't measurably changed in the last 60 years. Rotation speed has changed only the tiniest fraction of a second, that slowdown might be enough to lower the equator and raise the polls about the depth of a few millionths of a meter. Each of your points is worth asking but I strongly suspect, none of them have the effect your going for. – userLTK Feb 1 '16 at 3:03
• Unless I've misunderstood, one of the four primary plate actions taught in high school is subduction, which is the decline\destruction of a plate going beneath another. Examples include the Pacific Plate near Alaska and the Cascadia Zone in the NW US. So you can't say we don't consider the contrary at least. As someone suggested, the biggest issue if the plates sink is it has to go somewhere. So you'd have to see ocean floor rise rise (or rock density increase??? But since you propose water volume is unchanged, the water can still only cover the same total room, so that fails to explain it) – JeopardyTempest Oct 31 '16 at 23:50
• I'd be quicker to question the sealevel conclusions themselves (since you'd need quite comprehensive global data to make such planetary claims) than the geologic theories. Though I'm not certainly not versed with such measurements. But perhaps you should include the basis of those sea level claims in your question :-) – JeopardyTempest Nov 1 '16 at 0:02