A bunch of stuff I'll bet you never knew about sea level

You lose.
I’ve known about orthometric vs. geodetic heights for some time, but then, as a sea surveyor for some 35 years, so I should.
Some others above have mentioned actual sea level measurement, so here goes.
I had a chat at a trade show to some chaps from the Proudman Oceanographic Lab (one of the froremost labs in the world for tidal matters) about their charts displaying how sea level is rising due to Global Warming and asked them how they measured it. The answer came that they took them from a network od tide gauges, as I’d expected. Where were they? I queried? Placed like Sydney, Hawaii, Cape Town, Bombay, etc. So, on the forearc of growing mountain ranges, a bloody great shield volcano, etc., etc.?
In the UK, Land Levelling Datum is at Newlyn, in the southwest. By carefull levelling, this level has been carried accross the country. You use it to put in a tide gauge. When you’re doing surveys for a navigational chart, though, your average seafarer couldn’t give a monkey’s about Newlyn, he wants to know how much water he’s got under his keel. So you usually choose a zero datum such that the sea level will almost never get that low, and call it Chart Datum. Then you work out the relation between that and the land levelling datum, so that you can relate stuff on land to the water. Clear so far?
OK, how was the Newlyn Datum determined? By years of careful observations of Sea Level! So land levels are determined from sea level, and sea levels from land levels. ???
In addition to which, only about 18,000 years ago there was a couple of miles of ice sitting in the centre of Great Britain, which pushed it down. As the Earth has about the consistency of jelly at that scale, the land arond the edges bulged up in a Periglacial Bulge. Quite suddenly, the ice melted, so the Pennines are now rising and the periglacial bulge (including Newlyn) is sinking. Or would be if the whole island wasn’t also tilting to the south east.
That’s the UK, with probably the oldest records and expertise in tidal and sea level measurement. The rest of the world is later, with less well understood data.
You still want to believe in global sea level rises at all, let alone a few millimetres per century?

eo says:
November 27, 2013 at 12:07 am
Not so, the rotation is (more or less) constant, so Newton’s First Law of Motion applies.

Yes, Birdieshooter, thanks. We can see the details in the arrows and the trend values. http://tidesandcurrents.noaa.gov/sltrends/ is definitely a good map by NOAA’s Tides&Currents.

That’s the UK, with probably the oldest records and expertise in tidal and sea level measurement. The rest of the world is later, with less well understood data.
You still want to believe in global sea level rises at all, let alone a few millimetres per century?
I don’t think things are quite as bad as you make them out to be. People have been sailing the world for hundreds of years, and there is reasonably reliable tide gauge data from all over the world going back well over 100 years. The tide gauges, it is true, won’t reflect coastal uplift, but on average one expects uplift and sinking to about balance — some places may rise, but others will fall. Even without any effort made to identify those places/trends by other means, I rather trust the average of tide gauge data (more than I trust satellite data, actually — to many modern apples being compared to historical oranges).
It isn’t a few millimeters a century, either — it is a few millimeters a year, 2-3 cm/decade, some 20 cm/century. Again, the tide gauges might or might not do a good job of resolving a year’s changes on a scale of mm, but 2 cm is pretty hard to miss and 2 cm plus or minus 2 mm pretty easy to believe. That’s the kind of result summarized here:
http://en.wikipedia.org/wiki/File:Trends_in_global_average_absolute_sea_level,_1870-2008_%28US_EPA%29.png
One can worry about whether or not the satellite numbers in the last 15 years of this figure (that doesn’t seem to extend to 2013 for some reason) are comparable, and as has been pointed out in other posts/threads, some of the tide gauges used are probably rejectable on the basis of egregious coastal changes as opposed to actual SLR, but there is little doubt that the tide gauges themselves show a total SLR of around 9 inches over 140 years, and there isn’t any particularly good reason to doubt that the sea level is currently continuing to rise at a rate of around 3 mm/year. That rate is hardly unprecedented in the tide gauge data — there are steeper rises in the 1930-1950 early 20th century warming (that was almost certainly not driven by CO_2) — and it is not, actually, alarming. It is boring. If SL continued to rise at 3 mm/year for the rest of the century nobody would, or should, much care, any more than there has been any panic caused by a rise of about that rate in the last century. Coastal changes due to other factors (such as hurricanes or changes in tidal current patterns or due to silting) are usually a lot more important and happen a lot faster.
In a sense, whether the isostatic level of “the sea” is rising is irrelevant anyway. What we care about is whether or not it is rising on the coasts, and coastal gauges are a direct measure of this. What satellites show about the ocean bulging (or not) in the middle of the Atlantic is almost irrelevant.
The major fraction of SLR comes not from land-based ice melt, but from the thermal expansion of the water above the thermocline as it gradually warms post-Wisconsin glaciation. There are lovely and again, fairly reliable pictures of this:
http://en.wikipedia.org/wiki/File:Post-Glacial_Sea_Level.png
Note well the scale is twenty METERS — the thickness of the line is around a meter, and the entire changes in the former figure over a century aren’t even close to this. Note also the time scale of things like “the meltwater pulse” produced when the land mass glaciers rapidly melted, to be terminated by the Younger Dryas (return to partial glaciation for a thousand years or so). The slope of SLR on this timescale appears to still be rising as it approaches the present at a rate of a meter per thousand years, but note well that the data does not resolve to annual or even century scale changes — it is pretty sparse. Nevertheless, if you look closely you can see a single red dot corresponding to the MWP and a single dot corresponding to the LIA at the far right hand edge of the curve, suggesting that the current SLR is still dominated by “rebound” from the SLF (sea level FALL) of the LIA, and that the current level is within a hair of the average over several thousand years (where a “hair” on this scale means “within a meter” since smaller variations are invisible).
This is a very instructive figure, actually. If we return to glaciation, one of the first effects is going to be the leaving of all of the world’s major ports high and dry. Note that the change in SL over 22,000 years was a rise of 120 to 130 METERS, and that, if anything, the rise rate has come very close to levelling. OTOH, even in the worst of the warming rises, SL did indeed rise five to ten meters “overnight” (over a few hundred years). This makes Hansen’s claim of a 5 m SLR over a century “barely” believable, if one assumes that the entire continent of Antarctica and/or the entire Greenland plateau start to melt in a near-tropical breeze. More realistically, the rise rate during the Holocene Optimum, when temperatures were at their highest and there was still a lot of unmelted ice was around a meter/century (a rate that persisted for some 4000 years, BTW). EVEN with temperatures that persisted at or close to optimum levels from 7000 to 9000 years ago, SLR had long since tapered off since most of the temperate zone glaciers were gone and the remaining ones were largely immune to global warming.
Just one of many things that we don’t really understand and cannot model.
rgb

The The Budget of Recent Global Sea Level Rise 2005-2012 (.pdf, NOAA report, June 2012. Revised January 2013) is at http://ibis.grdl.noaa.gov/SAT/SeaLevelRise/documents/NOAA_NESDIS_Sea_Level_Rise_Budget_Report_2012rev1.pdf

Interesting video, Mr. Watts!

May seem like a stupid question to some but what happens on the sea bed?Is it an impermeable membrane or can water leak through it and if so what happens to it.To use a crude analogy is the basin a bit like a bath tub where you can let water out by pulling the plug?