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

Interesting. In the 1950’s I was taught tilt gravimeters reported, by actual measurements, lesser gravity toward mountains and greater toward oceans, because the continental rock was less dense than the sub ocean and the mountain was floating above more dense material with a keel extending into it.
Does anyone know which is correct?

Wait!
Did they say in the video that the model correctly estimates sea level to “one meter”????
How on God’s green earth do we come up with increases or, for that matter, decreases of mm/year?

And they never mentioned the gravitational effect of ofter of other celestial bodies has on sea level. That really can confound things.

Given how quickly the magnetic poles are shifting, north shifted as much during 2000-2010 as it did 1900-2000, one would guess the gravity fields shift as well. Are they continuously updating their database?
http://www.ngdc.noaa.gov/geomag/GeomagneticPoles.shtml
Check out the map at the bottom of the page, a better graphic with more recent (2010) info.

PS.- Thanks for all the interesting articles and comments.

I read somewhere (Daily Telegraph I think) last week that at any one time there are 100,000 ships at sea. With increasing trade and increasing sizes of ships I wonder what effect their displacement of water has had on sea levels. Any mathematicians out there willing to do a “back of the envelope” calculation?

Interesting – Joe is my wife Frederika Haskell’s nephew

The more you learn about sea level, this and Dr. Soon’s video presentation, the more you realize the sea is rising as much as the sky is falling.

Steve Garcia says:
November 26, 2013 at 2:18 pm
“If shrunk to the size of a billiard ball, however, the Earth is within the same roundness tolerance range of a standard billiard ball.”
But the Earth is not the size of a billiard ball! The Marianas Trench does show up with our standard measuring equipment. If the Earth was shrunk to the size of a billiard ball, and we were shrunk with it, our measuring equipment would still be able to detect the Marianas Trench.
So, the point of your post escapes me??
SR

Eric ah “With increasing trade and increasing sizes of ships I wonder what effect their displacement of water has had on sea levels.”
2.15 billion cubic meters divided by the surface area of the oceans equals about 6 microns (0.006 mm).
http://what-if.xkcd.com/33/
But the article goes on: you don’t have to worry about that six-micron sea level drop. The oceans are currently rising at about 3.3 millimeters per year due to global warming (through both glacial melting and thermal expansion of seawater).

Pippen Kool says:
November 26, 2013 at 2:49 pm
…………………
But the article goes on: you don’t have to worry about that six-micron sea level drop. The oceans are currently rising at about 3.3 millimeters per year due to global warming (through both glacial melting and thermal expansion of seawater).

Pippen, have you wondered what else might cause sea levels to rise?

Global groundwater depletion leads to sea level rise
Large-scale abstraction of groundwater for irrigation of crops leads to a sea level rise of 0.8 mm per year, which is about one fourth of the current rate of sea level rise of 3.3 mm per year.
http://www.un-igrac.org/publications/422

Here is the study.

Abstract – 26 OCT 2010
Global depletion of groundwater resources
[1] In regions with frequent water stress and large aquifer systems groundwater is often used as an additional water source. If groundwater abstraction exceeds the natural groundwater recharge for extensive areas and long times, overexploitation or persistent groundwater depletion occurs. Here we provide a global overview of groundwater depletion (here defined as abstraction in excess of recharge) by assessing groundwater recharge with a global hydrological model and subtracting estimates of groundwater abstraction. Restricting our analysis to sub-humid to arid areas we estimate the total global groundwater depletion to have increased from 126 (±32) km3 a−1 in 1960 to 283 (±40) km3 a−1 in 2000. The latter equals 39 (±10)% of the global yearly groundwater abstraction, 2 (±0.6)% of the global yearly groundwater recharge, 0.8 (±0.1)% of the global yearly continental runoff and 0.4 (±0.06)% of the global yearly evaporation, contributing a considerable amount of 0.8 (±0.1) mm a−1 to current sea-level rise.
http://onlinelibrary.wiley.com/doi/10.1029/2010GL044571/abstract

It’s not as simple as you previously thought!

Since tidal gauges all around the Baltic Sea have displayed isostatic rebound of the coastline, is it not safe to assume that entire basin is on the rise, displacing seawater out into the North Atlantic? Likewise, for most of northeastern Canada? These are just two examples of how rising or falling sea bottoms around the world would have a tremendous effect upon global sea level. How could any human caused SLR, if there is such a thing, even be determined?
Since coastlines likewise rise and fall, all that really matters now that continental ice sheets of North America and Eurasia are melted, is the rise or fall of local sea level. Global sea level rise is swamped by local sea level changes.
As those areas that are subsiding have been doing so since before industrialization and people have been able to cope with the local rise of the sea, this is an artificial crisis created to bolster that other artificial crisis – CAGW.
SR

Hmm We want to know if the sea level is rising and how much, so why measure it all over different places, if we took just three measurements in places that were not affected by gravity, isostatic rebound etc, like one in the Atlantic, one in the Pacific and one in the Indian ocean. wouldn’t that be more representative of true sea level rise.

The video, while raising many valid points, is only scratching the surface (sorry if that sounds like a pun) of the problems in trying to measure sea level changes. Prevailing weather patterns of high and low pressure, plus general wind direction and magnitude, can lead to significant changes in sea level at various locations. For instance, California levels are quite a bit higher during El Nino periods.
Geological effects include isostatic rebound at high latitudes, seismic movement, and human induced pumping of groundwater and/or petroleum.
Satellite measurements bring in a host of other issues. Fundamentally, they measure distance from the satellite to the sea surface by measuring the speed-of-light delay in bouncing a laser off the surface. But the speed of light varies with atmospheric conditions such as temperature and humidity that are not that precisely known.
The fundamental resolution of the satellite measurements is far above the annual trend they claim to measure. I believe it is only recently that this base resolution has gone below 1 meter. It is only by averages of averages of averages that can claim the precision to measure the trends.
Also, the satellite altitude is not that precisely known either. All of the gravitational variations that the video mentions do affect the satellite’s trajectory at this level of precision, as well as density variations in the very tenuous, but still real, atmosphere, there. So the measurements must all be calibrated against ground measurements.
Many other issues as well…

There is no problem with finding sea level. Go down to the beach and find the sea. (Look for something big, wet, and moving slightly. No, not that. Bigger, and made of water.) Found it? Now look at the top of the sea. Sea level is where to top of the sea is.

Sea Level Trends – NOAA Tides & Currents, at http://tidesandcurrents.noaa.gov/sltrends/sltrends.html might be of interest.
[Very interesting NOAA reference page, Thank you. Mod]

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?