Joe Hanson (via a retweet from Gavin Schmidt) tips us to an interesting video that gives some insight into the complexity of measuring sea level. A good use of 3 minutes follows. Watch.
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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
lemiere jacques says:
November 26, 2013 at 12:44 pm
I don’t know if i am the only one but to decide to measure sea level instead of sea volume makes the things even more messy?
quite hard to define a reference and an increase in sea level is only proportional to sea volume in first approximation.
Sea level is at least relatively measureable. Ocean volume is far less exactly known. We don’t have anywhere near the necessary mapping precision of marine basin topography yet for one thing. Besides, even with that quality mapping, in order to estimate the volume, you would still need an estimate of sea level. Ocean basins are not graduated like measuring cups.
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).
Re steve Garcia,
Is the emphasis of the spheroid not increased as the sea, being a fluid, will tend to be thrust out due to centrifugal force near the equator, and presumably drag some fluid from the poles to compensate?
Robert A. Taylor says:
November 26, 2013 at 12:09 pm
===========================================
You learned right. See: http://en.wikipedia.org/wiki/Gravity_anomaly
But that’s low resolution. At high resolution big marine volcanoes do attract the sea and raise sea level a little. –AGF
What I find absolutely astonishing about some Warmists is when they say stuff like:
I then remind them that this has bee happening since the last de-glaciation. Then I tell them that the rate has flattened in the last few thousand years and there is no concrete evidence for acceleration. Sure, they show me references and I show them references to the contrary.
Sea level rise is not an easy one to measure by any measure.
Pippen, have you wondered what else might cause sea levels to rise?
Here is the study.
It’s not as simple as you previously thought!
Will groundwater abstraction get worse if we don’t act now?
The abstract of the paper is here.
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
Very nice video. Of course, from the standpoint of CAGW, the key isn’t sea level, but sea level change.
You just might be!
Robert A. Taylor at 12:09 pm above asks about tilt gravimeters showing lesser gravity toward mountains and the apparent contradiction with the video.
Both are true. The difference is in where you take the measurement. The experiments in the 1950s were made based upon measurements relative to the surface. Those measurements would correctly show that continental rock is less dense (and thus has less gravitic effect).
The video’s calculations assume an external orbit uniformly above both the mountains and oceans. From that perspective, the less-dense mountain sits in a depression in the denser crust. But the ocean also sits in a depression in the crust. The sum of mountain plus crust is greater than the sum of ocean plus crust, leading to the video’s “puddling” of gravity around mountains.
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.
There is also the tidal effect of extra-terrestrial bodies such as the moon and the sun (and the great planets when they are in alignment.
What about erosion. When I visited the ancient city of Troy, which was a port a few thousand years ago I found it is now 14 kilometers from the sea. Apparently this is because the river that was next to Troy has dumped so much sediment from the mountains in Turkey into the sea at that point. It makes you wonder how much sea level rise is due to sedimentary increases. This would be accelerating as mankind develops cities and towns. Another source is deliberate dumping of waste into the ocean. The OK Tedi mine in Papua New Guinea comes to mind as 80 million tons of waste was dumped each year into the local river which in turn would have taken most of that to the sea.
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…
Speaking as sailor, I’ve yet to see a change to the lowest or highest astronomical tide measurements on any nautical chart. Nor have I managed to find detail that any have ever changed as a result of climate change.
http://thenewaustralian.org/?p=10285
To misquote Joe Strummer; London ain’t drowning and I….. live by the river!
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.
@Brent Walker “What about erosion. When I visited the ancient city of Troy, which was a port a few thousand years ago I found it is now 14 kilometers from the sea. Apparently this is because the river that was next to Troy has dumped so much sediment from the mountains in Turkey into the sea at that point. It makes you wonder how much sea level rise is due to sedimentary increases. This would be accelerating as mankind develops cities and towns. Another source is deliberate dumping of waste into the ocean.”
Maybe that’s part of the Anthropogenic part of it.People are always changing (i.e. Port Building, Levees,Dredging,Canal Building) their environment .But,i think that natural forces dominate the system.My comment on another article:http://wattsupwiththat.com/2013/11/26/real-pollution-not-carbon-pollution-increases-storm-clouds/#comment-1484975 ,IMO,says that also.What i mean is that the high pressure(more dense) cold weather(air) system causes an inversion layer where it pushes down on the lower level emissions.Warming is not caused by the anthropogenic forcing because of the natural atmosphere phenomena.Well,that’s what i’m thinking,Who really knows?
Thanks for the interesting articles and comments.
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]
Sea level rise due to subsidence seems incongruous.
And to top everything, we rely on sea level measurements, using the speed of light, from satellites in decaying ellipsoid orbits.
Then there are also these factors to consider: wave heights, currents, tides, winds, isostatic rebound, tectonic movements and seasonal changes in ocean temperature.
And we believe we can measure changes in sea levels to an accuracy of 0.01mm!?!
What about erosion. When I visited the ancient city of Troy, which was a port a few thousand years ago I found it is now 14 kilometers from the sea. Apparently this is because the river that was next to Troy has dumped so much sediment from the mountains in Turkey into the sea at that point.
The video missed one important aspect of sea level which is the rotational speed of the earth. The earth moves from west to east. Land mass are fixed to the earth while the sea being liquid is mobile. Long land mass such as the north and south america practically slams the water in the Atlantic Ocean and in the process increases the sea level and in the process creates a slight vacuum on the Pacific Ocean side reducing the sea level. The separation could be very short as in the Panama canal, it could have the same gravitational force, same force of attraction from the land mass within this small area, but the sea level could be different on each side of land mass.
Psalmon says: “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.”
And according to the IPCC (and no one else) that is very very fast.
I love Minute Physics.