[dgt20 (OP)]

What are some ways to prove that global warming does affect rising sea levels other than thermal expansion and metling of ice caps?

[alancalverd]

Obvously, melting ice caps will increase the volume of liquid water in the sea and reduce the overall area of dry land. The problem is that "sea level" is not definable!

The south and east of Great Britain are sinking under the weight of the North Sea, whilst the north and west are rising as the earth relaxes from the last ice age. The GB map reference sea level is nominally a mark at Newlyn Harbour, fairly close to the neutral axis of this tilt. However the European and North American tectonic plates are moving (hence the Atlantic Ridge and Icelandic volcanoes) so sea level at Reykjavik or New York, however you measure it, bears no useful relation to the mark at Newlyn.

[evan_au]

sea level at Reykjavik or New York, however you measure it, bears no useful relation to the mark at Newlyn.

I think if you measure the sea level in enough places (including Newlyn), you will find a general correlation that sea level is rising.

Surely, that is a useful relationship to know?

The Dutch certainly think it is real - a lot of their country is already below sea level!

[Bored chemist]

The problem is that "sea level" is not definable!

You may want to discuss that with the folk who run the GPS network and are in a position to measure it WRT the centre of mass of the Earth (which isn't moving from our point of view)

[alancalverd]

http://www.esri.com/news/arcuser/0703/geoid1of3.html gives a very readable account of GPS altimetry, which rather underlines my point that however you define sealevel, it diesn't have much universal value. For instance

 

However, zero elevation as defined by Spain is not the same zero elevation defined by Canada, which is why locally defined vertical datums differ from each other.

We use GPS altimetry for instrument approaches to runways thousands of miles apart, so it helps to have a universal datum geoid, but each approach is fraught with local corrections that vary from week to week in some places and certainly have to be revised globally every year because the earth's wobblature (I just made that word up, but I think it will become part of the geometers' vocabulary in time) keeps changing.

Now here's the problem: a tolerance of ± 10 cm on runway elevation isn't going to worry aviators much, but a 10 cm rise or fall in mean sea level could have a huge impact on coastal ecology, with all sorts of sequelae if people are involved. Fact is that human society is just too fragile for this planet.

[alancalverd]

I think if you measure the sea level in enough places (including Newlyn), you will find a general correlation that sea level is rising.

Surely, that is a useful relationship to know?

Not really. If you live on the Suffolk coast, it is disappearing a lot faster than MSL is rising, but new bits of Iceland appear every year. Some desert cities in the Arabian peninsula were seaports witin recorded history, whilst the posh bit of Herculaneum disappeared under the Mediterranean around fhe same time.

The Dutch certainly think it is real - a lot of their country is already below sea level!

About half the country always was.

[Bored chemist]

On a minute by minute basis  GPS altitude measurements are about as much use as a chocolate fireguard. They are regularly so wrong that they would crash a plane if you tried to use them to tell you where the runway was.

Averaged over a reasonable period they are pretty good.

People may be too fragile for this planet; that''s a poor excuse for making teh planet worse.

[alancalverd]

GPS altitude measurements are about as much use as a chocolate fireguard. They are regularly so wrong that they would crash a plane if you tried to use them to tell you where the runway was.

No problem so far. The great thing about a GPS precision approach is that you don't need a complicated,expensive and generally unreliable ILS transmitter on your jungle airstrip, though the bit in the plane is slightly more expensive than an ILS radio, so in principle you can publish a GPS approach to any airfield and use it at any time. The published safety minimums for WAAS/LPV GPS are the same as for ILS, and you'd have to be very bold, very confident, or very desperate, to attempt a landing with less than 200 ft vertical visibility whatever system you use.

[Bored chemist]

You can publish  the altitude of your airstrip- measured any how you like.
And if you look at the sort of precision you get with commercial GPS
https://www.singletracks.com/blog/gps/gps-elevation-accuracy-test-smartphone-apps-vs-dedicated-gps/
you will find that you might miss by 100 feet- which would kill you.

But, if you average the height measured by GPS over a month or so, you will get a pretty good answer.
 Apart from bits where cliffs are falling into the sea, the effect of global warming will be pretty much constant over that time period.

[alancalverd]

Which is why I specified WAAS/LPV GPS, not a smartphone.

But the fundamental question remains: sea level relative to what? As I said at the begining, the volume of sea water will increase, but the answer you get depends on where you measure it because the land is moving up and down too, and there are gradients in the sea, so whilst the answer is obvious, there is no practical means of experimental proof.

[puppypower]

Which is why I specified WAAS/LPV GPS, not a smartphone.

But the fundamental question remains: sea level relative to what? As I said at the begining, the volume of sea water will increase, but the answer you get depends on where you measure it because the land is moving up and down too, and there are gradients in the sea, so whilst the answer is obvious, there is no practical means of experimental proof.

If you look at satellite images of the earth's gravitational field, the earth is very lumpy in terms of gravity. Changes in the gravitational grid can cause the sea level to fluctuate, locally. 

It is hard to tell from the images below, but the polar caps appear to have ice where gravity is above average. If this ice was to melt, the loss of mass will lower gravity at the poles, and slightly increase gravity elsewhere The new sea level may appear less than expected since the forces of gravity goes up in the oceans.

Shifts in the earth's crust can change the map. Submergence of the crust, under the oceans, can entrain ocean water and deposit this water blow the crust into the mantle. A huge deposit of water have been found under the crust in the mantle, below south east Asia, the size of the Arctic ocean.

I would be more worried about this mantle water being released, than the poles melting. If that mantle water escaped, you would have a world wide flood of hot steamy water that would cause huge tidal waves as is gushed outward. While the loss of that much water under the crust, would causes plate movement never seen before. Huge mountains would collapse. The steam laden atmosphere would melt all the worlds ice, adding even more problems.

This appears to have happened before. A huge scar in the crust was found at the bottom of the Atlantic Ocean, where mantle material is exposed. This venting of mantle water, would have caused a catastrophic flood, with weeks or months of rain, to clear the skies of water vapor. If you like to worry, worry about something epic.



 

[Colin2B]

A huge deposit of water have been found under the crust in the mantle, below south east Asia, the size of the Arctic ocean.

I would be more worried about this mantle water being released, than the poles melting.

I wouldn’t worry if I were you.
This is not an ocean as we know it, Jim. The water is locked in moisture-containing rocks 400 to 800 miles (700 to 1,400 kilometers) beneath the surface.
It isn't an ocean because the water is a very low percentage of the rock, probably less than 0.1 percent.

[alancalverd]

On a minute by minute basis  GPS altitude measurements are about as much use as a chocolate fireguard. They are regularly so wrong that they would crash a plane if you tried to use them to tell you where the runway was.

Averaged over a reasonable period they are pretty good.

People may be too fragile for this planet; that''s a poor excuse for making teh planet worse.

Last time I looked at the chart (Tuesday) practically every commercial airport in Northern Europe had a published GPS approach and EASA (who admittedly are a bunch of donkeys but don't like breaking planes) were considering when to phase out ILS entirely. Not much wreckage visible to date, though the UK CAA shares your scepticism and at the last count we only had 5 approved approaches.

[chris]

In 2006, I interviewed for the journal Nature Isabella Velicogna, from the University of Colorado, about experiments she was conducting using the satellite GRACE. This is actually a pair of orbiting craft that register the distance between themseves by laser interferometry and infer the gravitational field through which each is passing and hence the mass attributable to that part of the Earth's surface. In this way ice mass can be tracked, and ice-loss calculated...

Greenland and Antarctica hold between them the largest reservoirs of freshwater ice on the planet. And, if global warming does cause sea levels to rise, this is where the extra water would come from. Now we have an idea of how fast the ice is melting and the numbers might be worse than we thought. Using the GRACE Satellite System Isabella Velicogna of the University of Colorado and also the Jet Propulsion Laboratory in Pasadena, has been looking at the ice in Greenland to find out whether this country is losing mass. Nature 443, 329–331 (21 September 2006) ; Nature 443, 277–278 (21 September 2006)

Isabella Velicogna: Between Greenland and Antarctica there's about 70 metres of change in sea level that can be caused if they both would melt. If all Greenland melted, which is very unrealistic to think that this would happen, that would cause 7 metres of sea level change.

Chris Smith: So, according to your measurements, how fast to you reckon Greenland's melting?

Isabella Velicogna: We have measured between April 2002 through April 2006, so that's about four years, and we are observing a change of about 248 cubic kilometres per year which would correspond to about 0.5 mm per year of sea level rise.

Chris Smith: Is that rate remaining constant or is it actually accelerating, how fast we're losing the ice in Greenland?

Isabella Velicogna: What we are observing – and I think that's what's interesting – is that it seems that there is an increase in mass loss after April 2004, and is quite significant. And so we are not able to say that this is going to keep going but, sure, we want to keep an eye on the ice and keep monitoring because this can keep going and this acceleration being constant can have an effect on everyday life in the long time frame.

Chris Smith: How are you actually making the measurements that you did to find that you'd lost 248 cubic kilometres of ice, in each year?

Isabella Velicogna: We're using gravity measurement and we use the measurement provided by satellite machine. It's called GRACE. And GRACE is composed by two satellites that orbit around Earth and they're attracted by the mass of Earth. In other words, what we measure is the changing distance between the two satellites so this distance changed because every satellite separately is subject to attraction of the mass underneath the satellite. So if one satellite, for example, gets closer to a mass anomaly which can be a mountain, it suggests an acceleration because it's subject to a stronger attraction. And so accelerating the distance between the first and the second becomes larger. And once it gets far from this mass anomaly, so you get far from the mountain, it just decelerates and so the distance becomes smaller again. And from the change in distance we can infer the change in mass underneath.

Chris Smith: And can you extend this to not just Greenland but, say, Antarctica too, which was the other major ice sheet you mentioned?

Isabella Velicogna: Yeah. So we study also the change of the ice mass poration in Antarctica and we found also that the Antarctic ice sheets seem to lose mass at a significant rate, in fact more than what it was expected from the last IPCC report, which is that big report that is compiling and putting together all the available information. And the last one, which was about four years ago, was projecting not so much change for the mass of the Antarctic ice sheet for the 21st century. And we are sure that there is a significant change and so this tells us that we should try to be careful and watch those big ice sheets because something is changing.

Isabella Velicogna of the University of Colorado and the Jet Propulsion Laboratory in Pasadena on how Greenland may be melting faster every year.