sea level rise - measured how, exactly?

The link below is a good overview. Note that there are multiple links to specific research from the article. The author is a Norwegian oceanographer who has published a couple of tidal gauge-based sea level rise studies.

http://www.skepticalscience.com/Visual-depictions-of-Sea-Level-Rise.html

Thanks!

I dip my balls in the ocean.

dkdkdkdkdkkdkdkddkkd wrote:

I dip my balls in the ocean.

Do you adjust for ball contraction in cold water?

crete wrote:

dkdkdkdkdkkdkdkddkkd wrote:

I dip my balls in the ocean.

Do you adjust for ball contraction in cold water?

Yes. You are wise

I'll admit I didn't know much on the topic until I started reading. I was suprised to learn that sea levels have risen something like 8 inches since 1870, over the period when global temperatures have risen something like 1 C. Here is a snapshot of inferred global sea level change since 1870 (from Sustain Sci (2008) 3:9–22; DOI 10.1007/s11625-008-0042-4):

Here is a plot of global temperature anomaly over the same period:

8 inches doesn't sound like that much; if that's what we expect over the next 100 years, I think we have time to get out of the way. :-)

Reading more, I gather people are predicting that with continued global warming, the rate of sea level rise will accelerate, so they are projecting much more than another 8 inches by 2100, more like as much as 0.8 m, or 32 inches. Here's the prediction (also from Sustain Sci (2008) 3:9–22; DOI 10.1007/s11625-008-0042-4):

I'm not sure global sea level does that good a job of following global temperatures, apart from the general trend. Here I've superimposed T anomaly over sea level with the same time scale, scaled to give the same overall trend:

Most of the multi-year or decadal variation in one parameter (T anomaly or sea level) is not seen in the other. Assuming the global temperatures follow the predictions, what basis do we have to expect the sea level trends will obey the predictions?

the steadily melting ice is a good basis.

Bad Wigins wrote:the steadily melting ice is a good basis.

Not looking for facile responses. Can't prevent them either I suppose...

Oh brother, here we go again. Do you really think that folks do not see through you by now? Seriously?

Oh Brothers wrote:Oh brother, here we go again. Do you really think that folks do not see through you by now? Seriously?

See through what? I'm happy to be called a shill, it's in my username...

It's not measured "exactly", just measured, just like the global temperature.

the shill wrote:

Assuming the global temperatures follow the predictions, what basis do we have to expect the sea level trends will obey the predictions?

If you're really interested, the IPCC WG1 "Physical Science Basis" document, Chapter 13:"Sea Level Change" is, remarkably enough, an extensive overview of the physical science basis for sea level rise as currently understood.

Bobby1 wrote:It's not measured "exactly", just measured, just like the global temperature.

With temperature, we have an absolute scale, or fixed frame of reference.

With sea level, we measure mean sea level relative to land. Maybe that seems straightforward, but in fact it's not. We don't have a fixed, stationary datum against which to measure mean sea level. We can make observations at tidal gauges, and plot these over time to extract trends of "relative" mean sea level at those gauges ("relative" meaning they are relative to a local datum). And we can look at the whole data set and infer some "global" average.

But the land, against which we measure "sea level," is hardly fixed in space, and in many cases moves more quickly than the inferred change in sea level, for a variety of reasons including isostatic adjustment following retreat of the glaciers (where I live and work was once under 200-300m of water not 10000 years ago, that's about an inch of movement per year, or an order of magnitude more than inferred sea level rise); tectonic ajustment (movement along faults can be of a similar order of magnitude), or local change of what I will call "young" or "active" landforms (aggradation from flooding, subsidence of soft soils, etc).

Hence my question about how it is measured. Recently satellites have been used to help, and I don't know how they correct for various factors (crustal adjustments, gravitational changes to the "shape" of the ocean, etc).

Anyhow, Citizen Runner's link pointed me to some great resources (I knew LR would come through!), and I can see that these factors are known and considered, to some degree.

So I am then left with wondering, first "so what?" (about 8 inches over a century...), and then, "OK if more would maybe be bad, how confident can we be it will be more?"

Looking for somebody with insights to add to the story.

Or chirp in to entertain, as you wish. I love this place, both for info and for entertainment. :-)

So you read how we measure it, right?

I think it is pretty self explanatiory.

What I don't get is, wouldn't the scientist who could conclusively disprove all of this stuff win a Nobel prize?

Overturning the body of science is something we would all dream of... cover of Nature and we everything. We'd make more money and get more grants than any conspiracy could give us.

djdjdmdmdmdmdmdmdmd wrote:(1) So you read how we measure it, right?

(2) I think it is pretty self explanatiory.

(3) What I don't get is, wouldn't the scientist who could conclusively disprove all of this stuff win a Nobel prize?

(4) Overturning the body of science is something we would all dream of... cover of Nature and we everything. We'd make more money and get more grants than any conspiracy could give us.

(1) Reading in progress, yes. It's not a simple story.

(2) Indeed. Go ahead and explain it for the rest of us then.

(3) Where does that come from? Who is trying to disprove something?

(4) See (3).

the shill wrote:

I'm not sure global sea level does that good a job of following global temperatures, apart from the general trend. Here I've superimposed T anomaly over sea level with the same time scale, scaled to give the same overall trend:

Image:

http://i61.tinypic.com/2j4evxt.jpg

Most of the multi-year or decadal variation in one parameter (T anomaly or sea level) is not seen in the other. Assuming the global temperatures follow the predictions, what basis do we have to expect the sea level trends will obey the predictions?

My understanding that up to this point, much of the sea level rise has been caused by thermal expansion of water. Air temperature is not a good proxy for water temperature, since air temperature changes much more rapidly due to its lower specific heat and mass. So overlaying two plots and expecting the two to move in concert is pretty naive.

I think djdjdmdmdmdmdmdmdmd is projecting onto you..

Anyway, 8 inches itself may not be a lot in many places, but in some places it is. For island nations 8 inches starts taking away land. For coastal areas even in the US, that's 8 inches less protection when a storm surge comes in.

When you start talking about 0.3m, then things get really interesting. A significant amount of land disappears, including a lot of agricultural land. Less land lead to less food. People working those lands have to move. People living near the coasts in other areas have to move. Beaches disappear, storm surges reach further inland..

dingle wrote:

[quote]the shill wrote:(1) My understanding that up to this point, much of the sea level rise has been caused by thermal expansion of water.

(2)...overlaying two plots ...is pretty naive.

(3)... expecting the two to move in concert is pretty naive

(1) I believe the ocean temperatures are considered to have followed similar general trends as global air temperatures. That said, the IPCC report shows thermal expansion being a lower contributor to sea level change than other factors up to at least 1970.

(2) Overlaying two plots is OF COURSE naive. It's a quick and crude way of comparing two sets of data and looking for similarities or patterns.

(3) I didn't "expect them to move in concert" but I admit I am surprised by the nearly complete absence of similarity at any scale shorter than the century scale.

Citizen Runner wrote:If you're really interested, the IPCC WG1 "Physical Science Basis" document, Chapter 13:"Sea Level Change" is, remarkably enough, an extensive overview of the physical science basis for sea level rise as currently understood.

Of course you are right, that's a very good suggestion and I've just dove back in for another look.