The Data:
 
The information in black on this page (above) is copied directly from NOAA's web site:
http://tidesandcurrents.noaa.gov/sltrends/MSL_global_trendtablefc.html.
(A copy of that web page is also available at http://www.webcitation.org/5lvz9LlBe.)
 
The numbers in red are values which I calculated from NOAA's data.
 
Note: The portion of this web page preceding the red line is in Microsoft Excel 2000 HTML spreadsheet format. So if you want to load the data into a spreadsheet you can just load the web page into Excel (version 2000 or later). Doing so will scramble the text after the red line, but the data and formulas above the red line will load correctly, and the numbers will be represented with greater precision than is shown on the web page.
 
The data on this web page is entirely copied and calculated from the referenced NOAA web page, but I've requested the raw data from which it is derived, and intend to revisit the topic if and when I obtain the raw data.
 
 
Summary:
 
159 stations are listed by NOAA. They had been in operation for an average of ~85 years. Mean Sea Level (MSL) rose at 117 locations, and fell at 41 locations.
 
The location with both the greatest total increase and fastest rate of increase was Galveston, TX, USA, where mean sea level rose by 2.10 feet over a period of 99 years.
 
The location with the greatest total decrease was Vaasa, Finland, where mean sea level fell by 3.00 feet over 124 years, but the location with the fastest rate of decrease was Furuogrund, Sweden, where mean sea level fell at a rate of 2.68 feet/century over 92 years.
 
The location with the median rate of MSL change was Prince Rupert, Canada, where MSL rose at a rate of 0.358 ft/century.
 
 
Insight #1:
 
There are two obvious ways of averaging the numbers. I've done both. One way is to weight each measurement location equally. The other way is to weight each station-year equally; that is, to weight each measurement location according to how long it was in operation, so that a station which was in operation for 100 years would have twice the weight of a station which was only in operation for 50 years.
 
It turns out that it doesn't make much difference in the result.
 
The average MSL change is +0.00200 ft/year if each station is weighted equally.
The average MSL change is +0.00150 ft/year if station-years are weighted equally.
 
0.00358 ft/year = 1.090 mm/year = 4.29 inches/century (median)
0.00200 ft/year = 0.611 mm/year = 2.41 inches/century (average1)
0.00150 ft/year = 0.458 mm/year = 1.80 inches/century (average2)
0.00175 ft/year = 0.535 mm/year = 2.10 inches/century ((avg1+avg2)/2)
0.00236 ft/year = 0.720 mm/year = 2.83 inches/century ((med+avg1+avg2)/3)
 
 
Conclusion #1:
 
According to NOAA's data, global average mean sea levels have been creeping up at a rate of only about 2-3 inches per century (0.5-0.7 mm/year). That is about one-third of the 1.8 mm/year (7.1 inches/century) rate which AGW alarmists typically claim.
 
 
Insight #2:
 
Although the global average MSL rate of change is only about 0.6 mm/year, most sites have historically seen much larger changes than that, because the average includes both locations where the sea level is increasing and locations where the sea level is decreasing. The "Absolute value of MSL Trend" column shows how much sea level change (up or down) is experienced by each site, and the average is about 0.00680 ft/year = 2.07 mm/year.
 
That means that most of the MSL trend at most sites is due to local conditions, rather than the global MSL trend.
 
The significance of that fact is that it means that changes in the global average rate of MSL increase are of much less significance than you might expect. This is shown by the last four columns, in which the effect of hypothetical increases in rate of MSL rise are calculated.
 
In fact, for 37 locations which have been experiencing large decreases in sea level, a doubling in the global average rate of MSL rise would actually reduce the rate of MSL change experienced at those locations.
 
For locations which have already been experiencing increases in sea level, an increase in the global average rate of MSL rise would increase the amount of change experienced at those locations, but often by a relatively small amount compared to the change which they have already been experiencing.
 
In the columns labeled "MSL Trend +0.6mm/yr" and "Absolute value of MSL Trend +0.6mm/yr" we calculate the local effect at each station of an approximate doubling in rate of global average mean sea level increase, from ~0.6 mm/yr to ~1.2mm/yr. The average amount of change experienced by the 159 locations would increase from the 20th century rate of .00680 ft/year to about 0.00786 ft/year, which is just a 16% increase.
 
In the column labeled "MSL Trend +1.2mm/yr" we calculate the local effect at each station of an approximate tripling in rate of global average mean sea level increase, from ~0.6 mm/yr to ~1.8mm/yr. The average amount of change experienced by the 159 locations would increase from .00680 ft/year to about 0.00911 ft/year, which is still only a 34% increase.
 
 
Conclusion #2:
 
Because of the large variation in rate of sea level change between different seaside locations, changes in the global average rate of sea level increase have much less effect on seaside communities that one might guess. Even a tripling in the global average rate of Mean Sea Level increase would cause only a 34% average increase in the rate of sea level change experienced by seaside communities.
 
 
How & Why:
 
The obvious question is, "How did the IPCC's AGW alarmists get it so wildly wrong?"
 
Or, first off, is it possible that the IPCC is right? I.e., is it possible that NOAA's list of 159 locations is so unrepresentative of the world as a whole, that it understates 20th century global average MSL rise by two-thirds?"
 
That seems very unlikely. The 45 NOAA-maintained stations and 114 PSMSL-provided stations represent what appears to be a pretty good geographic distribution (though Africa is noticeably underrepresented). They are the 159 stations designated in 1997 as GLOSS-LTT stations for monitoring long-term sea level trends. I'm aware of no evidence that the 159 locations were poorly chosen.
 
So how did the IPCC get it so wildly wrong? The answer apparently is that they've been adjusting the numbers with "correction factors," based on theoretical models rather than actual measurements, and their corrections drastically inflate the reported rate of sea level rise.
 
The best explanation I've found is by the late John L. Daly (whose untimely death the UEA CRU's Phil Jones infamously found "cheering"), here:
Testing the Waters: A Report on Sea Levels for the Greening Earth Society
 
Here's a key quote from Daly's paper:

"The impression has been conveyed to the world's public, media, and policymakers, that the sea level rise of 18 cm in the past century is an observed quantity and therefore not open to much dispute. What is not widely known is that this quantity is largely the product of modeling, not observation, and thus very much open to dispute, especially as sea level data in many parts of the world fails to live up to the IPCC claims."

This paper is long, but very readable, and well worth a careful reading.
 
 
Dave Burton
Cary, NC

Original version: 12/11/2009
Latest version: 12/30/2009
 

P.S. - For more climate info see:  http://www.burtonsys.com/climategate
 
Also, here's a short link to this page:  http://tinyurl.com/MSLavg,
and to the analysis section of this page:  http://tinyurl.com/MSLavg#notes
 

†  The column entitled "Total MSL change (feet)" is simply the calculated product of the "MSL Trend" (divided by 100 to convert ft/century to ft/year) and "Year Range" columns. Since the MSL Trend is determined (by NOAA) by linear smoothing, and there are substantial year-to-year fluctuations in MSL at most locations, the "Total MSL change" is not precisely the difference between the First Year MSL and the Last Year MSL. Rather, it is an intermediate value used to calculate the "average2" MSL Trend (in which each station-year is weighted equally; that is, in which each measurement location is weighted according to how long it was in operation).