Here’s a novel approach. Beenstock et al wondered if tide gauges were placed in any old spot around the world or were biased toward area where sea-level did more rising. They compared the location of tide gauges in the year 2000 to sea level rises and falls as measured by satellite altimetry. It turns out the placement seems to be independent (meaning anywhere). This is pretty important because the infernally tough thing about measuring sea levels is whether the land is subsiding or rising at the same time, and how to correct for that. If tide gauges are spread evenly (or quasi-randomly), it means we could average the lot instead of trying to correct and reconstruct each one individually. So that’s what they did – average (they did not reconstruct).
The consensus estimate is that sea-levels are rising by 2mm a year (and 3mm lately, with adjusted satellite data). Beenstock et al used 1,000 tide gauges and found global sea level rise was more like 1mm a year (very similar to the rise Nils Axel Morner found on that stable spot in Denmark). The conclusion is that sea level is rising slowly at 1mm a year, and that it hasn’t changed, and that local sea level rises and falls are very common, and that correcting for them is risky.
Although mean sea levels are rising by 1mm/year, sea level rise is local rather than global, and is concentrated in the Baltic and Adriatic seas, South East Asia and the Atlantic coast of the United States. In these locations, covering 35 percent of tide gauges, sea levels rose on average by 3.8mm/year. Sea levels were stable in locations covered by 61 percent of tide gauges, and sea levels fell in locations covered by 4 percent of tide gauges. In these locations sea levels fell on average by almost 6mm/year.
They blame the reconstructions for the difference (adjustments strike again):
We suggest that the difference between the two estimates is induced by the widespread use of data reconstructions which inform the consensus estimates. There are two types of reconstruction. The first refers to reconstructed data for tide gauges in PSMSL prior to their year of installation. The second refers to locations where there are no tide gauges at all. Since the tide gauges currently in PSMSL are a quasi-random sample, our estimate of current GMSL rise is unbiased. If this is true, reconstruction bias is approximately 1mm/year.
They found the placement of gauges is quasi random” — at least for recent times when there are hundreds of stations. Beenstock point out that there were not many gauges in 1900, and most of those were in areas where sea level was rising. The sampling was biased then. Hence the tide gauges from the 1800s overestimate how much sea levels rise globally.
Curious drop off in tide gauges after 2000?
It is clear from Figure 1 that not only were new tide gauges commissioned, but some tide gauges were also decommissioned. The number of tide gauges decreased sharply after 1995 because of reporting delays which are substantial.
Sea level rises and falls are regional
Beenstock et al note that tide gauges show rising trends (red dots below) in places which are right next to gauges showing the sea level is falling (blue dots). But most of the time the tide gauges show a trendless change (yellow dots).
From the conclusions:
Sea level rise is regional rather than global and is concentrated in the southern Baltic, the Ring of Fire, and the Atlantic coast of the US. By contrast the north-west Pacific coast and north-east coast of India are characterized by sea level fall. In the minority of locations where sea levels are rising the mean increase is about 4 mm/year and in some locations it is as large as 9 mm/year. The fact that sea level rise is not global should not detract from its importance in those parts of the world where it is a serious problem.
The location of tide gauges is not random. If their locations are positively (negatively) correlated with SLR, estimates of global SLR will be biased upwards (downwards). We show that the location of tide gauges in 2000 is independent of SLR as measured by satellite altimetry. Therefore PSMSL tide gauges constitute a quasi-random sample and inferences of SLR based on them are unbiased, and there is no need for data reconstructions. By contrast, tide gauges dating back to the 19th century were located where sea levels happened to be rising. Data reconstructions based on these tide gauges are therefore likely to over-estimate sea level rise.
We therefore study individual tide gauge data on sea levels from the Permanent Service for Mean Sea Level (PSMSL) during 1807 – 2010 without recourse to data reconstruction. Although mean sea levels are rising by 1mm/year, sea level rise is local rather than global, and is concentrated in the Baltic and Adriatic seas, South East Asia and the Atlantic coast of the United States. In these locations, covering 35 percent of tide gauges, sea levels rose on average by 3.8mm/year. Sea levels were stable in locations covered by 61 percent of tide gauges, and sea levels fell in locations covered by 4 percent of tide gauges. In these locations sea levels fell on average by almost 6mm/year.
Michael Beenstock, Daniel Felsenstein,*Eyal Frank & Yaniv Reingewertz, (2014) Tide gauge location and the measurement of global sea level rise, Environmental and Ecological Statistics, May 2014 [Abstract]