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Ronan Connolly On Sea Levels

March 21, 2014

By Paul Homewood




Dr Ronan Connolly has a thoughtful essay on his website, which goes into some depth on the subject of sea levels. It’s well worth a read. Although very detailed, it is still very readable.


The section that really took my eye though was the one on satellite measurements, in particular this bit:


So, are the satellite estimates reliable? Well, in order to answer that, we have to learn a little bit about how they were actually constructed.

Unfortunately, satellite altimeters don’t actually measure sea levels directly. Instead, they measure the length of time it takes light signals sent from the satellite to bounce back. In general, the longer the signal takes, the further the satellite is from the sea surface. So, in theory, this measurement could be converted into a measure of the sea surface height, i.e., the mean sea level.

However, the conversion is complicated, and a number of other factors need to be estimated and then taken into account. For instance, the distance of the satellite from the Earth’s surface varies slightly as it travels along its orbit, because the gravitational pull of the Earth is not exactly uniform – see the Wikipedia page on “geoid”, and the maps in Figure 19.

So, in order to convert a particular “satellite-sea surface distance” into a sea level measurement, the “satellite-Earth’s surface distance” also needs to be independently measured, e.g., using the DORIS system.

Another complexity is that light takes slightly longer to travel when travelling through water vapour than dry air. So, the water vapour concentrations associated with a given satellite reading also need to be estimated, and accounted for.

As a result, satellite estimates of sea levels involve the use of complex models, approximations, other measurements and calculations. Unfortunately, this means that if there are problems in any of those stages, it could introduce artificial biases into the estimates, possibly making them unreliable… or even worse, wrong.

Mörner, 2004 (Abstract; Google Scholar access) managed to track down a graph of the raw satellite trends from the TOPEX/Poseidon satellite up to 2000. When he looked at this graph (Figure 20), he didn’t see much of a “sea level rise”. Instead of the +2.8 or +3.1 mm/yr trends commonly reported, it appeared to him that sea levels had been essentially constant from 1993 to 1996. He agreed that from 1997 to 1999, there were considerable sea level changes. But, they comprised falls as well as rises, and were probably related to the unusual 1997-98 El Niño event.


Mörner, 2004 was a controversial paper, and several of the researchers involved with the TOPEX/Poseidon analysis objected to Mörner’s analysis, e.g., Nerem et al., 2007 (Abstract). However, surprisingly, these objections were not over his claim that the raw satellite data showed little trend. They agreed with Mörner that the original satellite data didn’t show much of a sea level rise. Instead, their objection was that he should have used their adjusted data. They felt the raw data was unreliable, and had developed a series of adjustments which they believed made the trends more realistic.

For example, Keihm et al., 2000 (Abstract; Google Scholar access) had decided that the TOPEX satellite was showing an instrumental negative drift of 1.0-1.5 mm/yr between October 1992 and December 1996. So, they adjusted the data by adding a positive trend of 1.0-1.5 mm/yr to that period. Chambers et al., 2003 (Abstract; Google Scholar access) decided that even more negative biases were introduced when the TOPEX satellite switched to its backup instrument in February 1999. So, they introduced more adjustments. This set of adjustments increased the apparent sea level rise from +1.7 mm/yr to +2.8 mm/yr. Neither set of adjustments affected the period January 1997-January 1999, but as Mörner had noted the raw data already showed significant variability for that period due to the 1997-98 El Niño event. Finally, they believe that an adjustment of +0.3 mm/yr is necessary to account for Peltier’s Glacial Isostatic Adjustments (see Section 4).

It turns out that almost all of the +2.8 mm/yr (or +3.1 mm/yr if Peltier’s post-glacial rebound adjustments are applied) sea level rise in the 1993-present satellite estimates are due to adjustments! The raw data (which no longer seems to be in the public domain) doesn’t show much of a trend, after all.

It is plausible that the unadjusted trends are unreliable, as Nerem et al., 2007 claimed. However, that doesn’t automatically mean that Nerem et al.’s adjustments are valid



 The paper by Chambers et al, used as an example, is here, and this is the Abstract:


 During the calibration of the Jason-1 altimeter, it was discovered that the Jason-TOPEX sea surface height (SSH) residuals contained a trend correlated with significant wave height (SWH), indicating an error in the sea state bias (SSB) model for one or both of the altimeters. After updating the SSB model for Jason the trend remained, which pointed to an error in the TOPEX model. Since two different TOPEX altimeters (TOPEX_A and TOPEX_B) have operated during the mission, we estimated new SSB models using data from each one. The estimated SSB model for TOPEX_B is significantly different than the model provided with the data, which was estimated using only TOPEX_A data. Replacing the SSB model on TOPEX with the new TOPEX_A and TOPEX_B models not only removes the correlation with SWH in the Jason-TOPEX SSH residuals, it also removes most of a bias between TOPEX_A and TOPEX_B that has been observed in tide gauge calibrations. The magnitude of the change between TOPEX_A SSH and TOPEX_B SSH is of the order of 10 mm globally, with TOPEX_B SSH increasing when the new SSB model is applied. The application of this improved model will increase the rate of observed global mean sea level rise from 1.7 mm/year when the original TOPEX data are used to 2.8 mm/year when the updated SSB models are applied.





Most people probably assume that satellite estimates of sea level change are based on accurate, actual measurements. They would likely be shocked to find that, instead, they are based on dodgy data, that is then subjected to massive adjustments, bigger than the observed changes.

  1. March 21, 2014 12:27 pm

    Another climate-related science miracle, 100% of adjustments going in the same direction.,

    Unsurprising, even.

  2. March 21, 2014 1:28 pm

    This is essentially the same scenario for precision GPS measurements. What I haven’t seen is correlation between satellite and benchmark water level measurements down on the ground (so to speak) – which is essentially verification of the adjustments applied. Maybe that’s been done – and other adjustments made to those terrestrial measurements to achieve “desirable convergence”.

    A whole can of worms there – to be sure… and when the raw data goes AWOL… one has to be suspicious – obvious – but worth stating I reckon.

  3. Joe Public permalink
    March 21, 2014 3:34 pm

    It has always amazed me that satellite ‘measurements’ of rising/falling sea surface level can be stated to 1/100th of a cm.

    From WikiP:- “From orbit 1,330 kilometers above Earth, TOPEX/Poseidon provided measurements of the surface height … to an accuracy of 3.3 centimetres.” [My bold]

  4. March 21, 2014 4:32 pm

    Thanks, Paul. Good article; Dr Ronan Connolly’s article is very well done.
    Living on an island in South Florida I have my personal tide gauge, made from experience while walking the beach at all tide levels and Moon phases. My personal result: No change in the last 40 years.
    According to the University of Colorado, it should have been rising about 3mm/year, that is 120mm total. This is 12 cm or 4.7 inches, I should have noticed as my parking lot is at sea-level and floods about half an inch in extremely high tides at Full Moon or New Moon.
    During hurricanes it can surely flood a lot more, so it is evacuated to escape the surge.

  5. March 21, 2014 10:30 pm

    A most comprehensive article, Ronan, which identifies the many factors and uncertainties required to estimate a “global” sea level trend. Just like “global air temperatures” this is fairly academic to me – I am more interested in what the effects are at specific locations.

    My plots of (annual mean) sea levels at 53 locations throughout the world identifies that the trend has been fairly constant for the past 200 years. Note that these are plots of “raw” data, without any adjustments.

    I suspect that residents of northern Scandinavia (where the sea level is dropping) are not very concerned about the threat of future sea level rise!

  6. March 22, 2014 3:14 pm

    I’m glad you all found our essay of interest. Paul, thanks for spreading the word about our website through this post! 🙂

    Omnologos, there does seem to be a popular belief amongst some researchers that if the data doesn’t match the theory, you should adjust the data… 😉 Needless to say, we’re not fans of that particular philosophy…

    TomO, there have been a few attempts in recent years to compare GPS measurements directly to tidal gauge records, e.g., Woeppelman et al., 2007. But, so far the available data is still quite limited and most of the studies have had to make unjustified assumptions, e.g., assuming that the satellite estimates are accurate.

    Joe, on a related note, if you look at the hourly tidal gauge data for a given station, the daily sea level difference between high tide and low tide can be several metres, e.g., see Figure 7 in the full essay Paul linked to. I think the fact that the reported sea level trends which people are discussing are only of the order of mm/year is often overlooked.

    Andres, it’s interesting to hear the results of your own “personal tide gauge”! It is in keeping with what we have found – many sites have shown little, if any trend, and the data is still too ambiguous to draw any conclusions over “global” trends…
    By the way, since you mention hurricanes, we also have a similar essay on hurricane trends on our blog that you might find interesting:

    Brian, agreed. It is the local sea level trends which are of most relevance for societies. For this reason, if we are to genuinely deal with the concerns about the sea levels near coastal areas, e.g., Bangladesh, then we should be putting much more focus on issues such as combatting local coastal subsidence (which can be several cm/year!), instead of putting all our emphasis on CO2 emissions.

  7. Bill Illis permalink
    March 23, 2014 1:25 pm

    I am most interested in how the tide gauges measurements are changing by year. What was the sea level increase from the tide gauges in 2011 or 2012 for example. Is the trend increasing?

    This turns out to be a very difficult problem, however, even though we have this huge database at PMSL and other places. Why can’t they just give us the data on a yearly basis. But they do not.

    I’ve downloaded the whole database before and consolidated it down to just yearly values (many tricks were needed) and came up with an average of about 1.4 mms/year more-or-less consistent from 1980-2009.

    In Ronan Connolly’s article, he links to a table of 500+ gauges from PMSL here which is updated for 2011 values in many cases.

    I went to the Wayback Machine to see how this table had changed over time. The 2010 version of this table was available.
    I consolidated it down again so that the same gauges were used in each and that only gauges that added new years were used ie, updated 2010 and/or 2011 but not taking any years out.

    300 gauges met this criteria and the average of these gauges was 2.26 mms/year over 1.7 years of updated data for 2010 and 2011.

  8. March 23, 2014 10:42 pm

    Bill, a useful presentation of historical sea level trends is that by NOAA. This presents plots of PSMSL and CO-OPS stations throught the world.

    The plots are of monthly data, but I wish that they also had plots of the annual mean values. The long-term trends are obvious and they are up to 2012 in most cases. They also show the average seasonal variation at each location.


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