Saturday, February 27, 2010

Sea Levels: Large Variances between Tide Gauges and Satellite Altimetry

Determining changes in global sea levels is an enormously complicated undertaking, with measurement error, calibration error, seasonal adjustments, and regional differences as four of the most significant problems to overcome. Different types of measurement achieve different results. For instance, Sea Level Expert Dr. Nils-Axel Mörner (see post just prior to this also) finds that careful analysis of historical tide gauge records (correcting for subsiding, tectonic shifts, etc) shows no significant global sea level rise during most of the 20th century, and also finds corroboration of this from the geologic and coral reef records in the field (if the sea level doesn't rise reefs have to grow laterally rather than vertically, etc.).

How about satellite altimetry measurements of sea levels? These have their own unique set of problems, including
  • Large divergences between GPS-corrected tide gauges and satellite altimetry at the same location (see below)
  • Use of two different satellites at different times, and two different altimeters "due to degradation in the original instrument" on TOPEX with different electronics and resultant measurement divergence
  • And factors mentioned in Nerem et al:
"Satellite altimetry is somewhat unique in that many adjustments must be made to the raw range measurements to account for atmospheric delays (ionosphere, troposphere), ocean tides, variations in wave height (which can bias how the altimeter measures sea level), and a variety of other effects. In addition, the sea level measurements can be affected by the method used to process the altimeter waveforms, and by the techniques and data used to compute the orbit of the satellite. Early releases of the satellite Geophysical Data Records (GDRs) often contain errors in the raw measurements, the measurement corrections, and the orbit estimates." Nerem et al also mentions other major problems such as drift in the TOPEX microwave radiometer, a change from the original TOPEX altimeter to the back-up altimeter in 1999 "due to degradation in the original instrument" which had "different electronics" from the original resulting in divergent measurements which had to be "corrected'.
What about the first point, that there are large variances between GPS-corrected tide gauges and satellite altimetry at the same location? Here are 2 graphs from the University of Colorado at Boulder Sea Level Change site:

TOPEX calibration




Jason-1 & Jason-2 calibration







From the site:
    The method of producing the tide gauge estimates of altimeter drift that we report here is described in detail by Mitchum (2000), and will not be discussed in full here. Briefly, the method works by creating an altimetric time series at a tide gauge location, and then differencing this time series with the tide gauge sea level time series. In this difference series, ocean signals common to both series largely cancel, leaving a time series that is dominated by the sum of the altimetric drift and the land motion at the tide gauge site. Making separate estimates of the land motion rates and combining the difference series from a large number of gauges globally results in a times series that is dominated by the altimeter drift. Since the difference series at separate time gauge locations have been shown to be nearly statistically independent (Mitchum, 1998), the final drift series has a variance much smaller than any of the individual series that go into it. Because of the relatively large number of degrees of freedom, this method outperforms calibrations from dedicated calibration sites, although it is only a relative calibration, meaning that it cannot determine any absolute bias. It can, however, detect change in a bias, either a drift or a step change. For the tide gauge calibrations, global mean sea level during the period of TOPEX-A operation is used as zero level for TOPEX-B, POSEIDON, and Jason.

    Ideally, one would want to include all of the available tide gauges in the calibration. A number of gauges, however, have a significant lag in reporting of records and are not available for the Jason calibration. On the other hand, some others do not extend backward through most of the T/P mission. We have restricted the ~100 available gauges to a set of 64 near real-time stations that span the majority of both the T/P and Jason missions, and will therefore provide a relatively consistent calibration for both.

As stated above "Since the difference series at separate time gauge locations have been shown to be nearly statistically independent (Mitchum, 1998), the final drift series has a variance much smaller than any of the individual series that go into it. Because of the relatively large number of degrees of freedom, this method outperforms calibrations from dedicated calibration sites, although it is only a relative calibration, meaning that it cannot determine any absolute bias." Looking at the individual GPS-corrected tide gauges in the two graphs above compared to the satellite altimetric measurement at the corresponding location shows very large divergences of up to 25mm at a given point in time. Yes, if you sum all the anomalies from the carefully selected subset of tide gauges compared to the satellite records it is statistically insignificant from zero, but the large variances on individual records suggests much more doubt in the accuracy of satellite altimetry and/or GPS-corrected tide gauges than is commonly held.

Considering the bias the blogosphere has identified with adjustments to the global thermometer records, similar biases might be present in the many adjustments made to the satellite altimetry data. Although these adjustments have been broadly described in Nerem et al, the raw satellite data and documentation of all the adjustments made is not available for independent analysis. As we have seen time and again inappropriate adjustments to the thermometer data, the raw satellite altimetery data and the hopefully fully-documented adjustments should be made publicly available for independent assessment and verification. It would also be useful to provide a list of the 64 tide gauges from the 100+ available which were selected for calibration of the satellite data, as well as the raw data and any adjustments made to those tide gauge measurements.


Related:

IPCC AR4 page on errors associated with satellite altimetry & tide gauges (note each pass of the TOPEX satellite only measures sea height to an accuracy of 80 mm at a 95% confidence level)
Background paper
poster showing unexplained tide gauge/satellite altimetry measurement divergences
Church & White paper 
poster on the oldest tide gauges of the southern ocean 
Basic Geology Part 3 Sea Level Rises during Interglacial Periods 

7 comments:

  1. Of course, the other well known factor affecting recorded sea levels is geological subsidence or emergence of the land mass, thus moving the tide gauges either up or down relative to mean sea level.

    There are many examples. However, this factor seems never to be acknowledged as a factor.

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  2. http://s8.postimg.org/9ysbkpw51/All_Sea_Level_Measurements_1960_2013.png

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  3. http://s2.postimg.org/xcp9tsz6x/Sea_Level_Measurements_PMSL_1930_1980_2009.png

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  4. http://wattsupwiththat.com/2013/07/03/new-study-using-grace-data-shows-global-sea-levels-rising-less-than-7-inches-per-century/#comment-1353336

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  5. Excellent lecture by Dr. Willie Soon explaining the problems with satellite & tide gauge measurements

    http://joannenova.com.au/2013/08/five-or-more-failed-experiments-in-measuring-global-sea-level-willie-soon/

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  6. http://stevengoddard.wordpress.com/2013/08/24/it-isnt-just-the-temperature-data/

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  7. http://paradigmsanddemographics.blogspot.com.au/2013/08/plain-dealer-example-of-worldwide.html

    ReplyDelete