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: