Paper on Satellite Sea Level Calibration

One of the interesting things about satellite data are the degrees and efforts taken to improve the calibration. For example, in 2004, Christy/Spencer corrected the UAH data for a systemic error that had show less global warming than their earlier data. About six months ago, a U. of Washington paper suggested there is may be another latent error, daily sampling, but this remains an open question. So I am not too surprised at this:

The rate of global mean sea-level (GMSL) rise has been suggested to be lower for the past decade compared with the preceding decade as a result of natural variability1, with an average rate of rise since 1993 of +3.2 ± 0.4 mm yr−1 (refs 2, 3). However, satellite-based GMSL estimates do not include an allowance for potential instrumental drifts (bias drift4, 5). Here, we report improved bias drift estimates for individual altimeter missions from a refined estimation approach that incorporates new Global Positioning System (GPS) estimates of vertical land movement (VLM). In contrast to previous results (for example, refs 6, 7), we identify significant non-zero systematic drifts that are satellite-specific, most notably affecting the first 6 years of the GMSL record. Applying the bias drift corrections has two implications. First, the GMSL rate (1993 to mid-2014) is systematically reduced to between +2.6 ± 0.4 mm yr−1 and +2.9 ± 0.4 mm yr−1, depending on the choice of VLM applied. These rates are in closer agreement with the rate derived from the sum of the observed contributions2, GMSL estimated from a comprehensive network of tide gauges with GPS-based VLM applied (updated from ref. 8) and reprocessed ERS-2/Envisat altimetry9. Second, in contrast to the previously reported slowing in the rate during the past two decades1, our corrected GMSL data set indicates an acceleration in sea-level rise (independent of the VLM used), which is of opposite sign to previous estimates and comparable to the accelerated loss of ice from Greenland and to recent projections2, 10, and larger than the twentieth-century acceleration2, 8, 10.

Source: http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2635.html​

Lay articles and interviews indicate there was a mismatch from increasing global ice loss and the decreasing rate of change. So this team went back and found a systemic bias in the data that brings the rate of change in line with the ice loss. It also reduces the satellite measure rate of change from ~3.2 mm/year to ~2.6 to ~2.9 mm/year. But like any such paper, it takes time to review and requires acceptance by the current data keepers, NOAA and U. of Colorado. So too, the recent U. of Washington paper on the UAH data has not yet resulted in the data keepers accepting and issuing new data sets.

What I enjoyed is seeing how a disconnect between ice loss observations and sea level rise led to this investigation. Why the sea level rise did not match the math models? Then a new method for dealing with tide records and satellite data calibration led to this paper. Accuracy trumps 'bragging rights' any day.

Bob Wilson

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