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Ocean Mass Trends (and Sea Level Estimates) from GRACE
Reference
Quinn, K.J. and Ponte, R.M. 2010. Uncertainty in ocean mass trends from GRACE. Geophysical Journal International 181: 762-768.

Background
The authors write that "ocean mass, together with steric sea level, are the key components of total observed sea level change," and that "monthly observations from the Gravity Recovery and Climate Experiment (GRACE) can provide estimates of the ocean mass component of the sea level budget, but full use of the data requires a detailed understanding of its errors and biases."

What was done
In an effort designed to provide some of that "detailed understanding" of GRACE's "errors and biases," Quinn and Ponte conducted what they describe as "a detailed analysis of processing and post-processing factors affecting GRACE estimates of ocean mass trends," by "comparing results from different data centers and exploring a range of post-processing filtering and modeling parameters, including the effects of geocenter motion, PGR [postglacial rebound], and atmospheric pressure."

What was learned
The two researchers report that the mean ocean mass trends they calculated "vary quite dramatically depending on which GRACE product is used, which adjustments are applied, and how the data are processed." More specifically, they state that "the PGR adjustment ranges from 1 to 2 mm/year, the geocenter adjustment may have biases on the order of 0.2 mm/year, and the atmospheric mass correction may have errors of up to 0.1 mm/year," while "differences between GRACE data centers are quite large, up to 1 mm/year, and differences due to variations in the processing may be up to 0.5 mm/year."

What it means
In light of the fact that Quinn and Ponte indicate that "over the last century, the rate of sea level rise has been only 1.7 0.5 mm/year, based on tide gauge reconstructions (Church and White, 2006)," it seems a bit strange that one would ever question that result on the basis of a GRACE-derived assessment, with its many and potentially very large "errors and biases." In addition, as Ramillien et al. (2006) have noted, "the GRACE data time series is still very short," and results obtained from it "must be considered as preliminary since we cannot exclude that apparent trends [derived from it] only reflect inter-annual fluctuations." And as Quinn and Ponte also add, "non-ocean signals, such as in the Indian Ocean due to the 2004 Sumatran-Andean earthquake, and near Greenland and West Antarctica due to land signal leakage, can also corrupt the ocean trend estimates."

Clearly, the GRACE approach to evaluating ocean mass and sea level trends still has a long way to go -- and must develop a long history of data acquisition -- before it can ever be considered a reliable means of providing assessments of ocean mass and sea level change that are accurate enough to detect an anthropogenic signal that could be confidently distinguished from natural variability.

References
Church, J.A. and White, N.J. 2006. A 20th-century acceleration in global sea-level rise. Geophysical Research Letters 33: 10.1029/2005GL024826.

Ramillien, G., Lombard, A., Cazenave, A., Ivins, E.R., Llubes, M., Remy, F. and Biancale, R. 2006. Interannual variations of the mass balance of the Antarctica and Greenland ice sheets from GRACE. Global and Planetary Change 53: 198-208.

Reviewed 21 July 2010