Background
The authors write that "estimates of global-scale sea level rise over the past century are mainly based on long tide gauge records," but that "the range of estimates published in the literature is rather wide," due to "the methods used to correct the tide gauge records for vertical displacements of the land upon which they are located." Hence, they set out to resolve this latter dilemma.

What was done
Wöppelmann et al. "analyzed GPS observations from a global network of 227 stations using a consistent processing strategy over the whole period from January 1997 to November 2006," noting that 160 of the stations they studied "are co-located within 15 km of a tide gauge." Assuming that land motion is essentially linear on the time span they considered," the GPS vertical velocities they derived were then used "to correct for the land motion affecting the tide gauge records to derive absolute (geocentric) changes in sea level."

What was learned
The eight researchers, hailing from France, Spain and the United Kingdom, obtained a global-average rate of geocentric sea-level rise for the past century, ranging from 1.55 to 1.61 mm/year, depending on whether or not one outlier (of 28 individual regions) was included or omitted from their analysis.

What it means
Wöppelmann et al. say their results are "in good agreement with recent estimates," citing the 1.7 mm/year value derived by Church and White (2006) and Holgate (2007), and that they are also "in good agreement with the sum of steric sea level and land ice contributions estimated by Leuliette and Miller (2009) over the recent period of 2003-2007 (1.5 mm/year) using altimetry, Argo, and GRACE gravity observations." Hence, it would appear that 20th-century sea level rise has not been in any way unusual, even over the most recent decade of supposedly unprecedented warmth.

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.

Holgate, S.J. 2007. On the decadal rates of sea level change during the twentieth century. Geophysical Research Letters 34: 10.1029/2006GL028492.

Leuliette, E.W. and Miller, L. 2009. Closing the sea level rise budget with altimetry, Argo, and GRACE. Geophysical Research Letters 36: 10.1029/2008GL036010.