The claim of a 6cm per decade rise is from their 1990 Executive Summary.
Note on the graphic 2012_rel4: Global Mean Sea Level Time Series (seasonal signals removed) – from this IPCC compliant Org – the claimed rate from adjusted satellite data is only 3.1mm per year – HALF of IPCC 1990.
But there is more – checkout the new Jo Nova article – Are sea-levels rising? Nils-Axel Mörner documents a decided lack of rising seas. Nils-Axel Mörner says the unadjusted satellite data show negligible trend and in his new pdf paper SEA LEVEL IS NOT RISING – has this stunning graphic.
So the rate of sea level rise is just an artifact of IPCC compliant adjustments.