In this project we address the centennial evolution of surface mass balance and in particular meltwater runoff from the Greenland ice sheet (GrIS). GrIS mass loss increased from ~150 Gt yr-1 in 2000-2005, to ~265 Gt yr-1 in 2005-2009 and ~380 Gt yr-1 in 2009-2012, contributing 20-30% to total global average sea level rise over the last decade. An increasingly large fraction of the mass loss (from ~55% in 2000-2005 to ~70% in 2009-2012) is caused by enhanced meltwater runoff. The rapidity of these recent developments is alarming, and we urgently need to address the following questions: how exceptional are recent GrIS surface melt events when viewed in a century-long perspective? What fraction can be ascribed to anthropogenic climate change, and what fraction to natural (decadal) variability? What is the role of transient firn processes and the albedo-melt feedback on meltwater runoff, and how does uncertainty in their representation affect the results? We address these issues by using the ECMWF 20th century global atmospheric re-analysis product, to be released in the summer of 2014, to force -in a fully transient fashion- the coupled regional climate/snow model RACMO2.3. This effort will provide a time series of GrIS surface mass balance, melt and runoff for the period 1900-2014, nearly doubling the length of the currently available time series and enabling us to provide physically-based answers to these pressing questions.