The cryosphere is in fast transition. The possibility that the ongoing rapid demise of Arctic sea ice may instigate abrupt changes on the Greenland Ice Sheet (GIS) is not tackled by current research. Ice cores from the GIS show clear evidence of past abrupt warm events,up to 15 degrees warming in less than a decade, possibly caused by disappearing se ice in the Nordic Seas..Arctic sea ice extent was in 2012 half of the 1979-2000 average. Satellite data document an increasing loss of GIS ice mass since 1990 and temperatures have risen markedly at the GIS summit. Strong transient changes in both Arctic cryospheric entities prompts the question: Is the dramatic decline in Arctic Sea Ice heralding a new phase of abrupt change, similar to those recorded in ocean sediments and ice cores? Such changes would have major consequences for the GIS mass balance and global climate and sea level. Ice2Ice will approach this complex problem by integrating 4 PI teams from three Nordic world class research centres comprising empiricists and dynamicists specialized in Arctic and Greenland atmospheric, oceanic and cryospheric sciences. With an innovative combination of synchronized records of GIS parameters, records of sea ice change and models ranging from global climate models to regional and process models, Ice2Ice will be the first concerted effort to tackle the question of the cause and future implications of past abrupt climate change in Greenland, the main hypothesis being that Arctic and sub-Arctic sea ice cover is key to understand past and future Greenland temperature and ice sheet variations. In Ice2Ice this will be done by:a)describing the nature, timing and extent of abrupt events across climate archives,b)resolving mechanisms behind the sudden demise of sea ice cover,c)identifying the risk that the ongoing rapid diminution of Arctic sea ice cover could give abrupt GIS changes in the future, d)determining the impacts of such changes for the GIS, Arctic and global climate.