There is an unknown but probably small probability that the West-Antarctic Ice Sheet (WAIS) will collapse because of anthropogenic climate change. A WAIS collapse could cause a 5-6 metre global sea level rise within centuries. In three case studies, we investigate the response of society to the most extreme yet not implausible scenario, a five-metre sea level rise within a century, starting in 2030. The case studies combine a series of interviews with experts and stakeholders with a gaming workshop. In the Rhone delta, the most likely option would be retreat, with economic losses, perhaps social losses, and maybe ecological gains. In the Thames estuary, the probable outcome is less clear, but would probably be a mix of protection, accommodation and retreat, with parts of the city centre turned into a Venice of London. A massive downstream barrier is an alternative response. In the Rhine delta (the Netherlands), the initial response would be protection, followed by retreat from the economically less important parts of the country and, probably, from Amsterdam-Rotterdam metropolitan region as well. These impacts are large compared to other climate change impacts, but probably small compared to the impacts of the same scenario in other parts of the world. This suggests that the possibility of a anthropogenic-climate-change-induced WAIS collapse would strengthen the case for greenhouse gas emission reduction.