The data used in Niemeier et al (2013, JRL) are mostly available via the Earth System Network. For additional data, e.g. G3S, you have to contact Ulrike Niemeier, as the server
http://implicc1.dkrz.de:8080/thredds/projectCatalog.html
was switched off.
The study was part of the GeoMIP intercomparison project. Most of the GeoMIP output is available through the Earth System Grid Framework.