While there is a growing research effort to assess the degree to which recent extreme weather events relate to historical anthropogenic emissions, understanding of the sensitivity of results to aspects of the experimental setup and the characterization of the event remain poorly explored. This poster describes the C20C+ Detection and Attribution Project, an international activity managed by LBNL that is designed to address this gap in understanding. It is producing large ensembles of atmospheric model simulations representing both the climate we have experienced and various estimates of the climate that might have been had human activities not interfered with the climate system. These ensembles will provide a novel resource for studying how anthropogenic emissions have contributed to the changing chance of extreme weather events over the past half century, as well as discerning changes in seasonal predictability and in the frequency of extreme events during that period. Most particularly, these ensembles will permit examination of the sensitivity of results to selection of atmospheric model, estimation of attributable ocean warming, state of interannual oceanic variability, and state of natural external forcings. As well as outlining the project, this poster will describe results from the first set of simulations produced with CAM5.1-1degree under the CASCADE project's contribution to the international activity.