In anticipation of the 2013 publication of the Intergovernmental Panel on Climate Change's fifth assessment report, many in the climate modeling community came together in 2008 under the banner of the Fifth Coupled Model Intercomparison Project (CMIP5). An international effort, this project sought to identify crucial research directions and laid down standardized parameters under which atmosphere-ocean coupled general circulation models (AOGCMs) should be tested so that they can be compared on equal footing. Andrews et al. assessed 15 models participating in the CMIP5 to find the most up-to-date measure of climate sensitivity and the remaining sources of model uncertainty. To test the AOGCMs, the authors looked at how the modeled top-of-the-atmosphere (TOA) radiative emissions from the Earth respond to an imposed change in atmospheric carbon dioxide concentration. Because TOA emissions depend on the global mean atmospheric and surface temperatures, this technique provides an effective way to investigate the feedback mechanisms built into each of the models. The authors assessed model runs for pre-industrial atmospheric carbon dioxide levels as well as those that simulated a sudden quadrupling in the concentration of atmospheric carbon dioxide—the amount that could be reached given a “do nothing” approach to mitigating carbon emissions.