Frame and Allen turned the problem on its head. Rather than focusing on atmospheric concentrations, they turned to emissions. What if humanity shut off human-caused carbon dioxide emissions? The duo tested the idea out on their climate model right there on the train. The result? Global temperatures remained stable at their new level. In short, the world would stop warming once humanity reached “net-zero” carbon emissions. “It was quite cool sitting on the train looking at these figures for the first time and thinking: ‘Wow, that’s a big deal,’”