Between 2008 and 2015, the United States was able to reduce carbon emissions while enjoying limited economic growth. Earlier this year in Science, President Barack Obama pointed to this data as evidence of “the irreversible momentum of clean energy” (10.1126/science.aam6284). But in a commentary published September 6 in the journal Joule, John Deutch, who has worked with the energy departments of several presidential administrations, urges cautious optimism. He explains the country experienced a short-term decoupling of emissions and economic growth that models suggest won’t sustain in the future or be enough to prevent climate change.