Fulfilling the promise of the 2015 Paris Agreement on climate change — most notably the goal of limiting the rise in mean global surface temperature since preindustrial times to 2 degrees Celsius — will require a dramatic transition away from fossil fuels and toward low-carbon energy sources. To map out that transition, decision-makers routinely turn to energy scenarios, which use computational models to project changes to the energy mix that will be needed to meet climate and environmental targets. These models account for not only technological, economic, demographic, political, and institutional developments, but also the scope, timing, and stringency of policies to reduce greenhouse gas emissions and air pollution.