This article, published in the journal Dædalus, argues that humans have a “budget” of a trillion tons of carbon we can emit before the probability of adverse consequences grows significantly, half of which has been spent. They use results from modeling using the En-ROADS software tool to study how changes in energy supply and demand might affect emissions and, in turn, climate. Topics discussed and modeled through EN-ROADS include carbon emissions growth in developing countries, energy efficiency, renewable energy, CCS, and other new technology.