Abstract Emissions scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are based on detailed energy system models in which demographics, technology and economics are used to generate projections of future world energy consumption, and therefore, of greenhouse gas emissions. We propose in this paper that it is useful to look at a qualitative model of the energy system, backed by data from short- and medium-term trends, to gain a sense of carbon emission bounds. Here we look at what may be considered a lower bound for 21st century emissions given two assumptions: first, that extractable fossil-fuel resources follow the trends assumed by “peak oil” adherents, and second, that no climate mitigation policies are put in place to limit emissions. If resources, and more importantly, extraction rates, of fossil fuels are more limited than posited in full energy-system models, a supply-driven emissions scenario results; however, we show that even in this “peak fossil-fuel” limit, carbon emissions are high enough to surpass 550 ppm or 2 °C climate protection guardrails. Some indicators are presented that the scenario presented here should not be disregarded, and comparisons are made to the outputs of emission scenarios used for the IPCC reports.