In a recent article, MacKenzie (Energy Policy 31 (2003) 1183) proposed an analytical approach based on a universal logistic growth curve to establishing, for each year, the minimum fraction of each country's CO2 emissions that can arise from non-fossil energy sources. The approach provides an alternative viewpoint on how to tackle the global CO2 emission problem in a more dynamic manner and has several attractive features. In this paper, we present from the analytical and energy analysis viewpoints a number of possible complications in implementing the approach Lis well as some possible refinements. (C) 2003 Elsevier Ltd. All rights reserved.