Zecca and Chiari (2010) have sought to challenge the findings of Nel and Cooper (2009), who argued that the impending peak and decline of fossil fuel production will most likely lead to a lower emissions trajectory than the majority of scenarios offered by the IPCC. Zecca and Chiari used their own model to produce a higher projection of atmospheric CO2 concentration, drawing on the conclusions of Archer (2005) to do so. In this short comment, we show that the model of Zecca and Chiari is an erroneous interpretation of Archer (2005). We present a model based on an improved interpretation of Archer's paper and demonstrate that the model still significantly overpredicts atmospheric CO2 with respect to historical observations of CO2 concentrations. Furthermore, whichever carbon cycle model one chooses to use (i.e. to convert a carbon emissions scenario into an atmospheric CO2 concentration), the primary concerns raised by Nel and Cooper (2009) -regarding an inability to achieve the IPCC's high emissions scenarios due to fossil fuel constraints - remain valid. These concerns are supported by a growing body of literature pointing to a peak in world oil and coal production within the coming few decades. (C) 2010 Elsevier Ltd. All rights reserved.