Remember 'Ecological Footprint' for cities ( en.wikipedia.org/wiki/ Ecological _ footprint ) ? Well that was not too long before 'Carbon Footprint' came along. However, the challenges are not too different. It is most important to be clear and transparent about 'what' , 'how', and 'why' the assessment is being made. It is often unclear what the carbon footprint is assessing. Is it the "gross" or "absolute" emissions, or is it the "net" or "relative" emissions ? While the former is really the footprint, the latter is a measure of the impact of an intervention, with respect to a counterfactual scenario. However, it is important to clearly define the intervention. The boundary of an intervention may vary depending on it nature and scale. For example, a new urban road may affect the whole network of roads in a city and hence have major indirect impacts in terms of carbon emissions, as compared to replacing pump sets at the municipal waste water treatment facility. In the parlance of GHG accounting this opens up the question of scope 1, 2 and 3 emissions - as defined by the GHG protocol (www.ghgprotocol.org) How the counterfactual scenario is defined can affect the assessment significantly ; and this relates to how the baseline or business-as-usual scenario is defined. The baseline could be considered "static" or "dynamic"? While the former assumes that without intervention the scenario would continue to be the same as before the intervention, the latter assumes changes that would occur over the time period of assessment. The time period could range from an average year of operation / implementation to the economic lifetime of the intervention; or even the time frame of loan repayment, if externally financed. Finally, the question boils down to why the footprint is being assessed. Is it to account for emissions or analyze the impact of interventions? While the former would call for a gross or absolute assessment, the latter requires the assessment of net or relative emissions.