A method of calculating the carbon or greenhouse gas footprint of a food composition includes determining the CO2 or greenhouse gas emission of each ingredient by associating a total energy value with the ingredient, determining the proportion of the total energy value contributed by each of at least one energy source, where each energy source has a characteristic CO2 or greenhouse gas emissions value, and then calculating a CO2 or greenhouse gas emission value for the ingredient based on the GHG emission values of each contributing energy source. Also, optimizing the CO2 or greenhouse gas emissions in the formulation of pet food employs this method.