Discussing resource depletion, Prof. Norris explained that the earlier methods of calculating depletions were inaccurate and resembled attempts to weigh frogs, literally. Just as frogs do not keep still while being weighed and keep jumping, it was very difficult to estimate existing resources, usage and left over balance since new discoveries of resources are also to be considered continuously. If one product life cycle uses one mega joule of coal and another
product life cycle also uses one mega joule of oil, both have same consequence in terms of fossil fuel depletion, as per the methods used in the 1980s. However, since then the method has been improved. For instance, as we use the established resources (proven reserves) of coal or oil and move to next one, they tend to be smaller, deeper, less assessable and more expensive to extract. The EcoIndicator-99 focuses on the fact that it takes more energy to extract usable resources (whether fuels or minerals) from lower quality deposits. In other words, he explained, the future generations have to pay a higher price to get lesser resources, if we deplete them now by reckless usage. The EcoIndicator-99 developed the scenario for quite far into the future for predicating the energy cost of extracting resources/substitutes in future.