**5. Do exergy measures accomplish the standards for a good environmental indicator?**

With exergy replacement cost we cannot measure the progress to sustainability but the progress to depletion, ultimate to Thanatia. It can be like a watch measure to death. We can decelerate death, but we cannot avoid it. Nevertheless, it can be a good policy guide since it can quantify the annual depletion of the mineral capital and explain crystal clear, what are the needed measures to stop it or at least to slow it. The only question is to prove that the indicator undertakes the requirements for a good one.

The Organisation for Economic Co-operation and Development (OECD) [17, 18] proposed a set of criteria for having a good environmental indicator: policy relevance, analytical soundness, and measurability.

Concerning policy relevance, a good indicator must be: (a) easy to interpret, (b) show trends over time, (c) be responsive to changes in underlying conditions, and (d) have a threshold or reference value against, which conditions can be measured.

Exergy as the available energy is easy to interpret since it is what laypeople call energy. As a matter of fact, we pay exergy not energy. The exergy replacement cost and the exergy cost indicators can show either aggregated or disaggregated trends over time just being responsive to any kind of variation in amounts of extraction, improvements in processes efficiency, substitution, recycling, and whatever changes in the element cycle. Finally, Thanatia as a threshold is the best provider of reference values to which evolutions on depletion can be measured. Therefore, our indicators are policy relevant according to OECD.

Concerning analytical soundness, indicators should be well supported in technical and scientific terms. It is obvious that exergy indicators are well based on the second law of thermodynamics.

Concerning measurability, indicators should be: (a) calculated from data that are readily available or available at reasonable cost, (b) data should be documented and of known quality, and (c) data and indicators should be updated at regular intervals.

The data for calculating exergy replacement costs must come from data provided from the physical SEEA tables. Assets providing amounts of extracted material, composition, ore grades, amounts of processed, smelted, refined chemicals, amounts of recycled material with its composition, etc., available in the PSU tables are what exergy costs need for their calculations. The data obtained for exergy replacement costs will be as reliable as the data provided by SEE accounts. And the calculations required are easily available with adequate computer programs. International agreements could be reached in order to update both data and indicators as well as improve interpretations and act accordingly. As exergy is an additive property, it has the capability of integrating and aggregating a large variety of causes of variation including how substitution, recycling, and nanotechnologies positively improve our global management of the mineral capital. Conversely, each country, company or mine could use the exergy replacement cost to account for the attained depletion level. And this cost can easily be converted into money units just by multiplying it by some previously agreed energy price. Money accounts are useful at the micro level from companies to countries, but at a global scale and throughout time, exergy accounts may give a clearer picture far removed from economic vagaries.

Finally, the proposed indicators are complementary with others, especially with cradle-tograve indicators that close the cycles of elements. All together could provide an overall measure of "unsustainability" and its yearly variation, which could be used as a policy lever.
