I have not found ISO 14031 very useful in providing guidance on measuring environmental performance and the fact that it has not been revised or updated in over 12 years suggest that others do not find great value in it either. Measurement of environmental performance is quite subjective and complicated. For example, which organization is the better environmental performer? The one that emits 100 tons per year (TPY) of CO2 of the one that emits 100000 TPY. What if these organizations are in different industries different sizes and different climates?
To begin to be meaningful the emissions need to be normalized. A simple normalized metric might be tons of CO2 per unit of production. This performance metric could also consider atmospheric influences like very cold or hot temperatures that require more energy use or base load emissions when production is either abnormally high or low. The normalizing performance calculation can become quit complex as it matures.
Once normalized a comparison of similar industries can provide valuable information about performance. Unfortunately it is difficult to get normalized performance information of similar industries because the metrics used by different companies are rarely the same.
That’s why certification/registration to ISO 14001 can be a better indicator of an organizations overall environmental performance. If the certifying auditors are doing their job they will insist on the organizations demonstrating commitment to systematically improving environmental performance. They will also expect to see evidence of actual (measurable) environmental performance improvement.
Certification to ISO 14001 is still the best way to ensure that an organization with which you desire some kind of relationship (you want to buy their stock or their product) is committed to improving environmental performance and is actively working to achieve better performance.