5. Discussion
Because we used our own indicator (called environmental efficiency index) as a proxy for environmental efficiency, we consider appropriate to comment upon its evolution for several countries. It is necessary to indicate the level of efficiency that has been attained by each country. It is obvious that we are looking for small values of the indicator, which indicate a high level of the environmental efficiency of renewable energy investments. We specify once again that the index is calculated as a ratio between effects (the CO2 emissions) and efforts (the investments), and therefore we can consider that this index is a truly efficiency indicator. Here, the effects need to be minimized for certain efforts; in most cases, an indicator of efficiency evaluation is calculated as the ratio between effects and efforts (expressing the amount of effect per one unit of effort); as a consequence, the value of the indicator should be as big as possible. However, in this case the effect is the emission of CO2 and our opinion is that this indicator should be as small as possible, which will mean an increased efficiency. Fig. 1 shows the evolution of the index for four suggestive countries Iceland, Latvia, Luxembourg and Norway, as they have the smallest values of the environmental efficiency index.