**5. Discussion**

The analysis in the previous section shows that twelve out of eighteen countries recorded average annual changes in eco-efficiency MPI greater than unity over the period 1997 to 2020 (Table 3). The average eco-efficiency MPI over this period for the G18 was, at 0.5%, low. Eco-efficiency leaders over this period include France, Great Britain, Russia, Germany, Italy, and the United States. Germany, France, Great Britain, and Italy benefited from the European Union's 2020 Climate and Energy Package, which aims to reduce greenhouse gas emissions 20% from 1990 levels, target 20% of EU energy from renewables, and accomplish a 20% improvement in energy efficiency by the year 2020 [37]. Great Britain's performance is partly due to fuel switching and reduced fuel consumption. Great Britain has moved to a cleaner fuel mix in electricity generation as coal was switched for natural gas and renewables [38]. Reduced fuel consumption by business and industry also contributed to the reduction in carbon dioxide emissions. However, Great Britain's decision to exit the European Union (BREXIT) may weaken the stimulus and incentive for further eco-efficiency improvements. In addition, Germany's success comes from cross-partisan policy consistency, shared goals between political leaders and renewable energy advocates, a strong social movement for renewable energy, and decentralized energy policies [39]. These results are consistent with the results of Midova et al. [40], who study low-carbon scenarios of six northwest European countries (Netherlands, Germany, France, Denmark, the UK, and Belgium). In ranking these countries on ten criteria regarding low-carbon energy scenario design, Germany comes out on top followed by the UK. France's reliance on nuclear energy for electricity generation has helped to reduce carbon emissions but has also reduced technological innovation for other renewable energy sources. [41]. Russia's growth in eco-efficiency is related to the modernizing of the economy after the breakup of the Soviet Union. The United States benefits from economy-wide technical progress and, at times, environmentally favorable US presidency.

The eco-efficiency laggards over the period 1997 to 2019 include Canada, China, India, and Indonesia. Canada is a developed country with a large resource extraction sector. China, India, and Indonesia are populous fast-growing countries where economic growth has taken priority over environmental stewardship.

Predicting eco-efficiency into the future under a BAU scenario shows that between 2019 and 2040, the average annual rate of change in MPI, catch-up, and frontier shift is forecast at −0.1%, −0.3%, and 0.2%, respectively. A slowdown in technical efficiency is predicted to be the main reason for the decline in MPI. However, these numbers are small, indicating that even for countries where eco-efficiency MPI growth is positive, the practical impact on eco-efficiency is likely to be insignificant.

There are some limitations to this research. The forecasts for the period 2020 to 2040 were conducted under a BAU scenario, which assumes existing data trends continue into the future and there are no major changes in policy or economic structure. Small changes in the growth rate (1% or 2%) of the DEA input variables and output will have a small impact on the efficiency scores and the MPI calculations. Large changes in energy policy, the energy mix, and CO<sup>2</sup> emissions reductions could lead to higher eco-efficiency than those reported in the BAU scenario. Then, the question becomes, how likely is it that these large changes occur? Recent research by the IPCC indicates that climate change is widespread, rapid, and intensifying [42]. A substantial increase in eco-efficiency would require the G18 to quickly enact long-term energy policy aimed at greatly reducing fossil fuel consumption. Future research could look into conducting further scenario analysis to account for major changes in clean energy policy. Additional analysis could also be conducted on the choice of DEA model.
