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Energy efficiency is a central target for energy policy and a keystone to mitigate climate change and to achieve a sustainable development. Although great efforts have been carried out during the last four decades to investigate the issue, focusing into measuring energy efficiency, understanding its trends and impacts on energy consumption and to design effective energy efficiency policies, many energy efficiency-related concepts, some methodological problems for the construction of energy efficiency indicators (EEI) and even some of the energy efficiency potential gains are often ignored or misunderstood, causing no little confusion and controversy not only for laymen but even for specialists. This paper aims to revisit, analyse and discuss some efficiency fundamental topics that could improve understanding and critical judgement of efficiency stakeholders and that could help in avoiding unfounded judgements and misleading statements. Firstly, we address the problem of measuring energy efficiency both in qualitative and quantitative terms. Secondly, main methodological problems standing in the way of the construction of EEI are discussed, and a sequence of actions is proposed to tackle them in an ordered fashion. Finally, two key topics are discussed in detail: the links between energy efficiency and energy savings, and the border between energy efficiency improvement and renewable sources promotion. [ABSTRACT FROM AUTHOR]

Many global transport sector decarbonisation studies assert that it is difficult for the transport sector to decarbonise and to contribute its proportional share to the ambitious climate targets set by the Paris Agreement. We challenge this argument by establishing that deep decarbonisation is possible in the transport sector, through original research that is anchored in a global meta-analysis of long-term transport sector emission pathways from over 500 bottom-up modelling estimates from 81 countries, rather than relying on aggregated regional data and modelling efforts. First, we translate the aspirational 1.5-degree Celsius (1.5DS) target to an indicative 2050 transport sector emission target of 2 GtCO2, based on proportional downscaling of existing economy-wide 2DS studies to a transport-specific 1.5DS target. We then compare this with mitigation potential derived from the aggregation of bottom-up estimates for business-as-usual growth and low-carbon scenarios from individual country studies, which we aggregate at national and global levels. This analysis suggests that in the absence of additional action, transport sector emissions could outpace earlier projections and thus become a major roadblock to avoiding dangerous climate change. Yet, if countries collectively maximise efforts to implement comprehensive low-carbon measures, the sector could achieve reductions approaching a 1.5-degree scenario. Realising the full mitigation potential of transport will require balanced implementation of low-carbon mitigation policies that avoid (or reduce) the need for transport trips; promote a shift toward more efficient travel modes; and improve performance of vehicles and fuels. The chances that such a comprehensive approach is taken will increase if countries, cities and companies establish medium- to long-term commitments to transport decarbonisation and accelerate short-term implementation of market-ready low-carbon transport measures. Setting more ambitious low-carbon transport target with mid-term implementation milestones, and closely integrating these plans with sustainable development objectives, can help to spur mitigation action consistent with a 1.5DS target. To conclude, we discuss potential limitations of a transport sector-specific analysis of emission pathways, and we offer recommendations for further refining pathways for the transport sector to realise Paris Agreement targets. [ABSTRACT FROM AUTHOR]