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Producción Científica, Valladolid (Spain) is a medium-size city (~300,000 inhabitants) that established a greenhouse (GHG) emissions reduction target in 2011 of 20% from 2010–2020. However, tracking the evolution of GHG in medium-size cities is challenging due to the general lack of compulsory data collection at this scale and issues with boundaries when attempting alternative estimates. Here, we propose and apply a novel method to estimate the evolution of GHG emissions due to energy consumption for the period of 2010–2019 in Valladolid, combining top-down and bottom-up data following a physical energy flows approach. The energy consumption of the city is estimated by main sectors and types of energies. The results show that, throughout the past decade, both total energy consumption and its sector end-use share did not significantly change: final energy consumption remained at around 24 MWh (86.5 GJ) per capita and was still highly dependent on fossil fuels, especially natural gas and oil products (over 70% of total energy supply). The GHG reduction by 2019 was ~11% with relation to 2010 and, thus, had not reached the set objective; in per capita terms, the GHG reduction was lower (~6%) due to population loss during the period. The trend, however, has not been monotone and has instead followed a U-shape strongly correlated with the economic crisis and subsequent recovery, suggesting that transition policies have had, at most, a modest effect on the overall results. The analysis shows, first of all, the limitations of statistical sources at a local level, both for energy and mobility, which do not allow more accurate results in identifying the main energy consumers to be reached; and, secondly, the need for strong decarbonization measures which have to be set urgently at all the relevant institutional levels. Reaching GHG neutrality in the city by 2050 requires reducing the GHG emissions by ~13%/year, which is ~20 times faster than for the 2010–2019 average of 0.6%/year.

Producción Científica, Whether renewable energy sources (RES) will provide sufficient energy surplus to entirely power complex modern societies is under discussion. We contribute to this debate by estimating the current global average energy return on energy invested (EROI) for the five RES technologies with the highest potential of electricity generation from the comprehensive and internally consistent estimations of their material requirements at three distinct energy system boundaries: standard farm-gate (EROIst), final at consumer point-of-use (EROIfinal), and extended (including indirect investments, EROIext). EROIst levels found fall within the respective literature ranges. Expanding the boundaries closer to the system level, we find that only large hydroelectricity would currently have a high EROIext ~ 6.5:1, while the rest of variable RES would be below 3:1: onshore wind (2.9:1), offshore wind (2.3:1), solar Photovoltaic (PV) (1.8:1), and solar Concentrated Solar Power (CSP) (, MEDEAS project, funded by the European Union’s Horizon2020 research and innovation programme under grant agree-ment no. 691287, LOCOMOTION project, funded by the EuropeanUnion’s Horizon 2020 research and innovation programmeunder grant agreement no. 821105