## 4 Discussion
[PERSON] et al. (2012) show that such a system can improve energy efficiencies by up to 15.1% [44]. Various studies highlight that traditional light sources in greenhouses are relatively energy inefficient and generally generate radiant heat and highlight that switching to LED sources can lead to considerable energy savings [45; 46; 47]. Studies generally show that improving the design, operation, and technology used in greenhouses can also significantly improve overall energy efficiency [43]. However, implementing these systems can be expensive and, due to variation in greenhouses, EEM interventions are often context specific, highlighting that further research in these areas would be useful [43]. Various studies investigate the transition to RES for greenhouse production, including the potential for: integrating solar technologies within greenhouse production [17; 32; 48]; biogas and bioenergy [49]; and geothermal [14; 16; 50]. Regarding geothermal, [PERSON] et al. (2016) find that geothermal are economically comparable with conventional systems [14] and [PERSON] et al. (2014) find that geothermal heat pumps are environmentally beneficial over conventional systems. Other recent research indicates that the installation of these can also be economically advantageous as compared to alternatives [51]. This suggests that the transition to RES, which is already occurring at a rapid pace in certain European countries, could open the door to major reductions in the dependency on fossil sources in the greenhouse sector. Similar to EEM measures, RES interventions are likely dependent on context specificity and economics, while various factors need to be taken into account to support a sustainable transition. For instance, one of the reasons fossil fuels are used to provide energy in greenhouses is that they are source of CO\({}_{2}\) that can be used to enhance productivity [4]. Taking local and international contexts into account there are various policy interventions that will likely support the transition to the use of RES and EEM in greenhouse production. In our view, these include increased taxes for fossil fuels, subsidies for RES adoption EEM interventions and higher carbon prices. For instance, direct subsidies as implemented in the Netherlands can accelerate this transition for greenhouse heating [52]. The new CAP is generally supportive of this by including a range of policies that support the targets of the Green deal including considerable funding for eco-schemes and climate supportive policies [53]. Despite our findings, there are relatively little data available and few studies were identified that investigated energy use in greenhouses in the EU, the studies that do exist employ varying methodologies and boundaries for measuring energy use ranging from LCAs to surveys to advanced models. Although this is similar to data and methodological issues experienced in other agricultural sectors [6; 54], in the greenhouse sector the lack of data is more considerable. In our view, the development of a widely accepted methodology for measuring energy use in agriculture is of utmost importance and its establishment will catalyse future studies on energy use. In addition, one of the limitations highlighted by this review is that greenhouse agriculture shows extreme variation but is often labelled under one term or category. In our view, an approach that further categorises greenhouse production into, for instance, low and high energy intensity greenhouses is useful for our understanding and for comparing greenhouse production across agro-climatic conditions. Our review also highlights a few other general areas for further research, including the potential of EEM and RES technologies, especially in the context of the European Green Deal, as well as on energy demands per crop and within different geographical and agro-climatic areas.