Global primary energy demand will most likely increase in the coming decades as a result of a growing world population and generally increasing prosperity. Historically, increasing energy demand has usually been associated with increases in greenhouse gas (GHG) emissions as the additional energy needs are met by the utilization of fossil fuels. However, in order to meet the efforts to curb climate change, both existing and emerging energy demands (and therefore energy supply systems) must be sustainably covered using renewable energy resources. In this context, two different development scenarios are presented in this paper: First, based on historical developments in recent years, the given expansion pathway of renewable energies is extrapolated until the year 2050 (bottom-up perspective). However, as this expansion does not meet the climate goals set to meet the 1.5 °C target, additionally a development pathway, which meets the GHG reduction goals in 2050 is developed (top-down perspective). Given the very high levels of energy delivered from (fluctuating) renewable energy sources needed for such a sustainable development path, comprehensive decarbonization (concerning fossil carbon) requires a much better interconnection of the different energy sectors (electricity, heat, mobility) compared to today; some of these necessities are also presented because they influence the further development of the energy system and thus the role of renewables considerably. All over, the paper shows that the already ongoing efforts to achieve the 1.5 °C target must be intensified dramatically; the already significant developments related to the installation of new systems to use renewable sources of energy are not enough to achieve the defined targets.
