## 6 Conclusions  
The longer-term energy future belongs to RE because in a few decades FF reserves and/or its EROI levels [95] will be too low to enable economies based largely on FF to function if business-as-usual policies continue. CDR and SG--if they ever do make a significant contribution to reducing climate forcing--are energy consumers, not producers. Nuclear energy is not seen, even by promoters, as having more than a marginal role in the future [44]. Nevertheless, in the short/medium term, RE will struggle to improve rapidly its share of energy production, for two reasons. First, as shown in Section 3, if EROIg is low, or the level of EROIg needed for economies to function is too high, the rate of growth of RE installations must be restricted to ensure sufficient net energy is available to power the economy. Second, as discussed in Section 4, our analysis suggests the urgent need for deep cuts in FF in OECD countries. This would produce surplus capacity in the power sector, discouraging rapid growth in RE power capacity. This review has shown the importance of considering all sustainability challenges Earth faces, not just CC, when evaluating how energy should be used in the future. This review showed, using the transport and agricultural sectors as examples, that non-CC challenges were often more serious. Emphasis also needs to shift from improving technical measures such as energy efficiency of transport (as measured by pass km or tonne km per primary MJ) or agricultural output (as measured by tonnes per primary GJ), to approaches requiring deep social and political changes. The UNEP Emissions Gap Report [96] similarly  
Figure 2: Distribution of nominal GDP (USD 2017), and 2019 CO2-eq emissions from transport, agriculture and energy. Note: Distribution presented as cumulative fractions of the global total (190 countries) for each sector, all ranked in order of increasing nominal GDP/capita. GDP/capita for the six most populated countries shown. The data are drawn from [82; 92]. summarized its message as 'a call for the rapid transformation of societies', noting the 'inadequate action' taken so far to resolve the climate crisis. Continuing to prioritise economic growth has so far not helped us achieve ecological sustainability and is unlikely to help in the future. The limitations of this study lie in unavoidable future uncertainty in total primary energy production, and RE's contribution, as well as global environmental and political/economic conditions in one or two decades of time. Moreover, RE is only one possible CC mitigation approach. As already mentioned, others include nuclear energy, SG, and various CDR techniques. It is at least possible that any one of these could be subject to a technological breakthrough. As a possible example of a RE breakthrough, [PERSON] [97] has recently reported on renewed enthusiasm for satellite solar power. Although first proposed many decades ago, supporters can point to the dramatic advances in both PV cells and space technology, although it is doubtful whether it could be deployed in the short/medium term to rapidly boost RE output. On the other hand, if a key breakthrough occurs in one of the other approaches, RE output growth in the short/medium term will be even slower. Despite further research, these uncertainties will remain. [PERSON] et al. [98] have pointed to a further problem which will likely impact global energy use: the economic costs of CC may have been seriously underestimated, because of missing risks excluded because of their uncertainty, and gave as examples 'the potential for climate change impacts to drive social discontent, dislocation and relocation, and instability and conflict, are all deeply uncertain, but potentially crippling'. Given the irreducible uncertainties we face, the likely best approach is to adopt the 'precautionary principle' [99].