Recent global modelling studies suggest a decline of long -distance trade in energy carriers in future global renewable energy systems , compared to today’s fossil fuel energy system. In contrast, we identified four crucial drivers that enable trade of renewable energy carriers . These drivers could make trade remain at current levels or even increase during the transition to an energy system with very high shares of renewables . First, new land -efficient technologies for renewable fuel production become increasingly available and technically allow for long -distance trade in renewables . Second, regional differences in social acceptance and land availability for energy infrastructure support the development of renewable fuel import and export streams. Third , the economics of renewable energy systems , i.e. the different production conditions globally and the high costs of fully renewable regional electricity systems , will create opportunities for spatial arbitrage. Fourth, the reduction of stranded investments in the fossil fuel sector is possible by switchi ng from fossil fuel to renewable fuel trade in exporting regions. The impact of these drivers on trade in energy carriers is currently under -investigated by the global energy research community . Therefore, we call for a major research effort in this field, in particular as trade can redistribute profits and losses of climate change mitigation and may hence support finding new partners in climate change mitigation negotiations . 2 The transition to a low carbon energy supply is possible with a range of technologies such as c arbon capture and storage, nuclear energy, and renewable energie s, according to recent studies with integrated assessment models (IAM) . Consistently, those IAM long-term scenarios that have shares of above 60% of renewable energies in the global energy mix in 2100, show a decline in long -distance trade in energy carriers ( Figure 1). In 2015, trade in fossil fuel s between six major regional groups , i.e. Africa & Middle East , Asia, Europe & North America, Former Soviet Union, Latin America & the Carribean, and Rest of World , amounts to a share of around 23% of global primary energy use*. Trade in the year 2100 declines to below 15% in 43 out of 44 scenarios . Mainly, this is a consequence of less fossil fuels in the supply mix . The long -distance trade of biofuels, wind or solar energy based hydrogen, or electricity – if considered – does not alter this trend. Consistently, the IAM modelling community therefore assumes that a global renewable energy system relies less on long -distance trade in energy carrie rs than today’s fossil fuel system , at least in relative shares. In contrast, we identified four drivers of long -distance trade in a global renewable energy system. These are linked to new technologies, social acceptance and associated land availability, e conomics of renewable energy systems, and the continued use of potentially stranded investments in the fossil fuel sectors. Despite uncertainties in the future development of these drivers , we believe that a major effort in the energy research community is necessary to better understand possible future trajectories of global energy systems. Figure 1: Long -distance trade in primary and secondary energy carriers between six aggregated world regions as share of global primary energy use. The black line shows historical observations of fossil fuel trade2,3. The coloured lines re fer to 44 different IAM scenarios4. All scenarios have a share of renewable energies in primary energy use above 60% and are in the range of 5 percentage points of the observed trade share in 2010. See appendix for details. * Biomass and biofuels trade is not accounted for. In 2015, the share of these energy carriers in long-distance trade was, however, less than 1 % of primary energy use1. 3 Driver 1: New technologies for producing renewable energy carriers New technologies will facilitate the future long-distance trade in renewable energy carriers. Such trade requires either installing inter -continental electricity grids for transmission of renewable electricity, or producing renewable fuels and transporting them with existing liquid or gaseous fuel infrastructure. Transmission grids allow economic benefits for trade over mid -range distances , e.g. between North- Africa and Europe. However, long -distance trade, e.g. between Europe and the US , needs a decrease of transmission costs by a factor of 5 to make it economically profitable5. As transmission grids are a mature technology, significant cost reductions of this order of magnitude are questionable. In contrast, liquid and gaseous fuels do not require installing new infrastructure. Currently, the only technically mature, large -scale option for long -distance trade in renewable energy carriers are biomass -based fuels, i.e. b iofuels . Yet, biofuels can sustainably re place only a minor share of foss il fuels in existing global energy systems, as photosynthesis has low solar energy to fuel efficiency6. This results in high land requirements and an associated reductio n in natural carbon stocks due to land -use change and land management7. New technologies for renewable fuel production, such as hydrogen produced from renewable electricity and possibly upgraded to methane or methanol, are associated with significantly lower direct land impacts . For instance, a process that derives hydrogen from electrolysis , using electricity from wind power (WP) , and CO 2 direct ly captured from air, could produce between 4 10 GWh and 6 80 GWh km-2 a-1 of methanol . In contrast, o ne of the most land-efficient biomass technologies which is commercially available, i.e. palm oil, can only produce around 6 to 7 GWh km-2 a-1 of biofuel ( Table 1). The high land efficiencies of new renewable fuel technologies make impacts on natural carbon stocks in the vegetation negligible . In addition, these technologies allow for produc tion on land with very low carbon stocks , such as semi -arid regions or even dese rts. They are therefore effective technologies in terms of climate change mitigation. An essential ingredient for most of these new renewable fuel technologies is hydrogen. It can be produced via electrolysis8, photolysis9, or hydrogenases by bacteria10. Hydrocarbons and other fuels can in turn be synthesised from hydrogen and a suitable carbon source through hydrogenation11, or generated photo- chemically from solar light, water, and carbon6. If the CO 2 is captured from the atmosphere12, these fuels are carbon- neutral. All of these technologies are still under development and not comm ercially deployed, with the exemption of hydrogen production through electrolysis, which is commercially available although still at low deployment levels globally13. For the further analysis , we focus on renewable fuels produced through electrolysis of water, assuming that the electricity is generated from wind, water, and solar energy (WWS), in particular Photovoltaics (PV ), WP, and hydropower . We call them WWS fuels in the following. Yet, o ur reasoning is also applicable to other renewable fuel technologies with high land -use efficiencies. Logistics does not constitute a major barrier to international trade of renewable fuels14. Methane can be traded globally with existing pipeline or l iquefied natural gas infrastructure, while methanol and other liquid fuels can be traded and distributed with the current infrastructure for fossil fuels15. Hydrogen is currently not liquefied for overseas transportation, despite liquefaction of hydrogen being a long known process16. At the current level of technological develo pment, energy needs and costs related to hydrogen liquefaction remain too high for commercialization. Yet, preliminary estimates of future costs allow to assume that in the future the full costs of renewable hydrogen including transportation can become cost-competitive with renewable methane13,17. Therefore, new low-cost synthetisation methods for hydrogen, methanol , and methane, based on renewable energies , will increase the opportunities for long -distance trade of renewable fuels. 4 Table 1: Energy generation per area for renewable fuels assuming Brazilian production characteristics. We use direct impacts on land for the estimation. For details, see appendix. Resource Process Product* Energy generation per area (GWh km-2 a-1)+ Lower bound Upper bound Commercially available technologies Palm Oil Transesterification Biodiesel 6.0 7.0 Electricity - Photovoltaics (PV) Electrolysis H2 35 110 Electricity - Wind Power (WP) Electrolysis H2 640 1000 Sugar Cane – 1st Generation Fermentation Ethanol 3.0 4.0 Technologies under development Algae Transesterification Biodiesel 13 13 Eucalyptus Gasification and Methane Synthesis Methane 11 12 Eucalyptus Gasification and Methanol Synthesis Methanol 8.9 9.9 Electricity - PV and CO 2$ Electrolysis and Methanation Methane 26 78 Electricity - PV and CO 2$ Electrolysis and Methanol Synthesis Methanol 26 76 Electricity – WP and CO 2$ Electrolysis and Methanation Methane 470 700 Electricity – WP and CO 2$ Electrolysis and Methanol Synthesis Methanol 470 680 Sugar Cane - 2nd Generation Fermentation and Gasification of Bagasse Ethanol 7.0 7.0 + We assume direct impacts of technologies on land as an indicator of land -uptake and competition with agriculture and forestry. The spacing area of wind parks and PV installations, which is more relevant for estimating production potentials, is larger (by two orders of magnitude for wind and by a factor of 2 for PV)18. For direct air capture of CO 2, land requirements are considered in the table , but contain a relatively high uncertain ty. * All production processes have different amounts of co -products (primarily heat and/or electricity). $ CO 2 is assumed to be taken out of the atmosphere using direct air capture . Driver 2 : Social acceptance of renewables and land availability Significant trade streams in WWS fuels only will be economically competitive and reduce carbon emissions , if some regions produce excess WWS . Currently, this is not the case, as no region is close to producing more WWS than its primary energy use ( Figure 2). Therefore, a global renewable energy system will need significant growth in WWS generation everywhere . If growth in WWS generation is faster than growth in energy use in some regions for extended periods , these regions may become able to generate WWS fuels. For example, if future WWS generation per area in Canada or Brazil converges to the current level of WWS generation per area in Germany - at 473 MWh km-2 a-1 – a little surplus of WWS fuels for export could theoretically be produced . Currently, WWS generation per area has not converged globally ( Figure 2), but late adopters of PV and WP have faster growth in these technologies than early adopters19. The growth in renewables is driven by multiple factors such as support policies20, economic growth, size of the electricity sector, and endowments with physical potentials19. Yet, the wide field of assessing land availability is a further important factor, often neglected in studies of energy systems21. While most regions in principle have sufficient land available for WWS generation due to low land requirements of WWS fuels (Table 1), a lack of social acceptance associated with the deployment of the associated infrastructure is observed already today in some regions . The relation between social acceptance and density of WWS infrastructure is therefore crucial for understanding their future spatial distribution. Acceptance may remain constant with the penetration level of WWS , if a strong shifting baseline phenomenon22 in the perception 5 of renewable infrastructure is present. In that case, convergence of WWS generation per area will be low globally, everything else equal , as regions with high energy use will also be able to deploy large amounts of WWS . If, in contrast , conflicts over new projects increase with the penetration level of WWS , the speed of convergence may increase . Trade in WWS fuels can develop under these conditions , as regions rich in land in relation to energy use will face less social conflicts when they increase the level of WWS generation above their level of energy use. In Europe, which has globally the highest WWS spatial density conflic ts due to critical impacts of large - scale infrastructure, in particular wind turbines , on the aesthetic perceptions of landscapes, and on the environment are already observed today23. Some conflicts are however also related to trust, and planning procedures and can be partly mitigated by better sharing of information, by participatory processes in decision making24, and by procedural justice25. Additionally , conflicts also are present in regions with much lower WWS generation per area, such as the United States26 and Brazi l, which is a promising exporting country from the Global South. There, the livelihood of rural populations particularly in the North -East of the country is negatively affected by the rapid expansion of wind parks and, consequently, territorial conflicts are triggered27. The evidence does not yet allow to draw clear conclusions with respect to the relation of WWS generation per area and social acceptance therefore. I n regions where WWS generation per area is much lower than in e.g. Europe, it seems , however, theoretically possible to mitigate those impacts at lower costs, if institutional capacities to deal with emerging conflicts are built- up. We conclude that one core aspect in understanding the future spatial distribution of WWS generation infrastructure is land availability and associated conflicts regarding access to and control over land . Existing theories of land -use change25 do not address the role of WWS generation infrastructure in the competition for land. Accordingly, th e most widely applied modelling approaches for future global energy systems do not assess land requirements for WWS generation infrastructure expansion in detail21. For understand ing the role of trade in globale renewable energy systems, a comprehensive assessment of these processes is, however, crucial and research in this field is therefore of fundamental importance. Figure 2: Primary energy use per area plotted vs. current WWS power generation per area. *Single countries also shown in the Figure (i.e. Brazil and China) are not included in the respective regions. See appendix for details. Observation: P rimary energy use is a rough indicator for final energy use , as it will likely fall with renewable electrification of most services. Jacobson et al.28 estimate a reduction of up to around 40% on average for energy systems with high shares of intermittent renewables. 6 Driver 3: Economics of renewable energy systems Future energy systems will likely be largely electrified28. However, some applications in transportation and industry will require liquid or gaseous fuels as the costs of fully electrifying these applications are prohibitively hig h. These applications include air transport, trucks, shipping and energy- intensive manufacturing industries29–32. Here , renewable fuels, tradable over inter -continental distances and storable at low costs, can provide a low-carbon alternative to electricity to allow deep decarbonisation. Moreover, renewable fuels could also have beneficial applications in future electricity systems. A series of studies has shown that, in principle, electricity systems with very high shares of variable renewables (VRES, i.e. PV and Wind) are possible on a country or continental level33–35. Different technological options in the energy system, such as sectoral integration36, spatial and technological diversification of VRES generation35, and integration of different generation and storage technologies37 allow to operate electricity systems almost fully based on VRES. Yet, the system levelized costs of electricity38 are lowest at VRES penetration well below 100%39–41, as depicted in Figure 3. WWS fuels have therefore the potential to lower system costs of highly renewable electricity systems, being a renewable, dispatchable source of electricity generation. Figure 3 compares marginal system levelized costs of electricity at different shares of VRES from three modelling studies for Europe. The marginal costs of the systems at various penetration levels of renewables are compared to cost estimates of electricity generation from WWS liquid fuels. Using dispatchable generation and reducing the share of intermittent electricity in the system has the potential to reduce system costs significantly and allows for a more efficient use of local VRES42. At future cost estimates of electricity generation from WWS fuels, these fuels would be competitive to VRES at a renewable share of about 80% or above in most scenarios (Figure 3). While renewable fuels ha ve the potential to decrease costs of highly renewable electricity systems , this does not necessarily imply long -distance trade in renewable fuels, as they might be produced locally. Yet, regions with high energy consumption are often not the ones that are best endowed with renewable resources43. Moreover, even with the same resource endowment, regions with high energy consumption have to ta p deeper into the available resources, meaning that more locations with less favourable conditions have to be accessed, resulting in increasing marginal costs of electricity supply from WWS. For instance, in Germany , full load hours of PV generation are a third of the best locations in Chile44; and hybrid PV -WP systems in Germany remain below 4,000 full load hours, while the same systems can reach more than 6,000 full load hours i n some parts of Africa, North and South America, and Asia43. Other factors, such as available infrastructure, regulation, labour, land and capital costs45, also influence the economics of renewable fuel projects. In consequence, differences in levelized cost of electricity between regions may be smaller than bio -physical differences would imply. For the example of Chile, production costs are only half of those in Germany46, despite three- fold full load hours. Global cost differences in the production of renewable fuels together with comparatively low transportation costs of renewable fuels (see Driver 1) make long -distance trade in renewable fuels economically viable. Such an undertaking would have the additional benefit of decreasing climate change mitigation costs, thereby fostering support for the necessary transition. 7 Figure 3: Marginal system levelized costs of electricity ( LCOE ) for varying levels of VRES in total generation for different scenarios in three different modelling studies39–41 (coloured lines) compared to lower bound for costs of generating electricity from WWS diesel16 (black dashed line). Scenarios are derived for the period 2035-2050. Marginal System LCOE are calculated according to Reichenberg et al .41. See appendix for details. Driver 4: Reduction of stranded investments in fossil fuel sector s The current fossil fuel energy system, consisting of infrastructure, institutions, and behaviour, is a major factor to lock our soci eties into a high -carbon world47. In particular coal and gas power pla nts, as well as oil fuelled vehicles, are substantial infrastructure assets that increase the lock -in effect48. Actors who produce , process and transport fossil fuels face the risk of huge stranded investments due to a full decarbonization of energy systems in the coming decades49. This causes lower incentives and weak commit ments to mitigate climate change. Such stranded investments could be partly avoided if the existing infrastructure serve s as a bridge to a low -carbon world . A sufficiently large renewable fuel sector will allow such continued use of adapted fossil fuel infrastructure, i.e. for transportation and distribution, and for end uses . Nevertheless, if renewable fuels are deployed at large scale, s tranded investments will remain high for fossil resource owners . The largest owners of unburnable resources under strong climate change mitigation are China and India, Russia, the Middle East, and the US50. These regions may, however, benefit from new opportunities arising with the use of renewable fuels, as they are endowed with substanti al potential for renewable fuel production43. To some extent, i ncome, generated from renewable fuel exports , may offset the cost of abandoning fossil fuel extraction. Hence, existing regional specialization in energy carrier production, e.g. as in the Middle East, may remain due to local sectoral lock -in effects, consequently increasing trade flows . Conclusions The four drivers indicat e current gaps in understanding the role of long -distance trade in a global renewable energy system. We believe closing the se gaps will assist in solving the challenges on the way to a sustainable low-carbon world. In particular, trade scenarios may help in closing future climate change mitigation deals , as a global energy system incorporating renewable fuel s will redistribute the gains and losses from the projected measures to mitigate climate change. They may therefore potentially increase acceptance by actors 8 involved in the production, transportation, or consumption of fossil fuels, who currently oppose strong global mitigation efforts. Nevertheless, important limitations associated with scenarios of trade in renewable fuels that range from the development of global trade and climate change agreements , to uncertain technological developments , and future forms and positioning of politics to steer energy transitions and energy democracy, have to be taken into account . Also significant normative questions play a role: the valuation of local impacts of different generation and production technologies in different world regions , concerns with security of supply, and the public opinion on trade have to be better understood and considered in scenarios . However, to largely neglect the factor trade in future global energy scenarios risks reduc ing the visibility of options to reach climate change mitigation targets. We call for a n extended research effort into future renewable fuel trade scenarios in the energy research community therefore, as closing the research gaps requires a major effort in improving existing modelling tools, theories, and data sets. Acknowledgements We gratefully acknowledge support from the European Research Council ( “reFUEL ” ERC -2017- STG 758149) and from Bio4Energy.