Renewable Energies
Sergio Tirado Herrero
Renewable energies are defined in the glossary of the IPCC Sixth Assessment Report (AR6) as any form of energy replenished by natural processes at a rate that equals or exceeds its rate of use (IPCC, 2023a). This category encompasses a wide range of energy sources and technologies, including solar photovoltaics (utility-scale and distributed), concentrating solar power, solar thermal energy, wind power (onshore and offshore), hydropower, biomass energy (i.e., traditional solid biofuels, biogas, and liquid biofuels), geothermal energy, and ocean energy. These different primary sources are transformed and processed into energy carriers, that is, directly usable forms of energy. Energy carriers can then be deployed to meet diverse human energy service needs including mobility, heating, cooling, and lighting of indoor spaces, food preservation and preparation, and operation of a wide array of machinery and devices across all sectors of the economy. Electricity stands out as a particularly versatile and high-quality energy carrier, since it enables the transformation of a broad range of primary renewable sources into energy services suited to multiple end uses.
Renewable energy is conventionally defined in opposition to non-renewable energy, namely energy from finite mineral resources located in the Earth’s crust whose rate of regeneration is negligible in relation to the use rate, which means that their total material availability declines with use. Primary non-renewable energy sources consist essentially of fossil fuels (oil, natural gas, coal, peat, and other unconventional sources) and uranium, from which nuclear fuel is obtained. Even if the renewable/non-renewable distinction refers explicitly to the issue of resource depletion, fossil fuel reserves – that is, the share of the resource base whose extraction is technically and economically feasible – have not declined significantly in recent decades (Our World in Data, 2025), despite longstanding peak oil predictions (Bardi, 2019).
The continued stability and even expansion of technically and economically recoverable fossil fuel reserves results from the ongoing exploration and discovery of new deposits, together with advances in extraction and conversion technologies that make it possible to exploit unconventional sources such as shale gas and tar sands. Even so, unconventional resources are becoming increasingly costly in both metabolic and economic terms owing to their lower quality and greater inaccessibility, which require growing amounts of technology and energy for extraction and processing. Similarly, uranium reserves are estimated to be sufficient to meet demand for centuries to come (NEA-IAEA, 2025). Even so, these remain finite resources in the strict sense, since the total quantity available on the planet is physically limited.
Although renewable energy is based on harnessing flows of energy or matter regenerated through biophysical processes at the planetary or solar system scale, renewability does not imply the absence of material limits to its expansion. Renewable energy systems depend on infrastructures and technologies built from other non-renewable mineral resources, and their deployment may generate irreversible impacts on biodiversity and ecosystems (Harjanne and Korhonen, 2019). That said, the infrastructures required to exploit non-renewable energy resources – especially fossil fuels – likewise depend on the massive use of mineral resources and result in biodiversity impacts on a far greater scale than those associated with renewables.
In the current context of the multiple crises of the Anthropocene and the accelerating anthropogenic disruption of the climatic equilibria inherited from the Holocene, the significance of renewable energy lies less in its reduced dependence on geologically finite resources than in its central role in climate change mitigation, and more specifically in reducing greenhouse gas emissions. Renewable technologies emit tens of times less carbon dioxide equivalent (tCO2eq) per unit of useful energy generated than coal, natural gas, or oil (NREL, 2021).
For this reason, long-term climate mitigation strategies depend on a large-scale increase in the share of renewable energy in the global energy mix, alongside improvements in energy efficiency across the full range of processes and economic sectors that make up final energy demand. Mitigation also requires emission reductions in land-use change and ecosystem transformation, as well as in non-CO2 greenhouse gases such as methane (CH4) and fluorinated gases (IPCC, 2023b). Virtually every energy use and emissions scenario consistent with the warming limits established in the Paris Agreement (2015) assumes that the majority of the energy consumed globally will eventually have to be obtained from renewable sources. Furthermore, primary renewable energy sources, which tend to be less spatially concentrated and which, in the case of solar radiation and wind, are a near-pure public good, offer more favourable conditions for advancing towards decentralized energy systems with less private ownership concentration and control over energy generation and transformation.
At the local scale, energy communities based on community-owned and community-governed renewable infrastructure constitute important practical expressions of energy democracy and energy sovereignty. Their transformative capacity within the wider energy system nevertheless remains limited at present, since ownership of renewable infrastructure continues to be concentrated largely in the hands of major energy corporations operating large-scale installations, while the effective contribution of energy communities to meeting total final energy demand remains modest.
At the national and regional scales, a greater share of renewable technologies based on own resources can enhance energy security in economies that remain dependent on imported fossil fuels. Renewables are currently estimated to account for only 15% of the global primary energy consumption (Ritchie et al., 2020). Nevertheless, projections suggest that by 2030 renewables will become the world’s leading energy source and will provide nearly 45% of global electricity generation. This expected growth is driven above all by the expansion of renewable electricity generation capacity through solar photovoltaic and wind technologies (IEA, 2025a). Although the share of renewables in the global energy mix is increasing rapidly, the current rates of growth remain insufficient, considering that fossil fuel consumption continues to rise (IEA, 2025b) and that meeting the objectives of the Paris Agreement would require a tripling of the installed renewable generation capacity by 2030 relative to 2022, whereas current trends point to an expected growth factor of 2.6 over that period (IEA, 2025a).
In this context of accelerated renewable expansion, it must be noted that renewable energy is not climatically neutral. When all life-cycle emissions are taken into account —including those from construction, operation, and decommissioning of technologies and infrastructures – renewable sources are estimated to generate between 10 and 50 grams of CO2eq per kWh, compared with 500 to 1,000 grams of CO2eq per kWh for coal-, oil-, or gas-based generation (NREL, 2021). In this regard, two related terms also require attention: clean energy and low-carbon energy. These refer to low-emission technologies, especially in terms of greenhouse gases, but which are by no means free from other socio-environmental impacts.
These two categories also include non-renewable sources such as nuclear energy, whose estimated life-cycle emissions per unit of electricity generated are comparable to those of renewable technologies (NREL, 2021). In the European Union, this has resulted in the formal inclusion of nuclear energy – together with natural gas – in the EU taxonomy for a sustainable energy transition, a decision that was widely criticized by academia and civil society (Pieńkowski, 2024) because of the multiple impacts associated with the nuclear fuel cycle. These include territorial and hydrological disruptions linked to uranium mining; intensive cooling-water use and the thermal pollution risk in aquatic ecosystems; low-probability, extremely high-impact accidents resulting in long-term contamination across extensive terrestrial and aquatic areas (e.g., Chernobyl and Fukushima accidents); and the radiological risks associated with the storage and management of radioactive waste over millennial timescales.
Critical perspectives call into question the actual effectiveness of the large-scale incorporation of renewables from a climate change mitigation perspective. In this vein, the environmental historian Jean-Baptiste Fressoz (2024) argues in More and More and More: An All-Consuming History of Energy that historically new energy technologies have served primarily to expand, rather than replace, existing energy sources. From this perspective, renewables are not displacing fossil fuels in the global energy mix but rather enabling further increases in total energy consumption. The so-called energy transition would thus be understood less as a genuine process of fossil fuel phase-out than as a dynamic of expansion and diversification within the energy system itself (see Ecosocial Transition).
Along this line, it has been estimated that the large-scale replacement of fossil fuel-based energy infrastructure with renewable systems would itself generate substantial emissions associated with the construction, operation, and maintenance of this new infrastructure. In some scenarios, such a transition would consume a significant share of the remaining global carbon budget compatible with limiting warming to 1.5°C above pre-industrial levels under the Paris Agreement (Slameršak et al., 2022). From a degrowth or post-growth perspective, these projections suggest that an energy transition capable of keeping the emissions under safe planetary boundaries would require not only an almost complete substitution of fossil fuels by renewables, but also a significant reduction in global energy consumption and, therefore, a profound reorganization of the energy systems on both the supply and demand sides.
Lastly, from an ecological justice standpoint, we need to consider the impacts of renewable energy on non-human living beings and ecosystems. Well-known examples include the effects of wind power on birdlife and the loss of biodiversity resulting from the conversion of natural ecosystems for biofuel production. These impacts are partly explained by the relatively low power density of renewable energy per unit of land area (i.e., the amount of energy produced per square metre of land), which means that renewable systems often require extensive spatial footprints to generate comparatively modest quantities of energy relative to fossil fuel-based systems (Harjanne and Korhonen, 2019). That said, other anthropogenic pressures on biodiversity are more significant than renewable technologies themselves, such as bird mortality caused by domestic cats and the large-scale threat that climate change poses to species and ecosystems worldwide (MIT Climate Portal, 2023). Ultimately, the scale and significance of the impacts of renewable energy on ecosystems and non-human life remain substantially lower than those associated with fossil fuel-based energy systems.
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