Recycling fossil infrastructure for cleaner energy transitions

Domain: biology
Published: 2026-03-15
Editorial outcome: major_revision

Abstract
The climate crisis mandates building renewable energy infrastructure faster, increasing the demand for primary materials with large environmental footprints. Sourcing these materials from urban mines can mitigate such impacts, but the potential of recycling depends on waste availability. Here, we use life cycle assessment and monetization of impacts to explore the environmental implications of recycling fossil infrastructure that may become obsoleted during the transition. We find that among many materials in fossil infrastructure, recycling steel and copper is particularly appealing, as their stocks (1.34 gigatons and 10.03 megatons) align with the projected energy transition demands (145% and 32% of median demand between 2020–2050, respectively). Recycling steel and copper in fossil infrastructure could save up to 1.95 gigatons CO2,eq and 11.69 trillion US Dollars in externality costs until 2050, while remaining competitive considering current production methods. Using recycled steel and copper would also reduce the carbon footprint of energy transition technologies—for example, wind and photovoltaic power—by one third.

Introduction
Meeting climate goals will require deploying 6.0–8.1 terawatt peak (TWp) of wind and 8.5–14.0 TWp of solar photovoltaic (PV) capacity in 2050, for which metals involving mining and refining operations with large environmental and social impacts will be needed. Recycling bulk metals from waste streams—such as demolished buildings, end-of-life vehicles, industrial equipment, and consumer goods—could reduce the demand for primary materials but the availability of waste is limited. Besides avoiding impacts, building renewable infrastructure with recycled materials could accelerate the transition by circumventing the slow process of opening new mines (16.5 years on average).

Previous studies showed that significant climate impact savings between 30% and 95% could be attained by recycling bulk metals compared to primary production. However, the limited availability of metal waste and its competing uses (e.g. buildings, vehicles, packaging, machinery) makes it necessary to find additional metal waste sources to further enhance circularity and reduce impacts in the energy transition. We focus here on how the energy transition can be supported by using recycled metals from fossil infrastructure in clean energy systems, as renewable energy systems are expected to replace fossil ones, thereby making the resulting waste from decommissioning fossil infrastructure available for recycling purposes. Specifically, we study materials contained in fossil infrastructures such as coal mines, oil and gas rigs, pipelines, and power plants. Some of these infrastructures can be repurposed using circular strategies other than recycling, including repurposing boreholes of oil and gas wells for geothermal energy, converting oil-based refineries into bio-refineries, reusing pipelines for hydrogen transport, constructing grid infrastructure (e.g. large-scale battery storage) on former power plant sites, carbon storage in depleted oil and gas reservoirs, and producing synthetic fuels on offshore gas and oil platforms. However, these options do not target the acceleration of building capacity in solar and wind, the dominant power sources in many transition plans. Although previous work estimated the material stocks in fossil infrastructure in aggregated material categories, the feasibility and broad environmental implications of their recycling, especially for building clean energy systems, are not yet well understood.

To fill this gap, we map global in-use stocks in the current fossil infrastructure for 22 materials, quantifying the recycling potential, and comparing the stocks of six materials with estimated material demand induced by transition pathways. We find that steel is the largest metal stock in fossil infrastructure (1.34 gigatons (Gt)) and is within the range of estimated demand in transition pathways aiming at 1.5°C and 2°C heating. Copper is another promising material to recycle (10.03 megatons (Mt) in fossil infrastructure), which could contribute around one third of the median energy transition demand. Using prospective life cycle assessment (LCA) and monetization factors (see Fig. 1 for method overview and Methods section for more details), we find that recycling steel and copper could save externality costs of more than 4.05 trillion USD (United States dollars in year 2023 equivalents), and decreases environmental impacts of steel and copper production in at least 17 out of 20 indicators—while remaining economically competitive.

Results
Fossil material stock estimate
We start by determining the amount of materials available in the current fossil infrastructure, finding that a total of 6.39 Gt (4.71–9.06 Gt, 95% confidence interval) of waste materials may become available through the transition. Concrete is the largest of these stocks, representing 2.01 Gt (1.24–3.42 Gt) and is mostly used for pavements and structural components. Yet, this stock is well below the estimated median demand induced by the energy transition of 6.49 Gt. Moreover, recycling of concrete is limited as only aggregates can be replaced by crushed concrete, still requiring new cement, the main driver of climate impacts for concrete.

Steel is the second largest stock, amounting to 1.34 Gt (0.84–2.26 Gt), matching cumulative steel demand estimates for energy transitions including energy generation (solar, wind, nuclear, biomass, hydro, and geothermal energy), transmission, and distribution infrastructure from 2020 to 2050 (145% of the median demand, 0.93 Gt (0.51–2.85 Gt) depending on the scenario, see Fig. 2a; material demand of grid-scale batteries is excluded since battery capacity in the transport sector (electric vehicles) may be leveraged). Steel is mostly used for structural purposes in oil and gas extraction equipment (46%), power plants (26%), pipelines (21%), and coal mines (6%) (Supplementary Methods 1). The five countries with the largest fossil steel stocks are China, Saudi Arabia, Russia, the United States, and Iran, with 164, 155, 152, 150, and 94 Mt steel respectively (see Fig. 3a), accounting for more than half of global stocks (53%).

Aluminum (Al) and copper (Cu) follow, used mainly in cables in fossil infrastructure (7.80 Mt (4.36–14.42 Mt) and 10.03 Mt (5.94–18.33 Mt) respectively), and could contribute up to 9% and 32% of the Al and Cu required in median projections of energy transitions. In contrast, stocks of nickel (Ni) and silicon (Si) in fossil infrastructure are insignificant in comparison to the induced demand (0.06 Mt and 0.83 kilotons (kt), 4% and 0.001% of median demand for energy transitions until 2050 according to ref. 30; see Fig. 2a).

For some of the materials contained in current stocks, no demand for the energy transition was reported in ref. 30 (e.g. for Zinc (Zn); see Fig. 2b), or they are not typically contained in renewable energy infrastructure, such as barite; however, alternative uses could be explored. For example, 338.36 Mt (58.64–1847.04 Mt) of barite—used in drilling fluids and representing at least seven years of annual production—could be repurposed as radiative cooling paints containing barite, while 10.42 Mt (6.00–18.32 Mt) of stone wool in power plants could be reused for insulating buildings or district heating pipes.

Demand for cobalt is also not reported in ref. 30 The future demand is driven by battery deployment and projected to reach almost 500 kt a-1 in 2050. In fossil infrastructure, cobalt is used in superalloys for gas turbines and can be recovered by leaching superalloy scrap. The total stock available in the current fossil infrastructure is 6.92 kt (median, see Fig. 2b), much lower than the expected cumulative demand for cobalt of 8.5 to 10.9 Mt from 2025 to 2050 (0.06% to 0.08% of demand). Therefore, recycling cobalt from fossil infrastructure might not play a major role, which is why we exclude it from further analysis.

We also exclude Al, Ni, and Si from further analysis, as recycling the stocks could at best contribute <10% of the median demand generated by the energy transition (9%, 4%, and 0.001% respectively). The potential of Al stocks in the energy transition is even lower when considering that most clean energy applications require wrought-alloy Al (e.g. module frames, mounting systems, cables), which only accounts for 64% of the total Al in fossil infrastructure (thus only around 6% of the median energy transition demand of Al could be supplied).

We next focus on investigating the environmental savings of recycling steel and copper stocks. We first compare steel and copper stocks with the available recycling capacity to evaluate the feasibility of providing large amounts of secondary raw materials.

Feasibility of recycling steel stocks and associated environmental impacts
The global capacity of electric arc furnaces (EAFs) and electric induction furnaces for steel recycling, with EAFs being today’s dominant technology for steel recycling (97%), can process a maximum of 743 Mt a-1, yet EAF steel is produced at 541 Mt a-1 globally, allowing for an additional 202 Mt of steel to be processed annually. The recycling capacity is expected to increase to more than 1,500 Mt a-1 until 2050 (in the Shared Socioeconomic Pathway 2 National Policies implemented (SSP2-NPi) scenario; Supplementary Methods 2), allowing for an idle capacity of more than 400 Mt a-1 if capacity utilization remains the same. If the steel scrap from fossil infrastructure were released evenly across the period 2025–2050, the annual additional steel scrap volume would then utilize less than 40% of the global idle recycling capacity in any year (see Fig. 3a, SSP2-NPi, never exceeds 74% in any other scenario, Supplementary Methods 2), thus likely not posing a constraint on absorbing the additional steel waste.

Because environmental impacts from both secondary and primary production routes do change over time—e.g. through technological learning, changing energy mixes, or decreasing ore grades—we evaluate the environmental benefits of recycling over time (between 2025 and 2050) and for six scenarios. These future trajectories are generated by the Integrated Assessment Model (IAM) Remind following SSP1, SSP2, and SSP5 in both NPi and Nationally Determined Contributions (NDC) variants. The life cycle inventory background database ecoinvent v3.9.1 is modified for each time step according to all IAM scenarios using Premise. Projected global mean temperatures until 2100 in the investigated scenarios range from 1.9°C in SSP1-NDC to 3.9°C in SSP5-NPi (see Supplementary Methods 3), with SSP2-NDC (2.3°C) and SSP2-NPi (3.1°C) being close to the currently projected increase of 2.5 to 2.9°C until the end of the century.

We find that during the period 2025–2050, recycling steel from fossil infrastructure would improve at least 17 out of the 20 LCA indicators across scenarios relative to the dominant primary production route (i.e., blast furnace and basic oxygen furnace; BF-BOF) (Fig. 3b for nine life cycle assessment (LCA) indicators and Supplementary Figures 18–19 for other indicators). More precisely, in the SSP2-NPi scenario, the largest impact reductions are found in metal depletion (92%), followed by particulate matter formation (72–78%), freshwater eutrophication (71–82%), and global warming potential (61–74%), with similar results for other SSP scenarios. In contrast, impacts from water use and ionizing radiation would increase slightly (5–8% and 14–23% respectively). In general, the temporal evolution of the relative steel impacts (secondary versus primary production) follow a monotonic decrease in ozone depletion, climate impact, biodiversity loss, and marine eutrophication, driven by efficiency gains and continuing defossilization of supply chains. The relative land use impact of steel production is increasing, due to increased use of biomass in lubricating oil production; however, this can be avoided by switching to green chemicals synthesized from atmospheric carbon and water.

Environmental impact reductions of steel recycling are mainly driven by lower demand for coking coal, mining of coal and iron ore, as well as production of alloying metals, all of which cause significant climate impact and eutrophication, and particulate matter emissions. Blast oxygen furnace slag contributes to human and ecosystem toxicity in primary steel production, and can be avoided in secondary production. Both higher water demand in the EAF route than in the BF-BOF route, and water consumption in hydroelectric power plants used for EAF electricity supply cause increased water use in secondary steel production. Substituting fossil energy in primary steel-making with electric energy in EAF generates a trade-off with ionizing radiation from nuclear power plant waste and Uranium production in the investigated scenarios (Supplementary Figures 20–22). This trade-off can be completely avoided when no nuclear power is present in the future electricity supply.

Feasibility of recycling copper stocks and associated environmental impacts
Regarding copper, the secondary production capacity was 12.7 Mt a-1 in 2023 and could reach more than 18.5 Mt a-1 in 2050 under the current growth rate (Supplementary Methods 2). If the current copper stock in fossil infrastructure were recycled evenly until 2050, it would require less than 30% of the idle copper recycling capacity in any year and scenario (utilization factor of 0.83; see Fig. 3c). Overall, copper recycling capacities will be sufficient for absorbing the additional copper scrap from fossil infrastructure.

Copper recycling reduces environmental impacts for all 20 LCA indicators when compared to primary production (Fig. 3d for nine LCA indicators and Supplementary Figures 23 and S24 for other indicators). In the SSP2-NPi scenario, the largest reductions are found in metal depletion, particulate matter formation, ozone depletion, and biodiversity loss (>99%, see Fig. 3d). Reductions in marine eutrophication (98–99%), ionising radiation (94–95%), water use (95–96%), and global warming potential (94–95%) are only slightly smaller. These benefits arise mainly from avoiding copper mining and its sulfidic tailings—which release toxic compounds and drive eutrophication—as well as from avoided emissions of smelting (Supplementary Figure 25). Over the period 2025–2050, the relative impacts of recycled copper decrease monotonically for 17 of the 20 indicators, primarily because declining ore grades make primary production progressively more burdensome (Supplementary Methods 4).

Total environmental benefits
Scaling environmental benefits to the total steel and copper contained in fossil infrastructure (investigating a timeframe of 2025–2050), between 1.68 and 1.95 Gt CO2,eq of emissions could be avoided depending on the scenario and year of intervention (>95% by steel recycling, see the Supplementary Figures 18 and 23), which is comparable to Russia’s greenhouse gas emissions in 2023. Between 49 and 80 billion m2a of land could remain unoccupied from 2025 to 2050 due to the reduction in land required for steel and copper production (e.g. for mining, foresting), representing roughly the land occupation of Luxembourg. Between 5.0 and 5.4 Mt PM2.5,eq particulate matter emissions could be avoided, equivalent to around three years of PM2.5 emissions in the United States in 2024. In regard to biodiversity loss, more than 9,600 species years could be saved.

Testing the robustness of our results with pessimistic assumptions regarding dilution of steel scrap with added primary metals to compensate for lower scrap quality and higher hexavalent chromium leaching from waste steel slag and dust, environmental impacts can still be saved, though at reduced levels. E.g. CO2,eq savings from steel recycling would be halved, and avoided particulate matter emissions decline from 3.35 Mt PM2.5,eq down to less than 1.85 Mt PM2.5,eq. However, steel recycling remains the ecologically preferable option for all indicators except for ionizing radiation and water use (Supplementary Figures 19–22), where impacts can become slightly larger for recycling if consumed electricity is partly produced with nuclear and hydropower. Also, human toxicity of secondary steel production can become larger than that of primary production due to increased levels of hexavalent chromium emissions (see the Supplementary Figures 20–22), which can be reduced with better treatments of EAF slag and dust.

We also test pessimistic assumptions for copper recycling, accounting for low-grade scrap entering an additional smelter step instead of directly going to fire-refining and casting (see Supplementary Methods 4 for details). When compared to recycling high-grade copper scrap, we find that copper recycling remains the preferable strategy in all indicators, yet environmental savings would be reduced particularly for ionising radiation (20–21%, depending on the scenario), fossil depletion (17%), and climate impacts (16–17%). The effect on all other indicators is <15% compared to the high-grade scrap case (Supplementary Figure 25).

Externality costs
Saving environmental impacts also avoids future externality costs for human health (HH) and ecosystem damage (ED) remediation (see Fig. 4a), which are typically not accounted for in production costs. Reducing impacts via recycling thus also leads to healthcare cost savings due to reduced pollution, less compensation for impaired ecosystem services, and lower demand for natural resources (see Methods section for details). Recycling fossil steel and copper stocks could avoid externality costs between 4.10 and 6.94 trillion USD2023 (Fig. 4a, SSP2-NPi scenario, 75–78% from steel recycling; between 4.05 and 11.69 trillion USD2023 across investigated SSP scenarios, Supplementary Figures 26 and 27), depending on the year of intervention (2025–2050). These cost savings are mainly realized by reducing damages to HH caused by human toxicity and particulate matter formation, as well as climate change.

Avoided externality costs correspond to 1.08% to 3.92% of that year’s gross domestic product (GDP) depending on the time of recycling and SSPs. Under pessimistic assumptions on EAF slag and dust treatment, externality cost savings from steel recycling can be realized in SSP1 and SSP2 scenarios, yet externality cost savings for steel diminish to zero in ‘fossil-fueled developments’ scenarios (SSP5; see Supplementary Figures 26). The lowest externality costs of steel and copper production are found in the secondary route in 2025, with costs increasing 0.7–4.1% a-1 or 8–221 USD2023 a-1 ton-1 steel and 0.7–3.9% a-1 or 4–275 USD2023 a-1 ton-1 copper on average depending on the scenario (Supplementary Figures 26 and 27).

The total costs of primary steel and copper production are dominated by externality costs (see Fig. 4b). Depending on the steel type (reinforcing, low-alloyed, or chromium steel; see Supplementary Methods 1 for split), externality costs can be as high as four to nine times the primary production costs, and even 14 times for copper (SSP2-NPi scenario). Recycling reduces externality costs, lowering the total costs of steel by 56–80% (depending on steel type) and by 97% for copper compared to primary production. Moreover, recycling could be economically competitive even when omitting externalities (–0.3 to –6.3% for recycled steel compared to primary route, plant average production cost depending on steel type; –58% for recycled copper from cables; see Supplementary Methods 5), suggesting a win-win strategy to reduce impacts while diminishing production costs.

Utilization in renewable energy systems
Recycling steel and copper from fossil infrastructure for the energy transition could lead to cleaner renewable energy technologies. For example, using recycled steel and copper for wind turbines allows reducing greenhouse gas emissions by 30.8% and 33.7% in on- and offshore systems respectively (in the year 2025, following an SSP2-NPi scenario, Fig. 5a), making externality costs of wind turbine construction drop by 63.5% and 54.5,% for onshore and offshore systems respectively (see Fig. 5b). Using all steel from obsolete fossil infrastructure to produce wind turbines would allow covering steel needs required for building 8.8 TWp or 10.0 TWp of offshore or onshore wind capacity respectively (see Table 1), i.e., 1.7 to 2.0 times the global wind energy capacity projected in Remind SSP2-NPi scenario in 2050 (and 1.1 to 1.7 times for reaching the targets of the International Renewable Energy Agency).

Similarly, while steel is currently used in solar PV systems only for screws and hooks in rooftop installations and for support pylons in open-ground installations, it could replace commonly used Al for mounting systems (for details see the Supplementary Methods 6). Replacing Al in mounting systems for solar PV with secondary steel and using recycled copper in cables would curb greenhouse gas emissions by 39.2% and 24.8% for open-ground and slanted roof PV system production, respectively. Furthermore, externality costs from open-ground and slanted roof systems would be reduced by 40.8% and 38.9% respectively. Using the steel from fossil infrastructure for constructing PV systems would allow covering the requirements (assuming the aforementioned replacement of Al with steel) for building 24.4 TWp and 45.1 TWp of open-ground and rooftop PV respectively. This is 2.5 to 4.7 times the projected installed PV capacity globally in the Remind SSP2-NPi scenario in the year 2050 (and 1.7 to 5.3 times the PV capacity in 2050 projected to reach climate goals; Table 1).

The potential of building clean energy infrastructure using copper from fossil infrastructure is significantly less than for steel. More precisely, the copper stock is sufficiently large to supply between 10% to 31% of the installed solar PV capacity projected for the year 2050 following the SSP-NPi scenario, and between 20% and 72% of the projected wind capacity (Table 1). Yet, overall, recycling steel and copper from fossil infrastructure could contribute significantly to the steel and copper demand of the energy transition and lower the environmental impacts and externalities costs of transitioning to solar and wind energy.

Utilizing recycled steel and copper can also reduce climate impacts and externality costs of other relevant infrastructure, such as electrolyzers for green hydrogen production and power transmission infrastructure (see Supplementary Figure 28; details on the inventories in the Supplementary Methods 4; we exclude batteries as their main application is expected to be primarily in the transport sector). The alkaline electrolyzer shows the largest reduction for climate impact (38.8%) and externality cost (27.1%), both driven by recycled steel use. In contrast, benefits for power transmission infrastructure construction are driven by recycled copper use, leading to a 36.2% and 83.3% decrease in climate impact and externality cost, respectively.

Methods
Material compositions
We take material intensities of fossil infrastructure from ecoinvent v3.9.1 cutoff, accounting for geographical variability, different sub-technologies, power plant classes, and fuel types as represented in ecoinvent. Intensities are normalized to capacity for coal mines and power plants, to length for pipelines, and cumulative output for oil and gas rigs. Uncertainties of material intensities are considered from (i) life cycle inventory (LCI) uncertainties in ecoinvent and (ii) the necessity of linking and matching infrastructure classes in ecoinvent versus the classes reported in Global Energy Monitor databases. Borehead lengths in oil and gas extraction facilities are commonly not reported, though an important driver of material intensities. All these sources of uncertainty are propagated to the final stock estimates using a Monte Carlo approach with 500 runs. For more details, see the Supplementary Methods 7. We only consider on-site materials, thus neglecting grid connection and buildings, and excluding operational material flows, such as water and chemicals, as they likely don't accumulate in infrastructure stocks.

Scaling to global stocks
To calculate total stocks, we consider currently operating, idle, or mothballed fossil infrastructure. Capacities that are already decommissioned, currently under construction, or planned are excluded. Material stocks and their uncertainties are calculated individually for each infrastructure worldwide. Material intensities for each material are scaled with the size of each infrastructure taken from Global Energy Monitor databases (for details, see Supplementary Methods 7). Data gaps occur for oil and gas wells and are filled by estimating cumulative production by multiplying the mean annual well production for a respective country by the well’s years of operation. The capacity of coal mines—where missing—is calculated from annual production values assuming a capacity factor of 0.75 (based on the range of more than 0.5 to 0.9 depending on year and region; total stocks are unsensitive to the capacity factor). Pipeline stocks are allocated to countries proportionally with the segment’s length in the respective country. Pipeline segments in international waters are assigned to the nearest country.

Lifecycle assessment of steel
The functional unit is 1.34 Gt (0.84–2.26 Gt) of steel produced, and the geographical scopes are globally weighted averages in the years 2025, 2030, 2035, 2040, 2045, and 2050. LCIs are modeled for three types of steel—reinforcing steel, low-alloyed steel, and chromium steel—using adjusted data from Premise v2.2.3. Premise is a Python package that translates the outputs of IAMs into changes in LCA background models, e.g. by adjusting for future energy mixes or efficiency changes of industrial technologies.

We model the primary and secondary production route with BF-BOFs and EAFs respectively, as they are the by far dominant production technologies (61.3% and 20.2% of global steel-making capacity, with the alternative open hearth furnace only 0.6% and the rest a combination of the three). Primary production consists of iron ore extraction, conversion to iron sinter and pig iron in blast furnaces, steel production via basic oxygen furnaces, and dust, slag, and sludge treatment. The secondary production model includes the collection, sorting, and pressing of steel scrap from fossil infrastructure, transport, melting in EAFs, and dust and slag treatment. For more details on system boundaries applied, transport distances, and process efficiencies, see the Supplementary Methods 4.

Low-quality steel scrap needs to be diluted with primary sponge iron and alloying metals to compensate for contamination. If steel scrap of the same type and quality is recycled in an unmixed waste stream—as could be possible for large stocks such as pipelines or other fossil infrastructure stocks—, the need for dilution can be reduced. To reflect these two possibilities, we introduce pessimistic and optimistic recycling scenarios, assuming low and high scrap quality and slag composition, respectively.

Several adjustments are made to the LCI data from Premise: 1) Scrap supply is modeled reflecting country-weighted transport distances from fossil infrastructure to the closest EAF plant (assumed to be the most economic option). It is assumed that scrap is burden-free as it is collected at fossil infrastructure (same as in Premise databases). 2) Ecoinvent v3.9.1 likely overestimates hexavalent chromium (Cr6+) contents in slag. We adjust Cr6+ emissions to values found in the literature (see Supplementary Methods 4), selecting the lower values for the optimistic scenario and upper values for the pessimistic scenario.

Lifecycle assessment of copper
The functional unit is 10.03 Mt (5.94–18.33 Mt) of copper produced as the current globally weighted average and projected for the period 2025–2050. We model LCIs both for the pyrometallurgical (accounting for 80% of today’s primary production) and hydrometallurgical route (20%) for primary copper production and the electrorefining route for secondary production (see Supplementary Methods 4 for details). Primary pyrometallurgical production consists of mining and beneficiation of copper ore concentrate, smelting, fire-refining, casting, and electrorefining. Hydrometallurgical copper production includes mining, crushing, and grinding of copper ore, leaching, solvent extraction, and electrowinning. For secondary copper production, we model scrap collection, transport, fire-refining, casting, and the electrowinning process. We also include a pessimistic scenario for secondary copper production—assuming low-grade copper scrap—, where scrap is smelted to increase purity before being fire-refined. For more details on system boundaries applied, assumptions, and data sources for process inputs and outputs, see the Supplementary Methods 4.

Impact and externality cost assessment
We evaluate 17 environmental midpoint indicators included in the ReCiPe 2016 impact assessment framework (hierarchical perspective, e.g. only impact mechanisms based on scientific consensus and impacts within 100 years) and three additional environmental indicators: Climate impact based on global warming potentials from the Intergovernmental Panel on Climate Change (IPCC) 2021 over 100 years, land use, and biodiversity loss. For details, see the Supplementary Methods 8.

Environmental impacts are calculated by converting LCI flows—including raw materials, energy, and water inputs as well as emissions to water, soil, and air—into impacts using characterization factors as in equation (1).

IMP m,f,j = \sum_{k} LCI k,f,j \cdot CF k,m (1)

IMP m,f,j is the impact for indicator m (belonging to the set of all disaggregated indicators M) associated with the functional unit f (of all functional units FU, e.g. different steels) in year j (belonging to the set of assessed years J). LCI k,f,j is the LCI input or output of the elementary flow k associated with supplying f in year j, and CF k,m is the characterization factor of k for indicator m.

To calculate externality costs, we multiply the endpoint indicator impacts—given in the unit DALY (Disability Adjusted Life Years) for HH and species years lost for ED—with monetization factors as in equation (2):

EXT e,f,j = \sum_{m \in M_e} MF e,j \cdot IMP m,f,j (2)

Here, EXT e,f,j is the externality cost in USD2023 generated in the endpoint externality class e (HH or ED; E) while supplying the functional unit f in year j. MF e,j is the monetization factor depending on the year j and externality class e (the cost of one unit of endpoint impact in HH or ED), and m denotes the disaggregated ReCiPe indicators, where M_e is the set of disaggregated endpoint indicators connected to the externality class e (M_e \subseteq M). Note that the ReCiPe endpoint indicator impacts on natural resources are already calculated in USD.

Monetization of impacts
To calculate the externality costs of environmental impacts, we follow the approach used in ref. 84 For the year 2018, the HH cost for one DALY is 176,624 USD2018, with a lower and upper bound of 148,288 USD2018 and 200,236 USD2018, respectively. The externality cost for one unit of species loss is 17,891,594 USD2018, with a lower and upper bound of 4,472,899 USD2018 and 44,728,985 USD2018, respectively. To estimate monetization factors in the future, we apply the benefit transfer concept based on the approach followed in ref. 86 and as in equation (3).

MF e,j = MF e,j' \cdot \left( \frac{GDP j}{GDP j'} \right)^{\epsilon_e} \forall e \in E, \forall j' = 2018, j \in J (3)

Where GDP j is the gross domestic product per capita in year j, and \epsilon_e is the income elasticity for the willingness-to-pay for each externality class e, which is 1.10 and 0.38 for HH and ED, respectively. GDP per capita values are calculated based on GDP and population prospects for respective SSP scenarios from 2025 to 2050 (data from ref. 89).

Total cost estimation
The total cost comprises of production costs and externality costs. Whereas externality costs are outputs of our LCA, production costs for steel are taken from the Global Steel Cost Tracker (GSCT) database, which contains steel production costs for individual steel plants in 12 countries covering around 90% of global steel production. We link each steel plant to the steel products they produce (e.g. reinforcing steel, chromium steel) using data from the GSCT database and then estimate the production costs for reinforcing, low-alloyed, and chromium steel individually, taking plant averages. For the estimated production cost of primary and secondary copper production, see Supplementary Methods 5.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper titled “Digesting fossil infrastructure for cleaner energy transitions”, analyzes the stock of 22 materials present in fossil infrastructure, including: mines, wells, refineries, and pipelines for coal, oil, and natural gas. It considers the demand for 6 materials of the energy transition (concrete, steel, aluminum, copper, nickel, and silicon) for electricity generation applications (photovoltaic and wind). After this analysis, it focuses on the benefits of steel recycling, as it finds that the stock from fossil infrastructure is of the same order of magnitude as the demand of steel for the energy transition. It considers important aspects for recycling, such as the global capacity of Electric Arc Furnaces (EAFs), the location of fossil infrastructure and recycling plants, and the approximate time needed to process this infrastructure. In addition, it compares the externality costs of recycling versus primary production.\n\nThe paper is very thorough and provides valuable information in the context of energy transition. Nevertheless, several aspects should be considered. In order of criticality:\n\nFirst, the paper only considers wind and photovoltaic power as renewable infrastructure, overlooking a key aspect of the energy transition: storage. The main disadvantage of renewables compared to fossil fuels is that renewables consist of an intermittent flow, whereas fossil fuels are a stock. In other words, fossil fuels are available on demand, while renewables are not. The necessary solution is storage. The energy transition will not succeed without storage, and therefore this aspect should be considered. One way to store electricity is through batteries, and the most efficient type (Li-ion) contains significant amounts of cobalt. For this reason, the statement in lines 53–55 (“For some of the materials contained in current stocks, no demand for the energy transition was reported in [30] (e. g. for cobalt (Co); see Fig. 2b), or they are not typically contained in renewable energy 55 infrastructure”) is surprising. Cobalt is often used in superalloys for gas turbines (see Figure S4), so this element could be recovered and reused in batteries. Moreover, cobalt is a conflict metal, critical, and very scarce in nature, so its inclusion is important.\n\nSecond, the paper only considers the recycling of steel, but why does it ignore the recycling of aluminum and copper? Figure 2a shows that the stock of copper and the demand for the energy transition are close. In addition, this metal is experiencing a significant decline in ore grade, which is causing increasing impacts on its extraction and could compromise its supply in the future. Furthermore, it is a strategic metal, as it is not only used in energy infrastructure but is also essential for digitalization. This paper would be much more complete if the recycling of these metals were included or it should be better justified why these metals have not been considered.\n\nThird, the paper only considers electrical technologies but ignores hydrogen infrastructure. Green hydrogen (produced via electrolysis) is key to decarbonizing applications that are difficult to electrify (such as high-temperature heat). It is also critical for the production of e-fuels, as acknowledged by the authors in lines 180–182 (“For example, the carbon footprint of green hydrogen, an energy carrier and fundamental molecule to activate the inert CO2 in carbon capture and utilization routes [60], could drop by as much as 30%”). However, this infrastructure is not considered in the study. Therefore, the authors should explain why these technologies are not included in the study.\n\nFourth, most of the steel corresponds to oil and gas extraction equipment (line 43). However, much of this infrastructure is located in very remote areas or consists of pipelines at some depth, which could make metal extraction challenging. How is the dismantling of this infrastructure being approached?\n\nFifth, to maximize the recycling of alloying metals in steel (some of which are critical, like Ni), it could be important to classify them in order to obtain steel of similar quality. Has this aspect been considered?\n\nLastly, regarding lines 108–109 (“Between 25 and 45 billion m2a of land could remain unoccupied”): does this figure account for the hectares that will be occupied by renewable technologies?\n\nIn conclusion, the title promises the digestion of fossil infrastructure for the energy transition but only focuses on electricity generation (wind and solar technologies) and steel recycling. Therefore, the authors should consider broadening the scope of the paper (including the recycling of more metals such as copper or cobalt and considering more technologies of the energy transition such as batteries or renewable hydrogen generation), or giving it a more specific title.", "reviewer_2": "General comment\nThanks for your invitation. There are several major logical gaps in this study that warrant careful attention.\n\nFirst, while the study devotes significant effort to discussing recycled steel, its title focuses on “digesting fossil infrastructure,” which appears somewhat inconsistent with the article’s main content.\n\nSecond, recycled steel from decommissioned fossil infrastructure can be utilized across a variety of sectors, including construction and transportation, not necessarily in renewable infrastructure. Why, then, is the analysis limited to energy transition infrastructure? A clearer logical connection between the recycling of fossil infrastructure and the development of clean energy systems needs to be established.\n\nThird, the study’s definition of energy transition infrastructure seems to include only wind turbines and photovoltaic systems —why are other forms of clean energy infrastructure excluded?\n\nFourth, even in the era of energy transition, it remains necessary to retain some fossil fuel infrastructure. Does this mean that the existing stock of fossil infrastructure does not need to be entirely “digested”?\n\nAbstract\n1)The first two sentences suggest that recycling can mitigate the environmental impacts of renewable energy infrastructure, which could make the readers think the topic of this study is about recycling renewable infrastructure. However, the focus of your study is actually on the recycling of fossil infrastructure. This creates a logical inconsistency that may confuse readers.\n2)Regarding the 1.35 Gt steel stock mentioned in the abstract—what is its source? Does it refer to the in-use stock within fossil infrastructure to be obsolete? Additionally, where does the projected demand for the energy transition come from, and what is the time frame it covers? I think you need to clarify clearly the sources of material demand for energy transitions throughout the whole paper.\n3)What is the time period over which the reported CO₂ emissions reductions and cost savings are calculated?\n4)The final sentence references 'socioeconomic and geopolitical issues,' yet these aspects are not addressed or supported by the findings presented in this study.\n\nIntroduction\n5)Line 26: the steel stock is (1.34 Gt) is a little different from the figure (1.35 Gt) in abstract.\n6)Figure 1: what is ‘soon-to-be-obsolete’ fossil infrastructure? How to determine which infrastructure is about to be obsoleted?\n\nResults\n7)Line 43-46: specific figures or tables need to be depicted to support these results, which can not be seen from Figure 2.\n8)Figure 2a: There are only 6 materials shown in Figure 2a, not 22.\n9)Figure 2b: Again, you need to define clearly the material demand for energy transitions. Actually, cobalt is a critical material used in batteries, which are also important for energy transitions, but you didn’t take cobalt as an energy transition metal in this study.\n10)Line 61: I think there needs a full explanation of why you chose steel to investigate the recycling benefits. The “relevance” here is too ambiguous.\n11)Line 67-68: how did you conclude it?\n12)Line 65: What does \"Mt/a\" mean—does it refer to \"Mt per annum\"? If so, comparing the ideal recycling capacity (in Mt/a) with the total steel stock is conceptually misleading, as the stock will not retire in a single year but over an extended period. A more appropriate comparison would be between the recycling capacity and the annual volume of steel retiring from fossil infrastructure.\n13)Line 71-77: How did you tell “recycling of steel from fossil infrastructure would take five to 14 years with existing idle recycling capacity” from Fig. 3a? I don’t see any information about recycling time in Fig. 3a. In addition, the definition of recycling time needs to be explained cautiously because the recycling capacity can increase, which will shorten the recycling time. In addition, it is confused and odd that you evaluate the feasibility of recycling steel stocks through a comparison between the recycling time and energy transition time. I think a better story should be told here.\n14)Figure 5: I understand that the aluminum is replaced by secondary steel so that reducing the carbon emissions related to aluminum. I suggest that you can separate the carbon emissions of steel in PV systems in the right bars (hatched areas) into two segments correspond to the recycled steel that replaces primary steel and the recycled steel that replaces aluminum, respectively.\n15)Will it increase the installation costs by replacing Al with steel in PV mounting system? If so, the way replaces Al with steel maybe not economically viable in practice.\n\nDiscussion\nLine 178-183: All the benefits are actually attributed to the recycled steel. It is obvious that using recycled steel can improve the environmental performance of their entire value chains, even in products other than renewable infrastructure."}, "author_response": "Response to Reviewers for: NCOMMS-25-31469-T\nNotes: Comments in blue | Replies in black | Actions in bold | Quoted text from manuscript in italic\nIndicated page, line, figure, or reference numbers refer to the revised manuscript and/or\nsupplemental information with changes highlighted.\nReviewer #1:\nThis paper titled “Digesting fossil infrastructure for cleaner energy transitions”, analyzes the stock of\n22 materials present in fossil infrastructure, including: mines, wells, refineries, and pipelines for coal,\noil, and natural gas. It considers the demand for 6 materials of the energy transition (concrete, steel,\naluminum, copper, nickel, and silicon) for electricity generation applications (photovoltaic and wind).\nAfter this analysis, it focuses on the benefits of steel recycling, as it finds that the stock from fossil\ninfrastructure is of the same order of magnitude as the demand of steel for the energy transition. It\nconsiders important aspects for recycling, such as the global capacity of Electric Arc Furnaces\n(EAFs), the location of fossil infrastructure and recycling plants, and the approximate time needed to\nprocess this infrastructure. In addition, it compares the externality costs of recycling versus primary\nproduction.\nThe paper is very thorough and provides valuable information in the context of energy transition.\nNevertheless, several aspects should be considered. In order of criticality:\nThank you very much for your feedback and suggestions, which we thoroughly addressed in this\nreview, as discussed in detail below.\n1.1. First, the paper only considers wind and photovoltaic power as renewable infrastructure,\noverlooking a key aspect of the energy transition: storage. The main disadvantage of renewables\ncompared to fossil fuels is that renewables consist of an intermittent flow, whereas fossil fuels are a\nstock. In other words, fossil fuels are available on demand, while renewables are not. The necessary\nsolution is storage. The energy transition will not succeed without storage, and therefore this aspect\nshould be considered. One way to store electricity is through batteries, and the most efficient type\n(Li-ion) contains significant amounts of cobalt. For this reason, the statement in lines 53–55 (“For\nsome of the materials contained in current stocks, no demand for the energy transition was reported\nin [30] (e. g. for cobalt (Co); see Fig. 2b), or they are not typically contained in renewable energy 55\ninfrastructure”) is surprising. Cobalt is often used in superalloys for gas turbines (see Figure S4), so\nthis element could be recovered and reused in batteries. Moreover, cobalt is a conflict metal, critical,\nand very scarce in nature, so its inclusion is important.\nThanks for pointing this out. As discussed more in-depth below, recycling the Co in current fossil\ninfrastructure might not play a major role. However, we think Cu does deserve attention and, as\nexplained in detail later in the document, have extended the calculations to account for the latter.\nIndeed, Co is a critical metal and much needed in NMC-type Li-ion batteries (nickel manganese\ncobalt), which are currently still the main type used for mobile applications (phones, laptops, etc.) and\nelectric vehicles. Yet, LFP alternatives (lithium iron phosphate), which avoid the use of Co (and Ni),\nare developing fast and are already used in >40% of electric vehicles as traction batteries [1]. This\nuncertain development makes it hard to predict the extent to which Co recovered from superalloys\ncurrently present in fossil infrastructure will find applications in energy storage. Furthermore, the\nprojected demand for Co is orders of magnitude larger than the Co available at present in current\nfossil infrastructure. As such, it seems sensible to conclude that its recycling from fossil infrastructure\nwill likely not play a major role.\nAction: Added explanation for excluding Co to section 2.1, lines 67 to 73.\n“Demand for cobalt is also not reported in [38]. The future demand is driven by battery deployment\nand projected to reach almost 500 kt per year in 2050 [41]. In fossil infrastructure, cobalt is used in\n1\nsuperalloys for gas turbines and can be recovered by leaching superalloy scrap [42, 43]. The total\nstock available in the current fossil infrastructure is 6.92 kt (median, see Fig. 2b), much lower than the\nexpected cumulative demand for cobalt of 8.5 to 10.9 Mt from 2025 to 2050 (0.06 to 0.08% of\ndemand). Therefore, recycling cobalt from fossil infrastructure might not play a major role, which is\nwhy we exclude it from further analysis.”\n1.2. Second, the paper only considers the recycling of steel, but why does it ignore the recycling of\naluminum and copper? Figure 2a shows that the stock of copper and the demand for the energy\ntransition are close. In addition, this metal is experiencing a significant decline in ore grade, which is\ncausing increasing impacts on its extraction and could compromise its supply in the future. Furthermore,\nit is a strategic metal, as it is not only used in energy infrastructure but is also essential for digitalization.\nThis paper would be much more complete if the recycling of these metals were included or it should be\nbetter justified why these metals have not been considered.\nThanks for raising this point, which prompted us to extend the calculations to include copper, as\ndescribed in detail next. We agree with the Reviewer that considering additional metals would\nincrease completeness, provided they could contribute significantly to the demand induced by the\nenergy transition. We focused on steel due to its high stock size compared to the expected demand of\nthe energy transition. We agree with the Reviewer that a similar argument applies to copper. Copper\nrecycling from fossil infrastructure could supply around one third of the expected cumulative demand\n(line 55), which is also significant. We, therefore, decided to include copper in our new analysis.\nHowever, we still believe aluminium could be omitted, since its stock in fossil infrastructure is only 9%\nof the expected cumulative demand (line 55) and would, therefore, play a minor role in reducing\nprimary aluminium production. Furthermore, most applications in renewable energy infrastructure\nrequire wrought aluminium (e.g., module frames, mounting systems, battery electrodes, cables),\nwhich cannot be produced with cast aluminium scrap. Wrought aluminium in fossil infrastructure\naccounts for 64% of the total Al stock, so the wrought Al stock in fossil infrastructure can only\ncontribute around 6% to the Al demand of the energy transition (lines 77 to 78).\nThe new results show that copper recycling could save up to 85 Mt CO (4% of combined steel and\n2eq\ncopper recycling savings; SSP2-NPi scenario, Fig. E3 and E5) and 1.7 trillion USD of externality costs\n(25% of total savings) in total (Fig. 4). We further find that utilizing recycled copper in clean energy\nsystems could significantly reduce the externality cost of manufacturing them; for example, the\nexternality costs of onshore wind turbine manufacturing would be reduced by almost 900,000\nUSD/MW (−45% compared to using primary copper, see Fig. 5b). We now also analyze the\nimplications of using recycled copper in electrolyzers and transmission networks. Since copper is not\nextensively used in electrolyzers, total externality costs are only minimally reduced (up to −2%),\nwhereas transmission network externalities could be reduced significantly (83%) due to the heavy\nreliance on copper in cables and transformers (Fig. E11).\nAction: We now include copper recycling capacity prospects and environmental impact\nreductions of copper recycling in section 2.2.2, lines 128 to 142:\n“Regarding copper, the secondary production capacity was 12.7 Mt in 2023 and could reach more\nthan 18.5 Mt in 2050 under the current growth rate (see SM sec. S3 for details). If the current copper\nstock in fossil infrastructure were recycled evenly until 2050, it would require less than 30% of the idle\ncopper recycling capacity in any year and scenario (utilization factor of 0.83; see Fig. 3c). Overall,\ncopper recycling capacities will be sufficient for absorbing the additional copper scrap from fossil\ninfrastructure.\nCopper recycling reduces environmental impact for all 20 LCA indicators when compared to primary\nproduction (Fig. 3d for nine LCA indicators and Fig. E5 and E6 for other indicators). In the SSP2-NPi\nscenario, the largest reductions are found in metal depletion, particulate matter formation, ozone\ndepletion, and biodiversity loss (>99%, see Fig. 3d). Reductions in marine eutrophication (98–99%),\nionising radiation (94–95%), water use (95–96%), and global warming potential (94–95%) are only\n2\nslightly smaller. These benefits arise mainly from avoiding copper mining and its sulfidic\ntailings—which release toxic compounds and drive eutrophication—as well as from avoided emissions\nof smelting (see Fig. E10). Over the period 2025–2050, the relative impacts of recycled copper\ndecrease monotonically for 17 of the 20 indicators, primarily because declining ore grades make\nprimary production progressively more burdensome (see Fig. S6a in the SM for details).”\nWe also extended the Methods to include copper (lines 350 to 361).\n“The functional unit is 10.03 Mt ([5.94–18.33] Mt) of copper produced as the current globally weighted\naverage and projected for the period 2025–2050. We model LCIs both for the pyrometallurgical\n(accounting for 80% of today’s primary production; [82]) and hydrometallurgical route (20%) for\nprimary copper production and the electrorefining route for secondary production (see sec. S5.2 in the\nSM for details).\nPrimary pyrometallurgical production consists of mining and beneficiation of copper ore concentrate,\nsmelting, fire-refining, casting, and electrorefining. Hydrometallurgical copper production includes\nmining, crushing, and grinding of copper ore, leaching, solvent extraction, and electrowinning. For\nsecondary copper production, we model scrap collection, transport, fire-refining, casting, and the\nelectrowinning process. We also include a pessimistic scenario for secondary copper\nproduction—assuming low-grade copper scrap—, where scrap is smelted to increase purity before\nbeing fire-refined. For more details on system boundaries applied, assumptions, and data sources for\nprocess inputs and outputs, see the SM Section S5.2.”\nWe further extended the original calculations to determine the environmental impact and\nexternality cost savings from copper recycling and expanded the Figures 1, 3, 4, and 5, as well\nas sections S1, S3, S5, S6, S8, S9, and S10 in the supplementary materials (SM) accordingly.\nWe further added the Figures E2, E5, E6, and E10 to show additional results for copper.\nRegarding aluminium, we now include the justification for excluding it from further analysis in\nsection 2.1, lines 74 to 79.\n“We also exclude Al, Ni, and Si from further analysis, as recycling the stocks could at best contribute\n<10% of the demand generated by the energy transition (9%, 4%, and 0.001% respectively). The\npotential of Al stocks in the energy transition is even lower when considering that most clean energy\napplications require wrought-alloy Al (e.g. module frames, mounting systems, cables), which only\naccounts for 64% of the total Al in fossil infrastructure (thus only around 6% of the energy transition\ndemand of Al could be supplied).”\n1.3. Third, the paper only considers electrical technologies but ignores hydrogen infrastructure. Green\nhydrogen (produced via electrolysis) is key to decarbonizing applications that are difficult to electrify\n(such as high-temperature heat). It is also critical for the production of e-fuels, as acknowledged by\nthe authors in lines 180–182 (“For example, the carbon footprint of green hydrogen, an energy carrier\nand fundamental molecule to activate the inert CO2 in carbon capture and utilization routes [60],\ncould drop by as much as 30%”). However, this infrastructure is not considered in the study.\nTherefore, the authors should explain why these technologies are not included in the study.\nThank you for this comment, which prompted us to perform additional calculations to cover this\nrelevant point. We had initially excluded the assessment of electrolyzers, as many previous studies in\nthe literature showed that the environmental impacts of hydrogen are dominated primarily by the\nimpact of the electricity powering the electrolysis (i.e., 90–96% of climate impact dictated by the\nelectricity source, and 58–88% in impacts of mineral resource scarcity [3]). Hence, making renewable\nelectricity (i.e. predominantly solar and wind) greener with the use of repurposed steel and copper will\nalso reduce the impacts of hydrogen produced using this electricity (sec. 2.4).\nthe production of electrolyzers. To address this, we analyze in the new version three relevant\nelectrolyzer technologies (proton-exchange membrane (PEM), alkaline electrolysis cells (AEC), and\nsolid oxide electrolyzer cells (SOEC)), finding that the carbon footprint of the electrolyzer could be\nreduced by 3%, 39%, 35% respectively, when recycled steel and copper are used (Fig. E11a).\nOverall, using electrolyzers based on recycled metals from fossil infrastructure for green hydrogen\nproduction with solar energy, would reduce the carbon footprint of hydrogenby 1–12% depending on\nthe electrolyzer (SM Fig. S13). Together with improvements in the footprint of solar infrastructure for\nelectricity supply, this would translate into reductions in the carbon footprint of green hydrogen in the\nrange 27–39% depending on the electrolyzer chosen (Fig. S13).\nAction: We assess the climate impact and externality cost reduction of electrolyzer production\nwhen using recycled steel and copper from fossil infrastructure and display the results in the\nExtended results section, Figure E11. We now also refer to our assessment of electrolyzers in\nthe main text, lines 225 to 231:\n“Utilizing recycled steel and copper can also reduce climate impacts and externality costs of other\nrelevant infrastructure, such as electrolyzers for green hydrogen production and power transmission\ninfrastructure (see Fig. E11; details on the inventories in the SM sec. S5.6; we exclude batteries as\ntheir main application is expected to be primarily in the transport sector [38]). The alkaline electrolyzer\nshows the largest reduction for climate impact (38.8%) and externality cost (27.1%), both driven by\nrecycled steel use. In contrast, benefits for power transmission infrastructure construction are driven\nby recycled copper use, leading to a 36.2% and 83.3% decrease in climate impact and externality\ncost, respectively.“\n1.4. Fourth, most of the steel corresponds to oil and gas extraction equipment (line 43). However,\nmuch of this infrastructure is located in very remote areas or consists of pipelines at some depth,\nwhich could make metal extraction challenging. How is the dismantling of this infrastructure being\napproached?\nThanks for raising this point. Indeed, much of the steel in fossil infrastructure is tied to oil and gas\nextraction, where dismantling practices vary by location and accessibility. Offshore stocks represent,\naccording to our findings, only ~2% of total steel (SM Fig. S1), with nearly half of the steel being in\nplatforms where retrieval is common and feasible (as is done e.g., in the North Sea [4]). Offshore\npipelines—around 1% of steel stocks—are often trenched on the seafloor and not recovered\n(SM sec. S11). Onshore pipelines—accounting for ~19% of steel—are technically retrievable, though\noften left buried due to cost unless regulations mandate removal (SM sec. S11). Likely inaccessible\nstocks include steel casing left in closed boreholes (~6% of total steel) and structural steel in\nunderground coal mines (~4%), which may remain in place to avoid ground instability (SM Fig. S1,\nSM subsec. S11.1). Overall, ~11% of steel is estimated to be inaccessible for recycling (SM sec. S11).\nAction: We have now added a comment on potentially inaccessible stocks in section S11 of\nthe supplementary materials (limitations section). Limitations are referred to in line 283 of the\ndiscussion section in the main manuscript.\n1.5. Fifth, to maximize the recycling of alloying metals in steel (some of which are critical, like Ni),\nit could be important to classify them in order to obtain steel of similar quality. Has this aspect been\nconsidered?\nThanks for this comment. This classification is indeed possible based on the data available, as\ndiscussed next. Most steel present in fossil infrastructure is construction steel (rebar, 94.6% to our\nfindings, see SM Fig. S1). This type of steel is used for beams, supports, buildings, pipes, and\ncasings. Specialized low-alloyed steel accounts for 3.8% and stainless steel (chromium steel) for\n1.6% of the total steel in fossil infrastructure (see SM Fig. S1). The location of different types of alloys\nis well known in large assets like fossil infrastructure. Hence, we assume that sorting will be efficient,\n4\nallowing to recycle most of the steel types without a substantial loss of quality. For recycling, we\nmodel the corresponding recycling process for each of the three steel types assessed. Mounting\nsystems for solar photovoltaic (PV), steel towers for wind turbines, and pylons for power lines all need\npredominantly construction steel, and could also use other low-alloyed steel, which matches the\nmajority of the stock present in the fossil infrastructure (98.4%).\nAction: We added a figure with the shares of steel by type to SM (section S1) and referred to it\nin line 189 of the main manuscript.\n1.6. Lastly, regarding lines 108–109 (“Between 25 and 45 billion m2a of land could remain\nunoccupied”): does this figure account for the hectares that will be occupied by renewable\ntechnologies?\nThis number refers to the land occupation reduction in the material supply chains. We have clarified\nthis further in the text, lines 147 to 149. Note that we now also include the avoided land occupation\nfrom copper recycling.\nAction: Clarified the source of land occupation reduction in lines 147 to 149:\n“Between 49 and 80 billion m2a of land could remain unoccupied from 2025 to 2050 due to the\nreduction in land required for steel and copper production (e.g. for mining, foresting), representing\nroughly the land occupation of Luxembourg [49].”\n1.7. In conclusion, the title promises the digestion of fossil infrastructure for the energy transition but\nonly focuses on electricity generation (wind and solar technologies) and steel recycling. Therefore, the\nauthors should consider broadening the scope of the paper (including the recycling of more metals\nsuch as copper or cobalt and considering more technologies of the energy transition such as batteries\nor renewable hydrogen generation), or giving it a more specific title.\nThanks to your feedback and the positive assessment of our piece.\nPrompted by the Reviewer, we have broadened the scope of our paper and performed additional\ncalculations to address the main concerns raised. The new version includes recycling of copper, while\ndiscussing the potential inclusion of Al and Co, omitted here due to their expected much less relevant\nrole in the same context. It also investigates the use of repurposed steel and copper in transmission\ninfrastructure and electrolyzers via additional calculations, originally omitted in the previous version.\nOur new results reinforce the finding that recycling fossil infrastructure can be a valuable strategy for\nreducing primary raw material demand for the energy transition (145% in steel and 32% in copper;\nline 46 and line 55) and mitigating environmental impacts, particularly when used for renewable\nenergy technologies (up to 39% and 36% climate impact reduction for electrolyzers and transmission\ninfrastructure, respectively; lines 229 and 231).\nBesides all of these changes, since we heavily focus on recycling, and to avoid potential\nmisinterpretation of the term ‘digesting’, we decided to propose a new title.\nAction: Besides actions taken to address comments 1.2 and 1.3 (which address similar\nconcerns), we change the title to “Recycling fossil infrastructure for cleaner energy transitions”.\n2.0. Thanks for your invitation. There are several major logical gaps in this study that warrant careful\nattention.\nWe thank the Reviewer for the thorough review of our manuscript and the many important and helpful\ncomments. We have carefully addressed all the comments raised by the Reviewer, which have\nenhanced the quality of our manuscript. Specifically, we have substantially enlarged the scope of the\nanalysis by including copper (and discussing the potential role of recycling cobalt and aluminum) and\nthe use of recycled metals in electrolyzers and grid infrastructure.\n2.1. First, while the study devotes significant effort to discussing recycled steel, its title focuses on\n“digesting fossil infrastructure,” which appears somewhat inconsistent with the article’s main content.\nThanks for pointing this out. We agree that our study has its focus on recycling materials from fossil\ninfrastructure, which does not become apparent from reading the title. The term “digesting” could be\nunclear to readers. We therefore retitled our manuscript to increase clarity.\nAction: Changed manuscript title to “Recycling fossil infrastructure for cleaner energy transitions”.\n2.2. Second, recycled steel from decommissioned fossil infrastructure can be utilized across a variety\nof sectors, including construction and transportation, not necessarily in renewable infrastructure. Why,\nthen, is the analysis limited to energy transition infrastructure? A clearer logical connection between\nthe recycling of fossil infrastructure and the development of clean energy systems needs to be\nestablished.\nThanks for the comment. Indeed, recycled steel and other materials could be used across various\nsectors. However, the decommissioning of fossil energy infrastructure is contingent on its replacement\nby renewable energy systems. Our focus is, therefore, on how the energy transition can be supported\nthrough the recycling of the assets that become obsolete, as this enables the dismantling of\nfossil-based systems. We also note that the recycled material could be used in other applications\n(e.g., buildings, machinery; line 246), leading to equivalent impact reductions. In this sense, our\nresults are general enough regardless of the final destination of the recycled material. For practical\nreasons, we limit the contextualization of the impact reduction values to uses in the energy\ninfrastructure, but only to put them into perspective. We could have adjusted the impact values of\nother systems relying on recycled material, too.\nHowever, when used for additional consumption instead of replacing non-recycled materials, recycled\nmaterials would overall not lower the environmental impact of society nor reduce supply constraints in\nthe energy transition.\nThat said, we agree that this logical connection could be made more explicit and have clarified it in the\nsecond paragraph of the introduction.\nAction: Clarified the connection between fossil infrastructure and the renewable energy\nsystem in lines 13 to 15.\n“We focus here on how the energy transition can be supported by using recycled metals from fossil\ninfrastructure in clean energy systems, as renewable energy systems are expected to replace fossil\nones, thereby making the resulting waste from decommissioning them available for recycling\npurposes.”\nWe also come back to the point of using recycled materials for other purposes in the\ndiscussion, lines 245 to 248.\n“Recycled materials may be redirected toward non-energy-related applications (e.g. buildings,\nmachinery), but if they merely supplement overall consumption rather than replace primary materials,\nthey will neither reduce society’s environmental impact nor alleviate resource constraints during the\nenergy transition.”\n6\n2.3. Third, the study’s definition of energy transition infrastructure seems to include only wind turbines\nand photovoltaic systems—why are other forms of clean energy infrastructure excluded?\nThank you very much for this question. A similar point was also brought up by Reviewer #1 in\ncomment 1.3. In the original manuscript, we focused on solar and wind energy as they are currently\nexpected to be the most scaled and deployed power technologies in many energy transition\nscenarios. We do agree, however, that other clean energy technologies, such as electrolyzers for\ngreen hydrogen production and power transmission networks, are also important. Hence, we now\ninclude them in our assessment. We exclude batteries from our assessment because their main\napplication is expected to be in the transport sector, less so in the energy sector, as we now justify in\nline 227.\nWe find that replacing primary steel and copper with recycled materials in electrolyzers and\ntransmission network infrastructure can reduce the carbon footprints by up to 39% and 36%,\nrespectively (lines 229 and 231). Further, externality costs could also be reduced, more precisely, by\nbetween 2% and 27% depending on electrolyzer type (mainly driven by replacing primary steel) and\nby 83% for medium voltage grid infrastructure (mainly driven by replacing primary copper; Fig. E11).\nAction: Performance of life cycle assessment on three electrolyzer types and power\ntransmission infrastructure, and inclusion of results in the Extended results section, Figure\nE11. Reference to additional results and justification for excluding batteries added in line 225:\n“Utilizing recycled steel and copper can also reduce climate impacts and externality costs of other\nrelevant infrastructure, such as electrolyzers for green hydrogen production and power transmission\ninfrastructure (see Fig. E11; details on the inventories in the SM sec. S5.6; we exclude batteries as\ntheir main application is expected to be primarily in the transport sector [38]). The alkaline electrolyzer\nshows the largest reduction for climate impact (38.8%) and externality cost (27.1%), both driven by\nrecycled steel use. In contrast, benefits for power transmission infrastructure construction are driven\nby recycled copper use, leading to a 36.2% and 83.3% decrease in climate impact and externality\ncost, respectively.“\nDetails on system boundaries and data sources added to supplementary materials, section\nS5.2.\n2.4. Fourth, even in the era of energy transition, it remains necessary to retain some fossil fuel\ninfrastructure. Does this mean that the existing stock of fossil infrastructure does not need to be\nentirely “digested”?\nWe thank the Reviewer for the question. Indeed, this is true; not all fossil infrastructure might be\nrecycled, as we clarify now in the manuscript. We are focusing on the potential of materials that can\nbe recycled, as we point out in line 28. It is certainly likely that parts of the fossil infrastructure are\nretrofitted for carbon capture, or are targeted by other circular strategies such as rethinking or reusing\n(thus, changing the function without full dismantling, e.g., gas pipelines to a hydrogen pipeline). This\nwill delay the available waste flow of materials, but will not reduce it, as even reused infrastructure will\neventually be decommissioned.\nHowever, around 11% of the existing steel stocks can be classified as inaccessible (also addressing\ncomment 1.4; SM sec. S11). Offshore pipelines—around 1% of steel stocks—are often trenched on\nthe seafloor and not recovered. Onshore pipelines—accounting for ~19% of steel—are technically\nretrievable, though often left buried due to cost unless regulations mandate removal (SM sec. S11).\nLikely inaccessible stocks include steel casing left in closed boreholes (~6% of total steel) and\nstructural steel in underground coal mines (~4%), which may remain in place to avoid ground\ninstability (SM subsec. S11.1). Overall, ~11% of steel is estimated to be inaccessible for recycling\n(SM sec. S11).\nsection S11 of the SM:\n“Recycling is one of several circular strategies that could be applied to obsolete infrastructures. Other\nstrategies, such as reusing pipelines in district heating networks, retrofitting plants for carbon capture,\nor reusing power plant sites for grid storage infrastructure, could potentially avoid more costs and\nimpacts or bring other advantages due to narrower cycles. Nevertheless, reusing will delay the\navailable waste flow of materials, but will not reduce it, as even reused infrastructure will eventually be\ndecommissioned. Future research can compare different circular strategies to find potential synergies\nor trade-offs.”\nAdded assessment of potentially inaccessible stocks in section S10 of the SM (limitations\nsection):\n“We are assessing the total potential of materials that could be recycled from fossil infrastructure, not\naccounting for material stocks in infrastructure that might be inaccessible. This might be the case for\nsubsea offshore infrastructure (pipelines, oil and gas rigs), however, only 2% of steel stocks and less\nthan 0.01% of copper stocks respectively are located offshore (see Fig. S1). Of these offshore stocks,\n45% of steel and 98% of copper are stored in oil and gas wells, for which platform retrieval after\ndecommissioning is common and feasible (e.g., in the North Sea; [43], although concrete jackets\noften remain). Offshore pipelines (around 1% of steel stocks) are often buried in the seafloor and not\nrecovered [44].\nOther fossil infrastructure might be inaccessible because it is buried, such as oil and gas borehole\nequipment, pipelines, and underground mines. In closed boreholes, part of the oil and gas borehole\nequipment (steel casing) is left permanently in the ground to stabilize the borehole, whereas in open\nhole and barefoot wells, steel casing is not always required [45]. Under the assumption that all\ndecommissioned wells need steel casing, we estimate the steel kept in ground as being 12% (3–23%)\nof gas and oil well infrastructure (see SM section S11.1 for details), representing around 6% (2–11%)\nof the total steel stock in fossil infrastructure. This fraction of the steel stock might be unavailable for\nrecycling.\nAdditionally, steel in underground coal mines is used for structural support and might lead to ground\ninstability if removed. Leaving steel in underground coal mines in place would reduce the available\nsteel stock in fossil infrastructure for recycling by another 4% (see Fig. S1).\nIn contrast, retrieval of onshore pipelines (18.6% and 0% of total global steel and copper stocks in\nfossil infrastructure) is technically feasible, although it might be uneconomical. Buried pipelines are\nallowed to be abandoned after use in some jurisdictions (e.g., in the US and the UK; [46, 47]). In\ncases where the economic value of the scrap does not compensate for the economic cost of retrieval,\nregulations would be needed that incentivize the removal of buried pipelines (or make removal\nmandatory) to ensure the availability of these stocks.\nIn total, the share of steel stocks that are inaccessible for recycling amounts to an estimated 11%\n(6–16%) and includes stocks in oil and gas well steel casings, underground mines, and offshore\npipelines. Almost all copper stocks are likely accessible since they are not needed for structural\npurposes.”\nLimitations are referred to in line 283 of the discussion section in the main manuscript.\nAbstract\n2.5. The first two sentences suggest that recycling can mitigate the environmental impacts of\nrenewable energy infrastructure, which could make the readers think the topic of this study is about\nrecycling renewable infrastructure. However, the focus of your study is actually on the recycling of\nfossil infrastructure. This creates a logical inconsistency that may confuse readers.\nThank you for pointing that out. We agree that the sentences can be misunderstood as referring to\nrenewable energy infrastructure recycling. We now clarify that we refer to recycling materials in urban\nmines for sourcing the required materials for renewable energy infrastructure.\n“Sourcing these materials by recycling material from urban mines can mitigate such impacts, but the\npotential of recycling depends on waste availability.”\n2.6. Regarding the 1.35 Gt steel stock mentioned in the abstract—what is its source? Does it refer to\nthe in-use stock within fossil infrastructure to be obsolete? Additionally, where does the projected\ndemand for the energy transition come from, and what is the time frame it covers? I think you need to\nclarify clearly the sources of material demand for energy transitions throughout the whole paper.\nWe now clarify that the steel stock refers to the in-use stock of fossil infrastructure, based on our own\nanalysis. The projected steel demand for the energy transition is sourced from [5], as stated in the\ncaption of Figure 2. The time frame for this demand spans 2020 to 2050, as indicated in line 46 and\nthe caption of Figure 2. We have also clarified in lines 45–46 that the demand covers technologies\nincluding solar, wind, nuclear, fossil, biomass, hydro, and geothermal energy systems, as well as\ntransmission and distribution infrastructure.\nAction: Specified focus on in-use stocks in line 27:\n“To fill this gap, we map global in-use stocks in the current fossil infrastructure for 22 materials,\nquantifying the recycling potential and comparing it with estimated materials demand induced by\ntransition pathways”\nChanged time frame of demand from “until 2050” to “2020 to 2050” in line 46. Clarified\ntechnologies included in the material demand projections of energy transitions in lines 45 and\n46:\n“Steel is the second largest stock, amounting to 1.34 Gt ([0.84–2.26] Gt), matching cumulative steel\ndemand estimates for energy transitions including energy generation (solar, wind, nuclear, fossil,\nbiomass, hydro, and geothermal energy), transmission, and distribution infrastructure from 2020 to\n2050 (145% of the median demand, 0.93 Gt ([0.51–2.85] Gt) depending on the scenario [38], see\nFig. 2a; material demand of grid-scale batteries is excluded since battery capacity in the transport\nsector (electric vehicles) may be leveraged [38])”\n2.7. What is the time period over which the reported CO₂ emissions reductions and cost savings are\ncalculated?\nWe report emissions reductions and cost savings for different possible years of recycling from 2025 to\n2050 (line 145), assuming that the total steel and copper stock is recycled within this time frame.\nAction: We clarified the time frame considered in the section on cumulative impact (line 144):\n“ Scaling environmental benefits to the total steel and copper contained in fossil infrastructure\n(investigating a time frame of 2025–2050),...”\nWe further clarified the time frame considered in the section on cost savings (lines 175 to 178):\n“Recycling fossil steel and copper stocks could avoid externality costs between 4.18 and 7.00\ntrillion USD (Fig. 4a, SSP2-NPi scenario, 75–78% from steel recycling; between 4.05 and 11.69\n2023\ntrillion USD across investigated SSP scenarios, Fig. E1 and E2), depending on the year of\n2023\nintervention (2025–2050).”\n2.8. The final sentence references 'socioeconomic and geopolitical issues,' yet these aspects are not\naddressed or supported by the findings presented in this study.\nWhile we briefly mention potential socioeconomic and geopolitical implications in the discussion, we\nbelieve that this topic can be omitted in the abstract. Therefore, we have removed the sentence from\nthe abstract.\nAction: Removed mention of socioeconomic and geopolitical implications from the abstract.\n2.9. Line 26: the steel stock is (1.34 Gt) is a little different from the figure (1.35 Gt) in abstract.\nWe thank the Reviewer for pointing out this issue, which we have amended.\nAction: Harmonized steel stock size to the correct 1.34 Gt throughout the manuscript.\n2.10. Figure 1: what is ‘soon-to-be-obsolete’ fossil infrastructure? How to determine which\ninfrastructure is about to be obsoleted?\nWe were referring to fossil infrastructure that might be phased out in the energy transition, and,\ntherefore, might not be used anymore. Under further consideration, we see that ‘soon-to-be-obsolete’\nis ambiguous, and therefore decided to remove the adjective.\nAction: Removed ‘soon-to-be-obsolete’ in the caption of figure 1.\nResults\n2.11. Line 43-46: specific figures or tables need to be depicted to support these results, which can not\nbe seen from Figure 2.\nThe Reviewer is right that the figures on the steel stocks by infrastructure type are not visible in\nFigure 2. We, therefore, added a figure to the supplementary material and referred to it in the main\nmanuscript.\nAction: Added figure with steel and copper stock by fossil infrastructure type to\nsupplementary material (Fig. S1). Added reference to figure in line 50.\n2.12. Figure 2a: There are only 6 materials shown in Figure 2a, not 22.\nThe Reviewer is completely right: the whole figure depicts 22 materials, but subplot “a” only depicts\nsix materials. We amended the number accordingly.\nAction: Changed to “six materials” in the caption of figure 2.\n2.13. Figure 2b: Again, you need to define clearly the material demand for energy transitions. Actually,\ncobalt is a critical material used in batteries, which are also important for energy transitions, but you\ndidn’t take cobalt as an energy transition metal in this study.\nIndeed, cobalt is used in NMC-type batteries mostly for mobile applications today. LFP-type batteries\nare emerging and may potentially replace the currently dominating NMC types [1]. In LFP, no cobalt is\nneeded. Yet, as this substitution is uncertain and IEA projects the demand for cobalt to double by\n2050, we included a potential analysis of cobalt recovered from super alloys currently embodied in\nfossil infrastructure. Though technically possible, recovering all cobalt from the fossil infrastructure\ncan at best contribute 10 kt of cobalt in total, which is much smaller than the current annual demand\n(280 kt/a) and the projected demand for 2050 (450 kt/a). Hence, cobalt recycling from fossil\ninfrastructure may not play a significant role in reducing the impacts from batteries. We have included\nthis discussion in the paper.\nAction: Included an analysis of Co in section 2.1 and justified its exclusion from further\nanalysis (lines 67 to 73):\n“Demand for cobalt is also not reported in [38]. The future demand is driven by battery deployment\nand projected to reach almost 500 kt per year in 2050 [41]. In fossil infrastructure, cobalt is used in\nsuperalloys for gas turbines and can be recovered by leaching superalloy scrap [42, 43]. The total\nstock available in the current fossil infrastructure is 6.92 kt (median, see Fig. 2b), much lower than the\nexpected cumulative demand for cobalt of 8.5 to 10.9 Mt from 2025 to 2050 (0.06 to 0.08% of\ndemand). Therefore, recycling cobalt from fossil infrastructure might not play a major role, which is\nwhy we exclude it from further analysis.”\n10\n2.14. Line 61: I think there needs a full explanation of why you chose steel to investigate the recycling\nbenefits. The “relevance” here is too ambiguous.\nThe original focus on steel is based on our findings mapping material stocks in fossil infrastructure. As\nFigure 2 shows, the steel stock is the only material stock in fossil infrastructure that closely matches\nthe steel demand induced by the energy transition. For all other materials, demand in the energy\ntransition is substantially larger than the corresponding availability in the fossil infrastructure.\nYet, following this and Reviewer #1’s comments, we have expanded the detailed analysis to copper\nand clarified further the selection of materials we consider for a detailed recycling analysis. We now\ninclude materials that are stored in the fossil infrastructure, which supply more than 10% of the energy\ntransition demand (145% for steel and 32% for copper; lines 46 and 55).\nAction: Explanation of material selection for further analysis is expanded in section 2.1, lines\n67 to 79.\n“Demand for cobalt is also not reported in [38]. The future demand is driven by battery deployment\nand projected to reach almost 500 kt per year in 2050 [41]. In fossil infrastructure, cobalt is used in\nsuperalloys for gas turbines and can be recovered by leaching superalloy scrap [42, 43]. The total\nstock available in the current fossil infrastructure is 6.92 kt (median, see Fig. 2b), much lower than the\nexpected cumulative demand for cobalt of 8.5 to 10.9 Mt from 2025 to 2050 (0.06 to 0.08% of\ndemand). Therefore, recycling cobalt from fossil infrastructure might not play a major role, which is\nwhy we exclude it from further analysis.\nWe also exclude Al, Ni, and Si from further analysis, as recycling the stocks could at best contribute\n<10% of the demand generated by the energy transition (9%, 4%, and 0.001% respectively). The\npotential of Al stocks in the energy transition is even lower when considering that most clean energy\napplications require wrought-alloy Al (e.g. module frames, mounting systems, cables), which only\naccounts for 64% of the total Al in fossil infrastructure (thus only around 6% of the energy transition\ndemand of Al could be supplied).”\n2.15. Line 67-68: how did you conclude it?\nWe arrived at the years needed to recycle the steel stock in the original manuscript by dividing the\npotential steel stock available (upper and lower estimates) by the current idle recycling capacity.\nHowever, as this is a static value, we changed the approach in the new version of the manuscript to a\ndifferent approach (see reply to comment 2.16).\nAction: See action taken for comment 2.16.\n2.16. Line 65: What does \"Mt/a\" mean—does it refer to \"Mt per annum\"? If so, comparing the ideal\nrecycling capacity (in Mt/a) with the total steel stock is conceptually misleading, as the stock will not\nretire in a single year but over an extended period. A more appropriate comparison would be between\nthe recycling capacity and the annual volume of steel retiring from fossil infrastructure.\nYes, “Mt/a” does indeed mean “Mt per annum” and refers to the global capacity of steel that can be\nrecycled globally within one year.\nOur assessment aims to assess whether or not recycling capacity may become a limiting factor for\nusing the materials recycled from the fossil infrastructure. We showed in the original manuscript that\nthe entire fossil steel stock can be recycled in a few years with the already existing and currently idle\nrecycling capacity. We understand that this approach can be misinterpreted since—as the Reviewer\nmentions—the whole stock is not retiring in a single year and the idle recycling capacity might change\nin the future (comment 2.17).\nWe, therefore, conducted additional calculations with refined assumptions, finding that the new results\nlead to similar conclusions. First, instead of relying on current recycling capacity, we now use\nprojections of recycling capacities until 2050. Moreover, we now assume an annual waste volume\n11\nstemming from the stock retiring evenly over the period 2025 to 2050. A constant retirement rate of\nfossil infrastructure until 2050 translates into a linear decline of fossil infrastructure stocks until\nreaching net zero in 2050.\nFollowing this refined analysis, it becomes apparent that in any year and scenario, the additional steel\nwaste flow utilizes less than 74% of the idle steel recycling capacity in any year and scenario (line 91).\nFor copper, which we now include, the additional copper scrap volume utilizes less than 30% of the\nidle recycling capacity in any year 2025 and 2050 (line 130). Thus, the recycling capacity might be\nenough to absorb the steel and copper stock from fossil infrastructure.\nAction: Changed Figure 3 and included a dynamic approach in section 2.2, with results for\nsteel described in lines 90 to 93:\n“If the steel scrap from fossil infrastructure were released evenly across the period 2025–2050, the\nannual additional steel scrap volume would then utilize less than 40% of the global idle recycling\ncapacity in any year (see Fig. 3a, SSP2-NPi, never exceeds 74% in any other scenario, see SM\nFig. S2), thus likely not posing a constraint on absorbing the additional steel waste.”\nFor copper in lines 129 to 132:\n“If the current copper stock in fossil infrastructure were recycled evenly until 2050, it would require\nless than 30% of the idle copper recycling capacity in any year and scenario (utilization factor of 0.83;\nsee Fig. 3c). Overall, copper recycling capacities will be sufficient for absorbing the additional copper\nscrap from fossil infrastructure.”\nAdded recycling capacity projections to the SM section S3.\n2.17. Line 71-77: How did you tell “recycling of steel from fossil infrastructure would take five to 14\nyears with existing idle recycling capacity” from Fig. 3a? I don’t see any information about recycling\ntime in Fig. 3a. In addition, the definition of recycling time needs to be explained cautiously because\nthe recycling capacity can increase, which will shorten the recycling time. In addition, it is confused\nand odd that you evaluate the feasibility of recycling steel stocks through a comparison between the\nrecycling time and energy transition time. I think a better story should be told here.\nAs we highlighted in our response to comment 2.16, we have now refined the calculations,\nconsidering varying recycling capacities and an annualized release of waste from fossil infrastructure.\nWe show that the annual steel and copper waste released from fossil infrastructure never exceeds the\nrespective idle recycling capacity in the period 2025 to 2050, therefore being sufficient for absorbing\nthis additional waste stock.\nAction: See actions taken to address comment 2.16.\n2.18. Figure 5: I understand that the aluminum is replaced by secondary steel so that reducing the\ncarbon emissions related to aluminum. I suggest that you can separate the carbon emissions of steel\nin PV systems in the right bars (hatched areas) into two segments correspond to the recycled steel\nthat replaces primary steel and the recycled steel that replaces aluminum, respectively.\nWe thank the Reviewer for this good suggestion. We agree that separating the contribution of\nrecycling steel replacing primary steel, and recycling steel replacing aluminium enhances the clarity of\nthe figure. We, therefore, modified Figure 5 so that the contribution of steel replacing aluminium is\ndistinct from the contribution of the recycled steel that replaces primary steel.\nAction: Adjustments to Figure 5.\n2.18. Will it increase the installation costs by replacing Al with steel in PV mounting system? If so, the\nway replaces Al with steel maybe not economically viable in practice.\nIndeed, we think steel mounting systems might influence the installation costs of PV due to two main\nreasons. First, steel mounting systems might be less expensive than aluminium mounting systems,\nreducing material costs. Secondly, installing steel mounting systems might take more time, thus\nincreasing labor costs.\nRegarding material costs, galvanized steel profiles are less expensive per kilogram compared to\naluminium profiles, thus we expect material costs to be slightly lower when using steel mounting\nsystems (decrease by 50(21–84) USD/kWp or 3.0(1.2–5.0)% of the total installation cost; sec. S7 in\nthe SM).\nEven under the assumption that the steel mounting system doubles the labor costs for mounting, this\nwould increase installation costs only marginally by 21(8–47) USD/kWp or 1.2(0.5–2.8)% of the total\ninstallation cost. The net cost savings of PV using steel mounting systems might, thus, be 28(−9–66)\nUSD/kWp or 1.7(−0.5–3.2)% of the total installation cost. This estimated cost benefit should be taken\nwith a pinch of salt in light of the lack of data collected from industry. Overall, PV installation costs\nmight remain similar regardless of the type of mounting system used.\nAction: Added cost analysis for PV installation with steel and aluminium mounting systems in\nsection S7 in the supplementary materials.\nDiscussion\n2.19. Line 178-183: All the benefits are actually attributed to the recycled steel. It is obvious that using\nrecycled steel can improve the environmental performance of their entire value chains, even in\nproducts other than renewable infrastructure.\nWe appreciate the reviewer’s observation and agree that recycled steel and copper is widely\nrecognized for its environmental advantages. However, our study goes beyond reiterating known\nbenefits. The novelty lies in quantifying the untapped potential of steel and copper embedded in fossil\ninfrastructure and demonstrating its strategic relevance for the energy transition. We think that our\nwork goes beyond previous studies in various ways:\n(i) We assess the feasibility of mobilizing steel and copper stocks, showing that global idle recycling\ncapacity is likely to be sufficient to absorb the released scrap without bottlenecks. (ii) We quantify the\nsystem-wide environmental and cost benefits of using this specific waste stream—including up to\n1.95 Gt CO avoided (line 145) and 11.69 trillion USD (line 177) in externality cost savings—across\n2eq\nmultiple climate scenarios and years. (iii) We demonstrate how recycled steel from fossil infrastructure\ncan enable the construction of up to 45 TWp of solar PV and 10 TWp of wind capacity (Table 1),\nexceeding global targets, and reducing the carbon footprint of these technologies by up to 41%\n(line 212).\nAction: The main novelty was higher stressed throughout the manuscript.\nLines 90 to 93:\n“If the steel scrap from fossil infrastructure were released evenly across the period 2025–2050, the\nannual additional steel scrap volume would then utilize less than 40% of the global idle recycling\ncapacity in any year (see Fig. 3a, SSP2-NPi, never exceeds 74% in any other scenario, see SM\nFig. S2), thus likely not posing a constraint on absorbing the additional steel waste.”\nLines 131 to 132:\n“Overall, copper recycling capacities will be sufficient for absorbing the additional copper scrap from\nfossil infrastructure.”\nLines 221 to 224:\n“Yet, overall, recycling steel and copper from fossil infrastructure could contribute significantly to the\nsteel and copper demand of the energy transition and lower the environmental impacts and\nexternalities costs of transitioning to solar and wind energy.”\n[1] Adamas Inside. LFP now commands 40 % of the global EV battery market.\nhttps://www.adamasintel.com/lfp-now-commands-40-of-the-global-ev-battery-market/ (2025)\n[2] Global Critical Minerals Outlook 2025. (International Renewable Energy Agency, 2025).\nhttps://www.iea.org/reports/global-critical-minerals-outlook-2025.\n[3] Zhao, G., Kraglund, M. R., Frandsen, H. L., Wulff, A. C., Jensen, S. H., Chen, M. & Graves, C. R.\nLife cycle assessment of H2O electrolysis technologies. International Journal of Hydrogen Energy 45,\n23765–23781 (2020).\n[4] OSPAR Commission OSPAR Decision 98/3 on the Disposal of Disused Offshore Installations (as\namended by OSPAR Decision 2024/01): Consolidated Text In OSPAR Commission Decisions and\nAgreements (2024). https://www.ospar.org/documents?v=57705.\n[5] Wang, S. et al. Future demand for electricity generation materials under different climate mitigation\nscenarios. Joule 7, 309–332 (2023)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for the opportunity to review the paper.\n\nThe main issues raised during the initial review have been resolved or clarified. The title of the revised version is more appropriate than the previous one. Furthermore, the authors have incorporated into their study the analysis of copper, hydrogen technologies, and transmission lines. They have also identified certain steel stocks that may not be easily accessible, as requested in the previous review.\n\nOn the other hand, the authors clearly explain that Al, Ni, Co, and Si are excluded from this study due to their limited contribution to the new renewable infrastructure. Finally, batteries have also been excluded from the study because, as the authors state, their primary use will be in the transport sector. However, it should also be noted that the transformation of the transport sector is indeed part of the energy transition. Beyond this minor clarification, I believe the authors have done a good job during the revision and that the article is ready for publication.", "reviewer_2": "The authors have made substantial improvements to the manuscript compared to the previous version, particularly by incorporating analyses on recycled copper from fossil infrastructure and secondary metal supply for electrolyzers and transmission networks. These additions significantly enhance the completeness and contribution of the study.\n\nNevertheless, several issues require further attention to improve clarity and accuracy:\n\nAbstract:\n(1) In the fourth sentence, the time frame for the \"projected demand for the energy transition\" should be explicitly stated.\n(2) In the final sentence, the term \"repurposing\" should be reconsidered, as it may not accurately reflect the focus of the study, which centers on \"recycling.\"\n\nIntroduction:\n(3) It should be clarified that only 6 out of the 22 materials are compared with their projected energy transition demand.\n(4) Lines 29–30: Including a similar comparative analysis for copper would help contextualize the subsequent sentence and clarify the rationale for focusing on steel and copper recycling.\n\nResults:\n(5) Line 45: The term \"fossil\" in parentheses does not align with energy transition technologies and may cause confusion.\n(6) Lines 51–53: A reference to the figure presenting these results should be provided.\n(7) Line 80: The title should also include the recycling of copper to accurately reflect the content.\n(8) Figure 3: The y-axis labels in subplots (b) and (d) are difficult to read.\n\nDiscussion:\n(9) Lines 245–248: The argument in this sentence is unclear. Regardless of the application, the use of recycled materials can displace primary metal production.\n\nOther Comments:\n(10) The formatting of numerical intervals (e.g., “[4.71–9.06] Gt” in Line 38) appears inconsistent with conventional style. Consider removing the square brackets for better adherence to standard practice.\n(11) In Lines 208 and 209, the full term \"aluminum\" should be replaced with its abbreviation (\"Al\") for consistency with the rest of the text."}, "author_response": "Response to Reviewers for: NCOMMS-25-31469A\nNotes: Comments in blue | Replies in black | Actions in bold | Quoted text from manuscript in italic\nIndicated page, line, figure, or reference numbers refer to the revised manuscript and/or\nsupplemental information with changes highlighted.\nReviewer #1:\n1.0. Thank you for the opportunity to review the paper.\nThe main issues raised during the initial review have been resolved or clarified. The title of the revised\nversion is more appropriate than the previous one. Furthermore, the authors have incorporated into\ntheir study the analysis of copper, hydrogen technologies, and transmission lines. They have also\nidentified certain steel stocks that may not be easily accessible, as requested in the previous review.\nOn the other hand, the authors clearly explain that Al, Ni, Co, and Si are excluded from this study due\nto their limited contribution to the new renewable infrastructure. Finally, batteries have also been\nexcluded from the study because, as the authors state, their primary use will be in the transport sector.\nWe thank the Reviewer for the careful assessment and the positive feedback. We are pleased that our\nrevisions have addressed the main issues to the Reviewer’s satisfaction. We appreciate the Reviewer’s\nrecognition of these clarifications and believe that the revised manuscript now presents a more coherent\nand targeted analysis.\n1.1. However, it should also be noted that the transformation of the transport sector is indeed part of\nthe energy transition. Beyond this minor clarification, I believe the authors have done a good job during\nthe revision and that the article is ready for publication.\nWe agree with the Reviewer that the transformation of the transport sector is an integral part of the\nbroader energy transition, namely the shift from fossil fuels as the dominant energy carrier to alternative\nfuels and electricity. We recognize that our initial justification for excluding batteries (line 49) could be\nmisinterpreted as suggesting that the transport transition is not part of the energy transition. To avoid\nthis misunderstanding, we have clarified the relation between the energy transition and the transport\ntransition.\nWe would like to thank the reviewer once again for the insightful comments, which have greatly\nenhanced the rigor, conciseness, comprehensiveness, and clarity of the revised manuscript.\nAction: Clarified that the transformation of the transport sector is part of the energy transition\nin lines 287 to 289:\n“Further, for simplicity, we do not focus on recycled materials’ utilization in the transport sector (e.g.\nelectric vehicles); nevertheless, the transport transition is also integral to the broader energy transition\n(e.g. energy storage in battery-electric vehicles).”\n2.0. The authors have made substantial improvements to the manuscript compared to the previous\nversion, particularly by incorporating analyses on recycled copper from fossil infrastructure and\nsecondary metal supply for electrolyzers and transmission networks. These additions significantly\nenhance the completeness and contribution of the study.\nWe thank the Reviewer for this positive assessment of our previous revision and for the additional\ncomments and suggestions, which we are happy to address below.\nAbstract:\n2.1. Nevertheless, several issues require further attention to improve clarity and accuracy:\nIn the fourth sentence, the time frame for the \"projected demand for the energy transition\" should be\nexplicitly stated.\nWe agree with the reviewer that adding the time frame would increase clarity, and we, therefore, added\nit to the abstract.\nAction: Added demand projection time frame (2020 to 2050) to the abstract.\n“Among many materials in fossil infrastructure, recycling steel and copper is particularly appealing, as\ntheir stocks (1.34 Gt and 10.03 Mt) align with the projected demands for the energy transition from 2020\nto 2050 (145% and 32% of median demand, respectively).”\n2.2. In the final sentence, the term \"repurposing\" should be reconsidered, as it may not accurately reflect\nthe focus of the study, which centers on \"recycling.\"\nThanks for pointing this out. Indeed, the term “repurposing” reflects several circular strategies, which is\nwhy we have now replaced it with “recycling” in the abstract and throughout the revised manuscript\nwhere it appeared in the same context (line 238) or with “retiring” where it appeared in the context of\ndecommissioning (line 265).\nAction: Replaced “repurposing” with “recycling” where it appeared in the context of recycling\nsteel and copper from fossil infrastructure and with “retiring” where it appears in the context of\ndecommissioning.\nAbstract:\n“These findings provide strong evidence of the benefits of recycling fossil infrastructure and underscore\nthe need for policies to expedite the energy transition.”\nLine 238:\n“While adopting greener production routes is often hindered by higher costs [61–63], recycling steel and\ncopper from fossil infrastructure could represent a win-win alternative.”\nLine 265:\n“Resistance to retiring may vary depending on the type of infrastructure.”\nIntroduction:\n2.3. It should be clarified that only 6 out of the 22 materials are compared with their projected energy\ntransition demand.\nWe recognize that the sentence in lines 27–29 could be misinterpreted as suggesting that all 22\nmaterials were compared with their projected energy transition demand. Indeed, only six materials are\nbenchmarked against their energy transition demand. We have revised the sentence to eliminate this\nambiguity and ensure the distinction is clear.\n“To fill this gap, we map global in-use stocks in the current fossil infrastructure for 22 materials,\nquantifying the recycling potential, and comparing the stocks of six materials with estimated material\ndemand induced by transition pathways.”\n2.4. Lines 29–30: Including a similar comparative analysis for copper would help contextualize the\nsubsequent sentence and clarify the rationale for focusing on steel and copper recycling.\nThe Reviewer is right, this paragraph (lines 29–31) refers to our results on steel recycling, while not\nmentioning copper, which is now included in the further analysis. We, therefore, added a comparative\nanalysis for copper.\nAction: Added a contextualized reference to the findings regarding copper stocks in lines 31–\n32:\n“We find that steel is the largest metal stock in fossil infrastructure (1.34 Gt) and is within the range of\nestimated demand in transition pathways aiming at 1.5 °C and 2 °C heating [38]. Copper is another\npromising material to recycle (10.03 Mt in fossil infrastructure), which could contribute around one third\nof the median energy transition demand.”\nResults:\n2.5. Line 45: The term \"fossil\" in parentheses does not align with energy transition technologies and\nmay cause confusion.\nThank you for spotting this. Fossil energy generation is, of course, not included in the clean transition\ntechnologies.\nAction: Removed “fossil” from the list in parentheses (line 47).\n“Steel is the second largest stock, amounting to 1.34 Gt (0.84–2.26 Gt), matching cumulative steel\ndemand estimates for energy transitions including energy generation (solar, wind, nuclear, biomass,\nhydro, and geothermal energy),...”\n2.6. Lines 51–53: A reference to the figure presenting these results should be provided.\nWe now include a reference to Figure 3a when listing the countries with the largest steel stocks in fossil\ninfrastructure (line 54).\nAction: Included a reference to Figure 3a in line 54.\n“The five countries with the largest fossil steel stocks are China, Saudi Arabia, Russia, the United\nStates, and Iran, with 164, 155, 152, 150, and 94 Mt steel respectively (see Fig. 3a),...”\n2.7. Line 80: The title should also include the recycling of copper to accurately reflect the content.\nThe Reviewer is completely right. Due to editorial policies, we had to remove the level of subsubsections\nand therefore split the subsection 2.2 into three subsections, one for “Feasibility of recycling steel stocks\nand associated environmental impacts”; one for “Feasibility of recycling copper stocks and associated\nenvironmental impacts”; and one for “Total environmental benefits”.\nAction: Split the former subsection 2.2 (line 85) into three subsections:\nLine 85\n“Feasibility of recycling steel stocks and associated environmental benefits”\nLine 128\n“Feasibility of recycling copper stocks and associated environmental benefits”\n“Total environmental benefits”\n2.8 Figure 3: The y-axis labels in subplots (b) and (d) are difficult to read.\nWe see that the y-axis labels in Figure 3b and d might be difficult to read, especially the subscripts.\nTherefore, we replaced the labels with variants without subscripts.\nAction: Replaced y-axis labels in Figure 3b and d.\nFigure 3b:\n“secondary steel impact as fraction of primary impact [-]”\nFigure 3d:\n“secondary copper impact as fraction of primary impact [-]”\nDiscussion:\n2.9 Lines 245–248: The argument in this sentence is unclear. Regardless of the application, the use of\nrecycled materials can displace primary metal production.\nIn our original argument, we focused on the energy sector, but certainly this does not imply that\ndisplacement only matters in energy-related contexts. In reality, displacement of primary production\nthrough secondary production is beneficial regardless of the sector.\nWe rephrased the sentence to avoid confusion.\nAction: Rephrased sentence in line 247.\n“We focus on the application of recycled materials in the energy sector, as fossil technologies need to\nbe replaced with renewable ones. However, we clarify that the benefits of recycling materials can be\nrealized regardless of the application sector as long as they displace primary production.”\nOther Comments:\n2.10. The formatting of numerical intervals (e.g., “[4.71–9.06] Gt” in Line 38) appears inconsistent with\nconventional style. Consider removing the square brackets for better adherence to standard practice.\nThank you for pointing this out. We changed the formatting numerical intervals to the more conventional\nstyle without square brackets (e.g. 1.34 Gt (0.84–2.26 Gt)).\nAction: Removed square brackets in intervals across the main manuscript and supplementary\nmaterials.\n2.11. In Lines 208 and 209, the full term \"aluminum\" should be replaced with its abbreviation (\"Al\") for\nconsistency with the rest of the text.\nThank you very much for pointing out this inconsistency. We replaced “aluminum” with its abbreviation\nin these occurrences.\nAction: Replaced “aluminum” with its abbreviation “Al” after its first appearance in the\nmanuscript (line 209 and line 210):\n“Similarly, while steel is currently used in solar PV systems only for screws and hooks in rooftop\ninstallations and for support pylons in open-ground installations, it could replace commonly used Al for\nmounting systems [56] (for details see the SM sec. S8). Replacing Al in mounting systems for solar PV\nwith secondary steel and using recycled copper in cables would curb greenhouse gas emissions by\n39.2% and 24.8% for open-ground and slanted roof PV system production, respectively.”\n4"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have successfully addressed all of my remaining concerns. The revisions are sufficient, and I have no further comments."}, "author_response": ""}]

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