Biomass carbon emissions from nickel mining have significant implications for climate action

Domain: earth_science
Published: 2025-01-08
Editorial outcome: major_revision

Abstract
Global nickel demand is projected to double by 2050 to support low-carbon technologies and renewable energy production. However, biomass carbon emissions from clearing vegetation for nickel mining are rarely included in corporate sustainability reports or considered in mineral sourcing decisions. Here, we compiled data for 481 nickel mines and undeveloped deposits to show that the footprint of nickel mining could be 4 to 500 times greater than previously reported (depending on the mine site), and thus the environmental impacts of nickel products, including batteries, have been underestimated to date. We found large variation in biomass losses among mines, and, in many cases, these unaccounted carbon emissions were significant relative to other Scope 1 and Scope 2 emissions from nickel extraction and processing. Reporting emissions from biomass losses from mining is key for strategic decision making on where to source nickel needed for effective climate action.

Introduction
Commitments to boost low-carbon energy production and end deforestation by 2030 were two key outcomes of the COP 28 conference of the United Nations Framework Convention on Climate Change1. Together, these measures could make substantial progress towards mitigating anthropogenic climate change and limiting global warming to 1.5 degrees Celsius2. However, their effects are not independent, and, without careful planning, progress on one could undermine the achievement of the other. Low-carbon technologies, for example, require production increases in mineral mining3. The mining industry is already responsible for ~10% of global energy-related carbon emissions each year4. In addition, mining directly and indirectly causes land use changes, and the associated biomass losses may result in further carbon emissions5–7. However, biomass carbon emissions are not generally reported by the mining sector8 and rarely feature in climate plans or decisions on where and how to source metals for low-carbon technologies3.

The minerals required for low-carbon technologies include aluminum, graphite, nickel, copper, zinc, lead, manganese, cobalt, and lithium3,9. For many of these minerals, large increases in mining production will be required to meet growing demand, even when recycling is taken into consideration. For example, The World Bank estimated that to develop the low-carbon infrastructure required to limit global warming to 2 degrees, graphite and lithium mining will need to increase by almost 500% by 20509. The minerals that have the highest estimated annual demands in 2050 for use in low-carbon technologies are aluminum (5.58 million tonnes), graphite (4.59 million tonnes), nickel (2.27 million tonnes), and copper (1.38 million tonnes)9.

Here, we focus on estimating biomass carbon emissions caused by clearing vegetation for nickel mining. Nickel is a silvery-white metal that is hard, ductile, and resistant to oxidation and corrosion. Today, nickel is primarily used in stainless steel and other alloys, as well as in specialized corrosion-resistant plating10,11. Nickel is also used to produce numerous low-carbon energy production and storage technologies, including batteries and infrastructure for geothermal, wind, solar photovoltaic, hydroelectric, and nuclear energy. Currently, ~15% of global nickel production is used in clean energy applications11,12. As battery use increases in future to aid the transition to a low-carbon economy, an increasing amount of nickel supply will be required. For example, one study estimates that the amount of nickel required for electric vehicles will reach 1.1 million tons (Mt) per year by 2030, which is approximately one-third of global annual production in 202312. Climate action, among other factors, is projected to see future nickel demand from energy technologies soar to 2.27 Mt of nickel per year by 20509. A recent analysis by the International Energy Agency predicts that cleantech demand for nickel will increase from 0.48 Mt (in 2023) to 3.4 Mt by 204011. Demand for other nickel use is also predicted to increase, from 2.5 Mt (in 2023) to 2.9 Mt by 204011. Since increasing end-of-life nickel recycling rates to 100% by 2050 would only increase recycled content rates in nickel products to 58%, continued supplies of nickel will need to be provided through mining9.

Global terrestrial nickel resources in 2018 (350.2 Mt nickel) are sufficient to meet increasing demand in the coming decades13,14. However, land use changes and associated biomass losses from establishing new nickel mines will depend strongly on where and how deposits are brought into production. More than half (60–70%) of terrestrial resources are located in nickel laterite deposits, which are concentrations of nickel that form in loose rock and soil through intense weathering in tropical climates14,15. To date, laterite mining has been secondary to the mining of magmatic sulfide deposits, which form in igneous rocks. Magmatic sulfides generally have lower land and energy processing requirements than laterites and are thus less expensive to mine, accounting for ~60% of current production14,16. However, magmatic sulfide deposits are declining in grade, and it is expected that laterite mining will comprise a larger proportion of nickel production in future14. In addition, recent technological developments have enabled battery grade (Class 1) nickel production from laterite deposits17–19.

Our goal in this study was to estimate the impact of nickel mining on biomass carbon storage, including potential future losses from undeveloped deposits. To do this, we compiled a database of 481 nickel mines and undeveloped nickel deposits, classifying them as either magmatic sulfide deposits (n = 276), laterite deposits (n = 173), or another type of deposit (n = 32) (Fig. 1; Data S1). For 18 magmatic sulfide mines and 18 laterite mines, as well as for the major mining fields of Sudbury Basin (sulfide deposits, ~11 mines) and New Caledonia (laterite deposits, ~30 mines), we were able to quantify land use footprints and cumulative nickel production data to 2020 to estimate land transformation factors (m2 of land per tonne of nickel; Data S2). In addition, we estimated the number of years (to 2020) that each nickel mine has produced nickel. We then used a global biomass dataset20 to quantify the average biomass density of unaffected vegetation surrounding the mine sites as a proxy of what was emitted through clearing vegetation for mining (see Methods and Data S2). Finally, we estimated potential biomass losses for developing 335 nickel deposits, under a range of land transformation factors (see Methods and Data S3).

Results
We found large variation in total land disturbance relative to nickel production for historic and current nickel mines (see Table 1, Fig. 2, and Data S2). For magmatic sulfide mines, land transformation factors range from 4 to 398 m2/t nickel (median: 30 m2/t nickel). For laterite mines, land transformation factors range from 7 to 229 m2/t nickel (median: 50 m2/t nickel), indicating that laterite mines generally have larger land disturbance than sulfide mines, although there are sulfide mines with very high land transformation factors.

Importantly, a land transformation factor of 4 m2/t nickel was determined for the Sudbury Basin, and a value of 10 m2/t nickel was determined for Norilsk-Talnakh in Russia, the two largest sources of nickel sulfide ore. The Sudbury Basin deposit has produced ~12 Mt of nickel since the 1880s while the Norilsk-Talnakh deposit has produced ~8.3 Mt of nickel since the 1930s13,14. In addition, a value of 20 m2/t nickel was estimated as an average for all laterite mining on the island of New Caledonia, where mining has occurred since the 1870s and produced ~5.2 Mt of nickel13. The value for all of New Caledonia is likely an underestimate, since it is challenging to map all areas that were historically disturbed. Unsurprisingly, values were lowest for underground nickel mines, such as Eagle Mine in the USA and Voisey’s Bay Mine in Canada.

Importantly, our land transformation factors are, in general, greater than those reported in previous studies used to quantify the environmental impacts of nickel21. The Nickel Institute reports values of 0.76 m2/t nickel for underground mines and 1.8 m2/t nickel for open pit mines22. The Nickel Institute values have been used in various academic studies examining the environmental impacts of nickel mining, for example in Nakajima et al.21. A 2009 report on metal mining environmental impacts for the Ecoinvent Database23, which has been cited in various academic studies on nickel mine land use24,25, contains a value of 1.7 m2/t nickel for open pit mines and assumes that the land footprint of an underground mine is negligible23. Using these literature land transformation factors in environmental impact assessments, including those of carbon emissions, greatly underestimates the environmental impact of nickel mining. For underground mines, the smallest land transformation factor determined in this study (see Table 1 and Data S2) is 4 m2/t nickel or ~5× the land transformation factor provided by the Nickel Institute, while the highest value of 398 m2/t nickel is >500× the Nickel Institute value. For open pit mines, the lowest value of 7 m2/t nickel is ~4× the values provided by the Nickel Institute and in the 2009 Ecoinvent report, while the highest value of 229 m2/t nickel is ~130× these values.

While the latest version of the Ecoinvent Database (https://ecoinvent.org/database/) contains updated assumptions that can generate land transformation factors of ~10–20 m2/t nickel, depending on the parameters selected, academic studies on nickel land footprints still primarily reference the older reports22,23. In addition, there is a lack of primary data from peer-reviewed scientific studies to inform the land transformation factors for nickel mines in the Ecoinvent Database.

In addition to considering land transformation factors, it is important to consider the biomass densities of vegetation located in nickel mining areas. A mine that has a high land transformation factor but is located in a region of the world that has a low biomass density can have a low impact on carbon storage (and vice versa). Determining the exact biomass densities for vegetation that has been lost as a result of global nickel mining is challenging since many nickel operations were brought into operation decades ago before satellite datasets were available and since the amount of deforestation that occurred is often poorly documented. Furthermore, even for mines that opened in recent years, the use of regional biomass datasets is limited by the lack of data harmonization between studies that have used different methods to determine biomass values.

To estimate biomass densities in a way that could facilitate a high-level global comparison of biomass densities for nickel mines, the globally harmonized biomass carbon density dataset of Spawn et al.20 was used. This dataset contains aboveground and belowground biomass carbon density values for the year 2010. Using this dataset, we estimated the biomass carbon densities of vegetation lost from mine land use by determining the average biomass carbon densities in buffers surrounding the mining operations (see Methods). We acknowledge that the actual biomass carbon densities that were lost could be different for many mine sites. However, our estimates provide a suitable high-level dataset to enable analysis of broad trends of biomass carbon losses from nickel mining in different parts of the world. We also believe that our analysis is conservative. For older nickel mining operations, the 2010 dataset is likely underestimating the biomass carbon densities of the vegetation that was lost due to mining since, in many parts of the world, there is a general trend of reduction of biomass densities due to, for example, deforestation due to agricultural activities. In addition, we are not considering the indirect land use change impacts of nickel mining, such as the development of towns and road networks to support mining operations.

Our analysis of biomass densities found that laterite mines, which tend to form in tropical environments, do not always have greater biomass values than magmatic sulfide mines. Total biomass carbon density (sum of aboveground and belowground values) of vegetation cleared for nickel mining ranges from 1.45 to 122.7 t C/ha (median: 36.6 t C/ha) for magmatic sulfide mines and from 2.5 to 141 t C/ha (median: 36.2 t C/ha) for laterite mines (Fig. 2 and Data S2). Some laterite mines are located in palaeo-laterite deposits that have low modern-day biomass densities, for example at Murrin Murrin and Ravensthorpe in Australia. Furthermore, some magmatic sulfide mines can be located in regions with relatively high biomass densities, such as Avebury in Tasmania, Australia, and Santa Rita in Brazil.

Total biomass carbon losses per tonne of nickel produced at mine sites depend on both the biomass densities of cleared vegetation and the land transformation factors. These values are highly variable for both laterite mines, which range from 0.054 to 6.91 t CO2e/t nickel, and magmatic sulfide mines, which range from 0.013 to 12.70 t CO2e/t nickel. Biomass carbon losses per tonne of nickel produced at some mines are similar to the emissions caused by nickel extraction and processing (see Data S2), which range from 0.9 to 10 t CO2e/t nickel for magmatic sulfide mines16 and 25 to 45 t CO2e/t nickel for laterite mines16,26. Our results illustrate the importance of accounting for biomass carbon losses when deciding where and how to source nickel in future.

Examining the characteristics of undeveloped nickel deposits globally can provide insights for strategies that could minimize future biomass carbon emissions from nickel production. The biomass densities of undeveloped nickel deposits differ among countries (Fig. 3C). Vegetation in areas coinciding with laterite deposits was generally more biomass dense (Fig. 3A), due to these deposits being formed in the tropics (for example, in Indonesia, Papua New Guinea, the Philippines, and Brazil). However, examples of undeveloped palaeo-laterite deposits exist in regions with low modern-day biomass carbon densities, such as in Western Australia (Fig. 1). There are also examples of magmatic sulfide deposits associated with vegetation of moderate to high biomass density (for example, in Canada and Scandinavia). Thus, the spatial distribution of biomass density must be considered in nickel sourcing decisions, particularly when supplying low-carbon technologies.

We used a sensitivity analysis to estimate potential biomass carbon storage losses for undeveloped nickel deposits for a feasible range of land transformation factors from 0.1 to 200 m2/t nickel (see Methods and Data S3). The results illustrate the range of expected biomass carbon emissions for each deposit with increasing land transformation factors (Fig. 4). We found emissions would remain low (relative to other reported Scope 1 and Scope 2 emissions) for deposits located in regions with low biomass densities, even when assuming large land transformation factors (Fig. 4). For deposits with greater biomass densities, emissions remain relatively low when utilizing smaller land transformation factors, such as those reported by the Nickel Institute. However, utilizing factors calculated in this study for actual mine sites can result in emissions of up to several tonnes of CO2e per tonne of nickel produced. For undeveloped laterite deposits with the highest biomass carbon densities, such as deposits in Indonesia and Papua New Guinea, biomass carbon losses could be greater than 10 t CO2e per tonne of nickel produced.

Methods
Compilation of global nickel deposits and sizes
Information on the locations, sizes, and characteristics of nickel deposits was compiled from the S&P Global Market Intelligence Metals & Mining Database (https://www.spglobal.com/marketintelligence/en/campaigns/metals-mining), which is one of the most comprehensive mineral deposit databases available and is commonly used by researchers to assess global trends in mining and mineral exploration44–46. The database was queried for nickel deposits with reserves and resources listed as of December 2020. Note that the terms “reserves” and “resources” are used in the mining industry to define the confidence that a commodity, such as nickel, can be brought into production from a mineral deposit (for example, see the descriptions in the Australasian Code for Reporting of Exploration Results, Mineral Resources, and Ore Reserves (JORC) website here: http://www.jorc.org/). For reserves, there is higher confidence that a commodity has value and can be economically, technically, and legally extracted. For resources, a commodity is potentially valuable, and there are reasonable prospects for eventual economic extraction. A total of 481 entries were returned by this query. Note that an additional 1829 nickel projects are listed in the database. However, most of these are early-stage exploration projects that have not yet proven the presence of economic nickel concentration through drilling and sample assay. These additional projects were not considered as part of this study.

The S&P database includes metadata under “Ore Body Type” that can be used to classify nickel deposits as either a magmatic sulfide deposit, a laterite deposit, or another type of nickel deposit. Classification of the deposits from the metadata requires geological knowledge. For example, ore types listed as “Komatiitic Magmatic”, “Layered Mafic-Ultramafic Intrusion”, and “Podiform (Ophiolite-Peridotite)” were all classified as magmatic sulfide deposits, which required an understanding that these types of rock will contain magmatic sulfide mineralization.

Only 236 of the 481 deposits had metadata under “Ore Body Type”. The remaining 245 deposits were classified based on information located elsewhere in the S&P database (for example, from a description of metallurgical processing under the “Comments” field for the deposit), from descriptions of the deposit in scientific literature, or from mining company websites or press releases (see Data S1). For a few deposits, there was not enough information to make a classification, and the deposit type was listed as “Unknown”. In addition, the 236 deposits with ore type meta-data in the S&P database were checked against other information. While most of the S&P ore type classifications were accurate, a few errors were noted and corrected.

Laterite deposits were straightforward to classify. However, magmatic sulfide deposits were more complex to classify since in igneous rocks, nickel can be present not only in magmatic sulfide minerals but also in hydrothermal minerals that form through alteration of the rocks. In addition, several large nickel deposits in the Sudbury region of Canada were formed by crustal melting caused by a large meteorite impact (the S&P database classifies these deposits as “Astrobleme” type). In this study, all nickel deposits present in mafic to ultramafic igneous rocks were classified as “Magmatic Sulfide” deposits, including those with nickel mineralization caused by hydrothermal alteration or a meteorite impact. While this is a simplification, it is justified because the nature of nickel deposits in igneous rocks is broadly similar and important to contrast with the different nature of nickel deposits in soil laterites.

In addition, information on production was compiled for as many nickel mines as possible. Information on cumulative production through the end of 2020 was sourced from the S&P Database, when available, and also from Mudd and Jowitt14 and Barnes et al.47. The number of years that a mine produced nickel (to end 2020) was also estimated, using the S&P Database and information obtained from company websites.

Determination of nickel mine land disturbance areas
To delineate the land occupied by nickel mining areas, we manually delineated mine areas from satellite imagery per methods previously established and described at length in Maus et al.44, Tang et al.48, Werner et al.49 and Tang and Werner50. In the interest of clarity, we briefly expand upon these methods here. Using the 2020 Google imagery available in Google Earth Engine and Sentinel-2 (grain size 10m), we delineated various features, including mine waste rock dumps, heap leach pads, tailings dams, pits, and milling infrastructure areas. We generally excluded features such as roads and remote airports unless such features were clearly in place due to the development of a mine. In cases where mines operated close to built-up areas, this process entailed distinguishing milling infrastructure from surrounding buildings. Revegetated areas, backfilled pits, or pits subsequently employed for non-mining-related water storage were identified using historical imagery as well as public reports. Such areas were not included in the land transformation factors. Beyond the mine area polygon data available from past studies44,48–50, an additional 31 sites were newly mapped or updated exclusively for this study, as well as updating delineation areas from those studies, based on more recent imagery. Additional processes of image validation are described in Werner et al.36 and Tang and Werner50. The land disturbance areas used in this study are provided in Data S2. Polygons of nickel mining areas are provided in a Google Earth file in Data S4.

Biomass carbon density and carbon loss calculations
To calculate biomass carbon densities in the vicinity of nickel mines and undeveloped nickel deposits, the global database of Spawn et al.20 was used. This database contains aboveground and belowground carbon biomass values that were determined from 2010 satellite imagery coupled with modeling techniques used to infer belowground carbon values. The values have been harmonized so that they are consistent from one region to another, which is the primary reason why this dataset was selected over other carbon biomass datasets. The use of the Spawn et al.20 dataset is appropriate for this study, which aimed to develop a global view and comparison of biomass losses. To assess biomass losses more accurately at specific nickel mine sites, detailed analysis of local records, including historic satellite data, should be conducted. The biomass values in the Spawn et al.20 dataset include all carbon stored in living plant tissue, such as stems, bark, and leaves, as well as in roots (the underground component). The values do not include organic soil matter, nor non-living plant tissues, such as leaves on the forest floor. The spatial resolution of the dataset is 300m.

For nickel mine sites, it was assumed that all biomass carbon was lost from the mining area polygons. To estimate what the biomass carbon densities were before mining, a buffer polygon of 5km was drawn around the mining area, and the average carbon densities within this polygon were calculated using the spatial statistics toolkit in QGIS, a mapping and spatial analysis software program (https://www.qgis.org/). If other mining areas fell within the buffer area, the areas for these were subtracted from the buffer polygon area before average biomass carbon densities were calculated. In addition, carbon values over bodies of water were converted from zero values to null values, so that they did not bring down the average carbon density values for biomass in a region. To check the appropriateness of using a 5km buffer, a sensitivity analysis was conducted with 1km and 10km buffers. As shown in Figs. S2 and S3, comparison of the 1km and 5km and the 10km and 5km buffers resulted in high R-squared values in all cases. The sensitivity analysis indicates that the carbon density values are not highly sensitive to the size of the buffer. Therefore, it was appropriate to use the results from the 5km buffers in this study.

After the carbon densities were determined, biomass carbon losses were estimated by multiplying the average carbon densities by the area from the mining area polygons. The carbon losses estimated in this study are provided in Data S2.

For undeveloped nickel deposits, average carbon densities were calculated for 5km radius circles around the S&P coordinates. A sensitivity analysis was conducted using 1km radius and 10km radius circles. As shown in Figs. S4 and S5, comparison of the 1km and 5km and the 10km and 5km buffers resulted in high R-squared values in all cases. The sensitivity analysis indicates that the carbon density values are not highly sensitive to the size of the buffer. Therefore, it was appropriate to use the results from the 5km buffer circles in this study. Potential biomass carbon losses for undeveloped deposits were estimated using an assumed land transformation factor (m2 of land disturbance per tonne of nickel produced). A sensitivity analysis was conducted for a range of land transformation factors, from 0.1 to 200 m2 per tonne of nickel produced. The results of this analysis are provided in Data S3.

Biomass calculation correction approach. We employed a multilevel weighted average approach to address the uncertainties inherent in both aboveground and belowground biomass potential loss estimations by incorporating both pixel-level and land cover-level coefficients of variation (CVs) into our calculations (Fig. S6). This approach addresses the variability in biomass measurements across different land cover types. The significance of this approach lies in its capacity to provide corrected biomass loss estimations, accounting for uncertainties at both the pixel and land cover levels within the buffer polygons. First, we transformed the uncertainty layers of biomass into CV values. We then calculated the pixel-level weights by taking the inverse of the squared pixel-level CVs, assigning greater importance to measurements with lower uncertainties. Next, we derived land cover-level weights from the inverse of the squared mean pixel-level CVs for each land cover (LC) class (using the ESA CCI Land cover 2010), introducing a nuanced weighting scheme that mirrors the average certainty of measurements within those classes. Following this, we calculated the weighted pixel-level mean (Eq. 1) and then proceeded with the weighted land cover-level mean calculation using the weighted pixel-level mean (Eq. 2). The resulting weighted average calculations at both the pixel and land cover levels yielded an overall weighted mean biomass value, providing a more robust estimation that captures the heterogeneity in biomass dynamics across diverse ecological settings where nickel mining is located. We also provide the original calculation of biomass loss without accounting for uncertainties, for comparison (Supplementary Figs. S7–S10 and Data S2–S3 under sheet labeled as ‘uncorrected’).

(Eq. 1) M_weighted,j = (sum_{i=1}^{n_j} W_{i,j} * Mean_{i,j}) / (sum_{i=1}^{n_j} W_{i,j})

Where:
W_{i,j} is the pixel-level weight
n_j is the number of pixels within land cover class j.
Mean_{i,j} is the mean biomass value for the ith pixel within the land cover class j.

(Eq. 2) M_weighted = (sum_{j=1}^{m} W_j * M_weighted,j) / (sum_{j=1}^{m} W_j)

m is total number of land cover classes within the mining polygon, W_j is the land cover-level weights, M_weighted,j is the Weighted Pixel-Level Mean.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The submitted manuscript quantifies the biomass carbon emissions from nickel mining around the globe and discusses the implications for climate action. Compiled datasets for 481 nickel mines and undeveloped deposits show that the impact of nickel mining might be greater than previously estimated. Understanding and quantifying the emissions from nickel mining is important for effective climate action. However, after careful reading, I think that the methodology has some flaws that are detailed below.\nGeneral comments:\n• Quantifying the impact of nickel mining in terms of carbon emissions needs to be done using rigorous statistical analysis. It was surprising to notice that no uncertainty estimation was done, while in reality, a combination of multiple uncertainties should have been performed. For example, is the biomass map used having an uncertainty layer? How was that uncertainty of carbon estimation performed? Estimating biomass carbon globally can have high uncertainty in many regions of the world where calibration data were not available. This uncertainty can reach or go beyond the actual value of carbon estimated, which would propagate into your results.\n• A series of decisions were made when constructing the database and interpreting imagery to delineate nickel mining. How do you validate the subjective choices along the workflow, and how do these impact the final results? How was visual interpretation done and validated? It is not clear how and what satellite images were used.\n• I think that by better integrating the age of the mine into the analysis will better reflect the reality and provide a better quantification of mining impacts. The large values of 3 to 500 times more emissions than previously reported are highly dependent on the age of the mine. Treating mines in different age classes should provide a better overview of the mining impact.\n• There seems to be a temporal mismatch between the mining database (up to 2020) and the biomass map (2010). When you overlap the two, you might get misleading results. How does this affect your analysis?", "reviewer_2": "The paper “Biomass carbon emissions from nickel mining have significant implications for climate action” reported and analyzed data from nickel mines to show the environmental impact of primary nickel flows, focusing on emissions from biomass loss. Interestingly, the paper provides a dataset collection for 481 nickel mines and undeveloped nickel deposits. The paper highlights exacerbating environmental issues caused by the growth of nickel mining. I would like to thank the author(s), as the paper addresses an important and timely topic. However, I have several major issues with the paper as presented.\n1. The first issue concerns the entire design of the analysis: Understanding the carbon balance in global mining is very important. Among several critical materials, you focused on Nickel. This must be strongly justified as to why other elements required for low-carbon technologies (e.g., battery technologies that you mentioned) are excluded from your analysis. Moreover, the analysis does not clarify which type of technologies (e.g., you mentioned one of the applications of nickel in batteries, but which type of battery?) have been considered in calculations. How does the demand for the given technologies influence the growth of global nickel production from mining?\n2. The second issue concerns the environmental assessment: life cycle assessment of materials is particularly sensitive to methodological differences, e.g., variations in allocation procedures can significantly change the environmental impact per kg of material. The authors well clarified the geology route for analysis and the database contains aboveground and belowground carbon biomass values. However, it is unclear how this study considers the nickel production analysis based on processing routes. Please provide this info in the section Methods.\n3. The paper focuses on identifying the problem. Author(s) provided a general discussion on strategies for reducing biomass losses. However, it is very important to provide some science-based solutions for sustainable practices directly connected to the obtained results. How the findings of this paper bring us awareness to provide solution(s) addressing the identified problem- growing biomass carbon emissions from nickel mining in this case. Suggest some strategies to enable future mining-based solutions for climate change mitigation.\nSome minor comments:\n1. It should be clarified how authors calculated carbon emissions to carbon equivalent.\n2. In the introduction the author(s) stated, “In addition, mining directly and indirectly causes land use changes, and the associated biomass losses may result in further carbon emissions”. How are direct and indirect carbon emissions calculated and considered in this study?\n3. You provided insights into patterns and correlations in the data by sensitivity analysis and the data analysis mainly presented in Supplementary Materials. However the analysis should be developed to test hypotheses or predict future outcomes. Why the estimation of potential future losses from undeveloped deposits is based on the year 2020? How this, as you stated, “Examining the characteristics of undeveloped nickel deposits globally can provide insights into strategies that could minimize future biomass carbon emissions from nickel production.”, can be extracted from your result? Addressing this question helps us determine the significance of the results or generalize the findings to other mines or environmental issues. Also, it should clarify the results without and with the growth rate of mining production, which is influenced by the demand for technology propagation.\n4. Discuss whether the proposed approach and analysis could be extended to other critical materials mining activities. Has a similar global assessment for other minerals been performed previously?"}, "author_response": "Response to Reviewers:\nFor the manuscript: Mervine et al. “Biomass carbon emissions from nickel mining have significant implicafions for climate acfion”.\nReviewer #1 (Remarks to the Author):\nThe submifted manuscript quanfifies the biomass carbon emissions from nickel mining around the globe and discusses the implicafions for climate acfion. Compiled datasets for 481 nickel mines and undeveloped deposits show that the impact of nickel mining might be greater than previously esfimated. Understanding and quanfifying the emissions from nickel mining is important for effecfive climate acfion. However, after careful reading, I think that the methodology has some flaws that are detailed below.\nRESPONSE: We appreciate the comments from Reviewer #1, which have helped us to improve the manuscript. Please see our responses to each point below.\nGeneral comments:\n1. Quanfifying the impact of nickel mining in terms of carbon emissions needs to be done using rigorous stafisfical analysis. It was surprising to nofice that no uncertainty esfimafion was done, while in reality, a combinafion of mulfiple uncertainfies should have been performed. For example, is the biomass map used having an uncertainty layer? How was that uncertainty of carbon esfimafion performed? Esfimafing biomass carbon globally can have high uncertainty in many regions of the world where calibrafion data were not available. This uncertainty can reach or go beyond the actual value of carbon esfimated, which would propagate into your results.\nRESPONSE: Uncertainty analysis has now been conducted and incorporated into the manuscript and supplementary informafion, using the uncertainty layers for both aboveground and belowground biomass from the Spawn et al. (2020) global biomass dataset. We add a descripfion of this process to our Methods (page 17, line 13) and add Figure S6 to illustrate the workflow to incorporate uncertainfies in the biomass layer into our results. We have updated all results reported accordingly. All major conclusions related to esfimafing carbon biomass emissions, including differences among deposit types, differences among countries, and likely future effects of developing new nickel deposits, remain true conducfing the uncertainty analysis.\nThere are also several other uncertainfies that were unable to be incorporated into our analysis. These includechanges in biomass densifies in the buffer areas since the older mines were opened and a lack of incorporafion of indirect biomass losses from surrounding infrastructure (such as the development of road networks and supporfing towns), as we describe further in response to Review 1’s comment 4. To address these factors, we now explain how these factors result in our results being conservafive (page 8, line 39).\n2. A series of decisions were made when construcfing the database and interprefing imagery to delineate nickel mining. How do you validate the subjecfive choices along the workflow, and how do these impact the final results? How was visual interpretafion done and validated? It is not clear how and what satellite images were used.\nRESPONSE: We have clarified our satellite image analysis in the Methods (page 15, line 37), nofing that we ufilized previously published methods (Werner et al. (2020) and Tang and Werner (2023)), where such validafion and visual interpretafion was rigorously undertaken. We have also slightly modified our calculafions to consistently use 2020 GoogleEarth imagery, as described in the Methods (page 15, line 41). This has resulted in minor changes to our land footprint calculafions for some mine sites, for example as listed in Table 1.\n3. I think that by befter integrafing the age of the mine into the analysis will befter reflect the reality and provide a befter quanfificafion of mining impacts. The large values of 3 to 500 fimes more emissions than previously reported are highly dependent on the age of the mine. Treafing mines in different age classes should provide a befter overview of the mining impact.\nRESPONSE: We thank Reviewer 1 for this suggesfion. Age of the mine is an important factor, although more important was the number of years the mine has been (or was) in operafion and producing nickel. We have added the number of years of operafion of the mine (to 2020) into Table 1 and have also added discussion on this topic to the Discussion secfion of the manuscript (page 12, line 26).\nThe text we have added is:\nAn important parameter that can influence the land transformafion factor is the number of years that a nickel mine has been in operafion (see Table 1 and Data S2). In theory, the land transformafion factor for a mine can reduce over fime as the mine produces more nickel. Indeed, some of the highest land transformafion factors that we determined are for mines that operated for a short fime, such as Bucko Lake in Canada (398 m2/t nickel, in operafion for 3 years) and Avebury in Australia (282 m2/t nickel, in operafion for 1 year). By the fime these operafions closed, the mine footprints had been established but cumulafive nickel producfion remained lower than planned. In addifion, some recently opened mines, such as Nova-Bollinger in Australia (opened in 2017) and Weda Bay in Indonesia (opened in 2019), may have land transformafion factors that decrease with fime as they confinue nickel producfion. These examples illustrate that there is risk in developing new nickel deposits, especially marginal ones. If nickel mines do not achieve full producfion, then their environmental impact per tonne of nickel producfion remains high. Ideally, governments should prevent the opening of new, marginal nickel deposits, where large biomass losses could be incurred relafive to nickel produced.\n4. There seems to be a temporal mismatch between the mining database (up to 2020) and the biomass map (2010). When you overlap the two, you might get misleading results. How does this affect your analysis?\nRESPONSE: Our analysis aimed to determine an average biomass value within the landscape surrounding each nickel mine or undeveloped deposit. We did not aftempt to quanfify the actual biomass losses due to the mining footprint and agree that if we had done this we would have needed to use a biomass layer that predated the establishment of the mine. To address this comment, we have clarified our methods (page 8, line 30), explained the assumpfion that the vegetafion cleared for mine expansion had a similar biomass value to the vegetafion surrounding the mine (page 8, line 35), and explained that any errors with this assumpfion (e.g. because surrounding vegetafion had been cleared or modified by other land use pressures) would likely underesfimate our reported carbon biomass losses and thus render our results conservafive (page 8, line 39).\nReviewer #2 (Remarks to the Author):\nThe paper “Biomass carbon emissions from nickel mining have significant implicafions for climate acfion” reported and analyzed data from nickel mines to show the environmental impact of primary nickel flows, focusing on emissions from biomass loss. Interesfingly, the paper provides a dataset collecfion for 481 nickel mines and undeveloped nickel deposits. The paper highlights exacerbafing environmental issues caused by the growth of nickel mining. I would like to thank the author(s), as the paper addresses an important and fimely topic. However, I have several major issues with the paper as presented.\nRESPONSE: We appreciate the comments from Reviewer #2, which have helped us to improve the manuscript. Please see our responses to each point below.\n1. The first issue concerns the enfire design of the analysis: Understanding the carbon balance in global mining is very important. Among several crifical materials, you focused on Nickel. This must be strongly jusfified as to why other elements required for low-carbon technologies (e.g., baftery technologies that you menfioned) are excluded from your analysis. Moreover, the analysis does not clarify which type of technologies (e.g., you menfioned one of the applicafions of nickel in bafteries, but which type of baftery?) have been considered in calculafions. How does the demand for the given technologies influence the growth of global nickel producfion from mining?\nRESPONSE: We agree that nickel is one of many minerals important for ana energy transifion (page 2, line 14).To jusfify our focus on nickel, we have now (1) described its relafive importance in energy transifions (page 2, line 19), (2) explained the fact that emerging sources of laterite nickel resources are often mined from biomass dense tropical forests and thus future biomass losses are potenfially significant (page 2, line 39), (3) explained that essenfial data needed to complete our analysis (i.e. cumulafive producfion data) is scarce for other minerals (page 14, line 14). We have also added in a descripfion of some key low-carbon technologies that require nickel (page 2, line 24), while also clarifying that our analysis did not explore scenarios of technology opfions on nickel demand and thus biomass losses. Indeed, nickel demand will ulfimately depend on the rate of energy transifions, the technology choices made, and the mineral demands of those choices, as described in the World Bank report cited (page 2, line 17).\n2. The second issue concerns the environmental assessment: life cycle assessment of materials is parficularly sensifive to methodological differences, e.g., variafions in allocafion procedures can significantly change the environmental impact per kg of material. The authors well clarified the geology route for analysis and the database contains aboveground and belowground carbon biomass values. However, it is unclear how this study considers the nickel producfion analysis based on processing routes. Please provide this info in the secfion Methods.\nRESPONSE: Our analysis does not include a Life Cycle Assessment (LCA) of nickel or nickel consuming products. The primary purpose of our research was to assess if carbon losses from biomass destrucfion are significant enough that they should be included in carbon accounfing for mine sites, and provide improved esfimates of land transformafion factors and average biomass values that could be used in LCA assessments of nickel products. Indeed, we find that our land transformafion factors are up to 500 fimes larger than those currently used in LCAs (page 4, line 15).\n3. The paper focuses on idenfifying the problem. Author(s) provided a general discussion on strategies for reducing biomass losses. However, it is very important to provide some science-based solufions for sustainable pracfices directly connected to the obtained results. How the findings of this paper bring us awareness to provide solufion(s) addressing the idenfified problem- growing biomass carbon emissions from nickel mining in this case. Suggest some strategies to enable future mining-based solufions for climate change mifigafion.\nRESPONSE: Our Discussion secfion makes several recommendafions to improve the carbon biomass losses of nickel mining. This include: (1) for mine sites to quanfify and disclose biomass losses as part of their scope 1 carbon emissions, which would promote transparency and trigger societal, government, and investor pressure on companies to reduce these emissions over fime (page 14, line 9)(2) for governments to prevent opening up of new marginal nickel deposits, where large biomass losses would be incurred relafive to nickel produced (page 12, line 38), and (3) for nickel sourcing decisions to incorporate informafion on the carbon biomass values (page 14, line 23).\nSome minor comments:\n4. It should be clarified how authors calculated carbon emissions to carbon equivalent.\nRESPONSE: To convert from C to CO e, we mulfiplied by 3.67, which is standard pracfice and based on the rafio of the molecular masses. We have added a note on this to the methods secfion (page 16, line 28).\n5. In the introducfion the author(s) stated, “In addifion, mining directly and indirectly causes land use changes, and the associated biomass losses may result in further carbon emissions”. How are direct and indirect carbon emissions calculated and considered in this study?\nRESPONSE: We have clarified that our study focuses on direct land use changes and added that these indirect land use changes would likely further increase the amount of carbon biomass lost due to nickel mining (page 13, line 20).\n6. You provided insights into pafterns and correlafions in the data by sensifivity analysis and the data analysis mainly presented in Supplementary Materials. However the analysis should be developed to test hypotheses or predict future outcomes. Why the esfimafion of potenfial future losses from undeveloped deposits is based on the year 2020? How this, as you stated, “Examining the characterisfics of undeveloped nickel deposits globally can provide insights into strategies that could minimize future biomass carbon emissions from nickel producfion.”, can be extracted from your result? Addressing this quesfion helps us determine the significance of the results or generalize the findings to other mines or environmental issues. Also, it should clarify the results without and with the growth rate of mining producfion, which is influenced by the demand for technology propagafion.\nRESPONSE: We thank the reviewer for this useful direcfion for future research though, and we pick up on this as an important avenue for future explorafion in the discussion/conclusions, for example on page 13, line 9. The purpose of the research was not to predict future emissions from nickel producfion but rather to point out a gap in current carbon accounfing (not including biomass carbon emissions) and to emphasize the point that biomass densifies around nickel mines and deposits is highly variable, which leads to highly variable carbon emissions from biomass losses (page 2, line 23). We agree that it is important for other researchers to incorporate biomass loss esfimates into other studies focused on predicfing environmental impacts from nickel use considering different technology scenarios, since the analyses to date would have underesfimated this environmental impact.\n7. Discuss whether the proposed approach and analysis could be extended to other crifical materials mining acfivifies. Has a similar global assessment for other minerals been performed previously?\nRESPONSE: Yes, the proposed approach and analysis could be extended to other crifical materials, such as copper and aluminium. To date, no similar global assessment has been done for other minerals, which makes our study novel. We have added addifional text to the manuscript discussion suggesfing that other researchers conduct similar analysis for other minerals (page 14, line 14)."}, {"round": 2, "reviewer_comments": {"reviewer_2": "Generally, I am happy with the corrections and modifications provided in the revised manuscript according to previous comments.\n1. Please as a minor comment consider to modify the sentence (page 20, line 14) “Better data on global mining production is essential for….”. Perhaps to rewrite “A comprehensive and diverse dataset on global mining production is still missing for …\n2. I see that your results are not sensitive to demand of nickel for energy technology options as you answered in the comments. In page 2, line 24, you clarified the important role of nickel in energy transition. But your statistical example is limited to battery use (…approximately 6% of global nickel production …). Please modify this part generally by providing the demand growth of nickel in other sectors to avoid misunderstanding your study focuses on battery demand. For example, IEA provided the estimations of nickel demand by sectors and scenarios between 2020 and 2040.", "reviewer_3": "The manuscript presents noteworthy results by highlighting the significant, yet often overlooked, biomass carbon emissions from nickel mining. This study quantifies the land transformation factors and biomass carbon losses across various nickel mining sites, demonstrating that the environmental impacts are substantially greater than previously reported. This work is significant to the field of environmental science and related fields, as it provides new insights into the carbon footprint of nickel mining, which is critical for climate action strategies. Compared to established literature, this study offers a more comprehensive and nuanced understanding of the environmental impacts of nickel mining, filling gaps in existing research. The conclusions are well-supported by the data presented, but some areas, particularly the categorization of emissions, need revision for accuracy. The methodology is generally sound and meets expected standards in the field, with detailed methods provided for reproducibility. However, some improvements in categorizing emissions and integrating established guidelines are necessary for clarity and accuracy.\n1. Scope 1 and Scope 2 Discussions\nThe discussions on Scope 1 and Scope 2 emissions are not aligned with standard carbon accounting protocols. Specifically, emissions from land clearing should not be categorized under Scope 1 or Scope 2, which are typically reserved for direct emissions from owned or controlled sources and indirect emissions from purchased energy, respectively. Including land clearing under these scopes misrepresents the actual emission sources.\nThe authors should revisit the carbon accounting guidelines to correctly categorize emissions from land clearing. The focus should shift to Scope 3 emissions, which cover indirect emissions, including those from land use changes. This adjustment will provide a more accurate representation of the carbon footprint associated with nickel mining.\n2. Carbon Emissions in National and Global Targets\nThe suggestion to include carbon emissions from nickel mining in national and global targets is noted but lacks novelty. The linkage between mining and deforestation is well-documented, and existing forestry carbon projects already address these issues as part of their efforts to reduce deforestation and associated emissions.\nhttps://iopscience.iop.org/article/10.1088/1748-9326/acd408\nhttps://www.forestcarbonpartnership.org/sites/fcp/files/DriversOfDeforestation.pdf_N_S.pdf\nThe authors should elaborate on how their findings can enhance current frameworks and contribute to new policy implications. A detailed analysis of the potential integration of these findings into existing national and global carbon targets would strengthen the manuscript's contribution to the field.\n3. Life Cycle Assessment (LCA) vs. Supply Chain Assessment\nWhile Life Cycle Assessment (LCA) offers a comprehensive view of environmental impacts, its application to nickel mining may not provide the necessary granularity for specific supply chain emissions analysis. LCA may overlook specific stages of the supply chain where significant emissions occur.\nIf you wish, the authors should conduct a detailed Supply Chain Assessment focusing on each stage of the nickel mining process. Following this, an LCA can complement the findings by offering a holistic view of environmental impacts, thereby providing both detailed and broad perspectives on the issue.\n4. IPCC Guidelines and Carbon Stock Estimation\nThe manuscript does not mention the IPCC Guidelines or the tiered approach (Tier 1, Tier 2, Tier 3) for estimating carbon stocks in forests. This omission is critical as the IPCC Guidelines provide standardized methods for carbon stock estimation, which are essential for ensuring consistency and accuracy in carbon accounting.\nThe authors should integrate IPCC Guidelines into their methodology to provide a standardized approach to carbon stock estimation. Including these guidelines will enhance the robustness and credibility of their carbon emission estimates.\n5. Additional but Specific Comments\nPage 2, Line 5: The language should be officially acceptable. Revise to: \"Commitments to boost low-carbon energy production and end deforestation by 2030 were two key outcomes of the COP28 of the UNFCCC.\"\nPage 13: Clarify the term \"offsetting\" to explicitly refer to \"carbon offsetting\" for better understanding."}, "author_response": "[OFFICIAL]\nResponse to Reviewers:\nFor the manuscript: Mervine et al. “Biomass carbon emissions from nickel mining have significant implicafions for climate acfion”.\nREVIEWER COMMENTS:\nReviewer #2 (Remarks to the Author):\nGenerally, I am happy with the correcfions and modificafions provided in the revised manuscript according to previous comments.\nRESPONSE: We thank the reviewer for reviewing the manuscript a second fime.\n1. Please as a minor comment consider to modify the sentence (page 20, line 14) “Befter data on global mining producfion is essenfial for….”. Perhaps to rewrite “A comprehensive and diverse dataset on global mining producfion is sfill missing for …”\nRESPONSE This change has been made to the text.\n2. I see that your results are not sensifive to demand of nickel for energy technology opfions as you answered in the comments. In page 2, line 24, you clarified the important role of nickel in energy transifion. But your stafisfical example is limited to baftery use (…approximately 6% of global nickel producfion …). Please modify this part generally by providing the demand growth of nickel in other sectors to avoid misunderstanding your study focuses on baftery demand. For example, IEA provided the esfimafions of nickel demand by sectors and scenarios between 2020 and 2040.\nRESPONSE: We have updated the manuscript on page 2, line 24:\n“Here, we focus on estimating biomass carbon emissions caused by clearing vegetation for nickel mining. Nickel is a silvery-white metal that is hard, ductile, and resistant to oxidation and corrosion. Today, nickel is primarily used in stainless steel and other alloys, as well as in specialized corrosion-resistant plating (10, 11). Nickel is also used to produce numerous low-carbon energy production and storage technologies, including batteries and infrastructure for geothermal, wind, solar photovoltaic, hydroelectric, and nuclear energy. Currently, approximately 15% of global nickel production is used in clean energy applications (11).\n(12). As battery use increases in future to aid the transition to a low-carbon economy, an increasing amount of nickel supply will be required. For example, one study estimates that the amount of nickel required for electric vehicles will reach 1.1 million tons (Mt) per year by 2030, which is approximately one-third of global annual production in 2023 (13). Climate action, among other factors, is projected to see future nickel demand from energy technologies soar to 2.27 Mt of nickel per year by 2050(9, 10). Recent analysis by the International Energy Agency predicts that cleantech demand for nickel will increase from 0.48 Mt (in 2023) to 3.4 Mt by 2040 (11). Demand for other nickel use is also predicted to increase, from 2.5 Mt (in 2023) to 2.9 Mt by 2040 (11). Since increasing end-of-life nickel\n[OFFICIAL]\nrecycling rates to 100% by 2050 would only increase recycled content rates in nickel products to 58%, continued supplies of nickel will need to be provided through mining (9).”\nReviewer #3 (Remarks to the Author):\nThe manuscript presents noteworthy results by highlighting the significant, yet often overlooked, biomass carbon emissions from nickel mining. This study quantifies the land transformation factors and biomass carbon losses across various nickel mining sites, demonstrating that the environmental impacts are substantially greater than previously reported. This work is significant to the field of environmental science and related fields, as it provides new insights into the carbon footprint of nickel mining, which is critical for climate action strategies. Compared to established literature, this study offers a more comprehensive and nuanced understanding of the environmental impacts of nickel mining, filling gaps in existing research. The conclusions are well-supported by the data presented, but some areas, particularly the categorization of emissions, need revision for accuracy. The methodology is generally sound and meets expected standards in the field, with detailed methods provided for reproducibility. However, some improvements in categorizing emissions and integrating established guidelines are necessary for clarity and accuracy.\n1. Scope 1 and Scope 2 Discussions\nThe discussions on Scope 1 and Scope 2 emissions are not aligned with standard carbon accounting protocols. Specifically, emissions from land clearing should not be categorized under Scope 1 or Scope 2, which are typically reserved for direct emissions from owned or controlled sources and indirect emissions from purchased energy, respectively. Including land clearing under these scopes misrepresents the actual emission sources.\nThe authors should revisit the carbon accounting guidelines to correctly categorize emissions from land clearing. The focus should shift to Scope 3 emissions, which cover indirect emissions, including those from land use changes. This adjustment will provide a more accurate representation of the carbon footprint associated with nickel mining.\nRESPONSE: We respecffully disagree with the reviewer that emissions from land use changes from nickel mining should be classified as Scope 3 emissions. While it has been historically true that land use change emissions from mining have often been categorized as Scope 3 and have been overlooked by mining companies in their Scope 1 reporfing, the most recent guidance released by the Greenhouse Gas Protocol states that emissions from land use change occurring on lands that a company owns or controls should be classified as Scope 1 emissions (see the document “Land Sector and Removals Guidance”, available online here:hftps://ghgprotocol.org/sites/default/files/2022-12/Land-Sector-and-Removals-Guidance-Pilot-Tesfing-and-Review-Draft-Part-1.pdf, for example the definifions in the table provided on page 108). While this new guidance document from the GHG Protocol is currently in the draft and tesfing phase, it is expected that in the future it will become standard pracfice for companies to report emissions from land use changes on lands they own or control as Scope 1 emissions.\nWe have added text to the discussion secfion to more explicitly address this topic (page 14, line 3):\n[OFFICIAL]\n“Our study has illustrated the need to include the impact of biomass carbon storage losses in Scope 1 emissions calculations for mine sites. Historically, emissions from land use changes have often been categorized as Scope 3 emissions and reported in national inventories rather than by individual companies, with the exception of certain industries, such as forestry (28). Currently, mining companies generally do not include emissions caused by biomass losses in their Scope 1 reporting, and these emissions are also generally not captured by Scope 3 reporting. For example, a Scope 3 reporting guidance document recently published by the International Council on Metals and Mining provides detailed guidance on best practices for mining companies to report on emissions from upstream and downstream supply chain activities but does not mention land use change emissions reporting (29).\nRecognizing that there is a lack of guidance regarding how companies should report emissions from land use changes and that companies commonly do not report these emissions, the Greenhouse Gas Protocol, which publishes one of the most widely used corporate reporting standards for carbon emissions, recently released a draft “Land Sector and Removals Guidance” document (30). In this new guidance document, the GHG Protocol dictates that companies should report emissions from land use change occurring on lands a company owns or controls as Scope 1 emissions. Emissions from land use changes associated with energy purchase should be reported as Scope 2 emissions, while emissions from land use changes within the company’s value chain (upstream and downstream supply chain) should be reported as Scope 3 emissions. Furthermore, the GHG guidance document specifically mentions mining as a “land-intensive activity” where emissions from land use changes are essential to include in Scope 1 reporting since these emissions can be significant (30). The new GHG Protocol guidance document is expected to come into effect in 2025, at which time all mining companies will be expected to report Scope 1 emissions from land use changes at their operations.”\n2. Carbon Emissions in Nafional and Global Targets\nThe suggesfion to include carbon emissions from nickel mining in nafional and global targets is noted but lacks novelty. The linkage between mining and deforestafion is well-documented, and exisfing forestry carbon projects already address these issues as part of their efforts to reduce deforestafion and associated emissions.\nhftps://iopscience.iop.org/arficle/10.1088/1748-9326/acd408\nhftps://www.forestcarbonpartnership.org/sites/fcp/files/DriversOfDeforestafion.pdf_N_S.pdf\nThe authors should elaborate on how their findings can enhance current frameworks and contribute to new policy implicafions. A detailed analysis of the potenfial integrafion of these findings into exisfing nafional and global carbon targets would strengthen the manuscript's contribufion to the field.\nRESPONSE: We have added addifional context to the discussion secfion (starfing on page 14, line 3) that outlines how nickel mining companies should collect befter reporfing data for biomass losses and that newly-released draft guidance from the GHG Protocol, a carbon accounfing standard commonly used by companies, requires reporfing on these emissions at a company (rather than just at a nafional or global)\n[OFFICIAL]\nscale. The draft guidance is expected to come into effect in 2025, which means that nickel companies (and companies using nickel in their supply chain) will be expected to account for these emissions as part of their carbon accounfing. Currently, most nickel mine sites have poor data records to safisfy these new reporfing requirements, and we point out that mines urgently need to obtain and report more robust datasets.\n3. Life Cycle Assessment (LCA) vs. Supply Chain Assessment\nWhile Life Cycle Assessment (LCA) offers a comprehensive view of environmental impacts, its applicafion to nickel mining may not provide the necessary granularity for specific supply chain emissions analysis. LCA may overlook specific stages of the supply chain where significant emissions occur.\nIf you wish, the authors should conduct a detailed Supply Chain Assessment focusing on each stage of the nickel mining process. Following this, an LCA can complement the findings by offering a holisfic view of environmental impacts, thereby providing both detailed and broad perspecfives on the issue.\nRESPONSE: Conducfing Supply Chain Assessment is beyond the scope of this study, which focused on idenfifying the significance of biomass carbon emissions in nickel mining. We agree that including biomass carbon emissions are important to include in both LCA and Supply Chain Assessment. We have added specific comment on this on page 15, line 34:\n“Biomass carbon storage losses should be included in Life Cycle Assessments and considered in Supply Chain Assessments for nickel products (39, 40). Due to the high variability of biomass carbon storage losses for different nickel mines, a single value should not be used to represent carbon storage loss per tonne of nickel. Instead, there should be values for individual mine sites, if feasible, or at least for particular countries or regions. The variability of biomass carbon storage losses highlights the importance of understanding nickel supply chains and tracing metal from mine site to final end use.”\n4. IPCC Guidelines and Carbon Stock Esfimafion\nThe manuscript does not menfion the IPCC Guidelines or the fiered approach (Tier 1, Tier 2, Tier 3) for esfimafing carbon stocks in forests. This omission is crifical as the IPCC Guidelines provide standardized methods for carbon stock esfimafion, which are essenfial for ensuring consistency and accuracy in carbon accounfing.\nThe authors should integrate IPCC Guidelines into their methodology to provide a standardized approach to carbon stock esfimafion. Including these guidelines will enhance the robustness and credibility of their carbon emission esfimates.\nRESPONSE:\nWe agree with the reviewer that use of Tier 3 (local datasets) and, if not available, Tier 1 (global) and Tier 2 (country-level) datasets is preferable for esfimafing biomass carbon losses at mine sites. However, it was not possible to use this approach for esfimafing losses from historic nickel mining, since many nickel mining operafions (including the largest mines in operafion) started operafions prior to the development of modern satellite monitoring of biomass losses and there are poor historic records tracking deforestafion. Thus, we had to use a proxy approach looking at surrounding surviving biomass and\n[OFFICIAL]\nesfimafing the historic biomass losses. This enabled us to produce a broad global dataset, which illustrates our point that these emissions are significant for many nickel mines and that mines should befter track and report these emissions.\nWe have added menfion of IPCC guidelines for the fiered approach for esfimafing carbon stocks to the discussion on page 14, line 30:\n“We have demonstrated in this study that Scope 1 emissions from land use changes at many nickel mining operations can be significant relative to other Scope 1 and Scope 2 emissions. Considering the new GHG Protocol land use change reporting requirements, there is an urgency for nickel mining companies to collect appropriate data to track carbon emissions from land use changes at their operations. Data that should be tracked include: 1.) the exact boundaries of areas where vegetation has been cleared, 2.) the biomass densities of the cleared vegetation, and 3.) any recovery of biomass through reforestation activities. Data on land areas should be straightforward to collect through normal survey methods. In this study, we have used a global satellite dataset of biomass densities to estimate how biomass emissions from nickel mine sites in order to produce a broad global comparison. However, mining companies should collect more accurate local biomass datasets, using a combination of satellite and/or aerial remote sensing and on-the-ground field surveys. Specifically, we recommend that mining companies follow Intergovernmental Panel for Climate Change guidance for Tier 3 level data (local activity data) collection as much as possible (31). Where it is impossible to collect Tier 3 level data (for example, at nickel mine sites that have already been in operation for many years), then it may be necessary to rely on lower confidence estimates of carbon emissions, for example a Tier 1 modelling approach.”\n5. Addifional but Specific Comments\nPage 2, Line 5: The language should be officially acceptable. Revise to: \"Commitments to boost low-carbon energy producfion and end deforestafion by 2030 were two key outcomes of the COP28 of the UNFCCC.\"\nRESPONSE: The text has been modified to more accurately describe the COP28 event.\nPage 13: Clarify the term \"offsefting\" to explicitly refer to \"carbon offsefting\" for befter understanding.\nRESPONSE: This change has been made to the text."}, {"round": 3, "reviewer_comments": {"reviewer_3": "Thank you for addressing my concerns with thoughtful revisions and convincing justifications. I appreciate the clarity you've provided regarding the classification of emissions, particularly with respect to Scope 1, and your integration of the latest guidelines and frameworks, which have significantly strengthened the manuscript.\nI am now happy to recommend your paper for publication and believe it will make a valuable contribution to the field. Thank you once again for your careful attention to the review process."}, "author_response": ""}]

Decision letter