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the Creative Commons Attribution 4.0 License.
| 06 Aug 2024
Review article: Understanding the placement of fire emissions from the Brazilian Cerrado biome in the atmospheric carbon budget
Abstract. Estimating fire emissions in the Brazilian Cerrado requires a comprehensive approach, combining data, fire and vegetation modelling techniques, and policy. Although high quantities of global fire emissions come from the Cerrado, research in this area is still overlooked when compared to other savanna countries. This study systematically reviewed 69 papers on fire emissions from the Cerrado. The aim was to provide insights into the placement of the Cerrado in the atmospheric carbon budget and support improved estimation of the Biome’s carbon balance. Our review finds that, in the Cerrado, studies often focus on quantifying fire dynamics parameters and emissions, and that a holistic approach is required to estimate fire emissions, which is hindered due to the difficulty in valuing the qualitative aspects of fire. Evidence suggests a rise in interest in understanding fire emissions in the Cerrado, reflected in the increased number of studies throughout the years. More research is required to understand the aspects of fire dynamics in the Cerrado, how these reflect fire emissions locally and globally and potential mitigation activities. This could be achieved by including fire management representation in land surface models and using observational data to constrain and assess their utility.
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RC1: 'Comment on egusphere-2024-2348', Anonymous Referee #1, 18 Sep 2024
General comments
The main goal of this manuscript is to perform a systematic and comprehensive literature review on fire emissions in the Brazilian Cerrado, and put these emissions into perspective of the global carbon budget. Given that Brazil is the largest contributor to land use (LU) emissions, the role of fire in LU emissions, and Cerrado as the largest contributor to Brazil's annual burned area, this assessment is relevant and of interest to the scientific community. To my knowledge, there is no such assessment yet, and this study could potentially contribute significantly to understanding the current state of fire emission estimates in the biome and identify research gaps. Unfortunately, I don't think the authors were successful in doing so. I will detail my main concerns with this manuscript below:
- The research question. The question that the authors use to guide their review process is the following: "How compiling published material on fire emission in natural areas of the Cerrado can provide a better understanding of the placement of these emissions in the atmospheric carbon budget?". This question is not mentioned again in the manuscript and it is left unanswered in the Discussion. Additionally, its formulation is not in line with the main goals of the manuscript. For instance, the question refers to "natural areas of the Cerrado". If this were to mean areas of intact native vegetation, the authors would need to provide a keyword for this, as the vast majority of papers that are mentioned do not focus on natural areas, and are often estimates for the entire biome or specific land cover types. I also believe that "global carbon budget" would be more appropriate than "atmospheric carbon budget".
- At no point in the manuscript, are the 69 papers disclosed. This is a major concern, as the papers found through the PRISMA method are never listed. The authors refer to many papers throughout the text, but the reader does not know if these papers are those included in the literature review, or just part of a discussion. There is no list, even in Supplementary Material, of the papers, along with their respective topic (fire dynamics parameters, emission estimates, and fire management and policy) and study design (empirical, review, and perspective). Moreover, these classifications are explained in the Results section (e.g. lines 210-214) rather than in Methods. The authors also divide the papers according to study area (Global, Tropical region, South America, Brazil, Cerrado) which is never mentioned in Methods.
- Given that the paper is about fire emissions, there is a lot of discussion about fire patterns and drivers. There are many papers, especially in the "fire dynamics parameters" category, that do not evaluate emissions. Although burned area and fire intensity are parameters used to estimate emissions, discussing fire patterns and their climatic and human drivers should not be a main focus of this literature review. The authors also select burned area as the sole parameter to estimate fire emissions in Figure 5 (Section 3.1), to then explain how Fire Radiative Power (FRP) can also be used in the last paragraph of Section 3.2. This shows a lack of grasp of some of these concepts: for instance, the authors introduce FRP in Section 3.2 as if it was not the same parameter as "fire intensity" in Section 3.1; they also mention that "FRP considers (...) area affected by fire" and that it uses "MODIS active fires are inputs" which is, at best, vey badly worded.
- The manuscript needs to be substantially re-organized. The Introduction fails to provide background to the importance of fire emissions in the Cerrado, both in the national and global context. The role of fire emissions in the global carbon cycle/budget should also be highlighted, along with the role of Brazil in the LULC emissions as the highest emitter (see the Global Carbon Budget 2023). Information on how carbon emissions are estimated worldwide should be included (e.g. what data and methods are usually employed), so that the reader can better understand results found in the literature review. Moreover, as a tropical savanna, the Introduction could also leverage on information from other tropical savannas worldwide. As mentioned previously, the Methods section is missing key information (e.g. that the analysis only considers papers up to 2022, or how the trend line in Figure 2 is estimated and its significance level), and in the Results section is hard to distinguish between description of papers found through the review process and discussion (e.g. lines 163-165, 272-278, 279-282, 330-333, 375-379, 396-398, 402-405, 441-460). Lastly, the Discussion should include limitations on current methods and estimates, especially those found in the review process.
- Most importantly, the authors fail to deliver on their main goal as it is not clear the importance of Cerrado's emissions to the global carbon budget. The authors also fail to conclude what is obvious for the reader: that there is barely any literature on fire emissions in the Cerrado, especially if comparing with other biomes worldwide. They also do not discuss the mitigation potential for Brazil in LU emissions, and the impact and importance of such policy changes in keeping to the 1.5ºC goal (see Roe et al., 2019 in Nature Climate Change).
Specific comments
- Line 441: I believe the authors are confusing emission factors with carbon emissions.
- Line 327: "low fuel moisture and low flammable biomass" if there is low fuel moisture, there should be high flammability. Please clarify.
- Line 380: "GFED relies on the study done by (...) to quantify emissions worldwide" GFED doesn't rely on Van Der Werf et al. (2017). Its fourth version is described in that paper. Moreover, "small burned areas detection derived from MODIS" seems to entail that GFED did not rely on MODIS, which is incorrect. Please clarify how small burned areas were included in the GFED dataset (which also relies on active fire information).
- Line 558-559: how are fire emissions a sink of CO2?
Technical corrections
- Standardize units throughout the manuscript (e.g. Pg year-1 or Pg per year)
- Please write biome in lowercase.
- Line 502: "we found" should be "Van Der Werf et al. (2017) found"
- Authors' contribution is missing an author.
- Either "burnt area" or "burned area". Both are used in the manuscript.
- Figure 3 has 25 papers in the Cerrado, while the text mentions 26 (line 182).
- Figure 3 and 4 could be merged into one.
- Figure captions need to be much more detailed.
- Line 489: dos Santos et al. (2021) found that fire management reduced LDS in 3 PAs of the Cerrado, not in "areas of the Cerrado", this should be clear.
Citation: https://doi.org/10.5194/egusphere-2024-2348-RC1 -
RC2: 'Comment on egusphere-2024-2348', Anonymous Referee #2, 08 Oct 2024
The authors provide a comprehensive review of the literature on emissions from fires in the Brazilian Cerrado biome. The article effectively discusses fire dynamics parameters, emission estimates, and fire management and policy, which are critical not only for the scientific community but also for various sectors involved in fire mitigation, adaptation strategies, and control efforts. Given Cerrado’s strategic importance within South America and its role in the global carbon budget, the article addresses relevant issues with broad implications. While the article is innovative and presents a novel assessment of the Brazilian Cerrado, I believe that some aspects of the scientific question were explored in a rather superficial manner.
Below are specific points I would like to raise:
Introduction and Brazilian Cerrado biome characterization: The introduction provides a useful overview of the geographical, seasonal, vegetative, and emission characteristics of the Cerrado. However, it overlooks two crucial aspects: (1) The Cerrado’s role in water resource availability in Brazil, as it is responsible for surface water in 8 of the 12 major Brazilian hydrographic regions, and how climate change and extreme fire events could impact the hydrological cycle. (2) The socio-biodiversity of the Cerrado, shaped by its peoples’ socio-cultural relationships with nature, is highly relevant when considering the connection between ancestral knowledge and integrated fire management. I believe these points would enhance the introduction.
Fire-climate feedback loop: In the introduction, you discuss how fire and climate regulate one another and can form a positive feedback loop. However, there is no mention of the interaction between droughts and heat waves, which amplifies fire risks. Recent research highlights the importance of understanding compound drought, heatwaves, and fires, which I consider essential to this work's context.
Distribution of studies: You classify the studies by location range, from global to local scales, indicating that the number of studies is higher for the Cerrado and global levels. I was curious about the spatial distribution of the institutions involved in these studies. Are they predominantly Brazilian or located in the Cerrado region? In other words, who is driving research on fire impacts in the Cerrado?
The decline in publications in 2022: The sharp drop in publications in 2022 is striking. Could this reflect a shift in focus toward another biome, such as the Pantanal? A simple analysis of publication trends in other biomes could provide insight. Also, might the pandemic have affected research outputs? While I understand this is not the article’s focus, the significant drop warrants more than a brief mention.
Critical fire year in 2020: Your findings show that 2020 was the most critical year in terms of burned area. Is there any information on what caused this increase? Could it be related to drought and exacerbated heat, or perhaps changes in government policy or legislation? This point deserves further discussion in the text.
Fire management focus: You identify that only 8% of papers focused on fire management, and state that "this review captured no studies quantifying the amount of fire emissions mitigated by fire management in the Cerrado." This seems to contradict the statement that "three prominent topics identified were fire dynamics, emission estimates, and fire management". I believe adjusting the scientific question or the criteria for topic selection is necessary. Additionally, while there may be no studies on integrated fire management reducing emissions in Brazil, may research outside Brazil, such as in Australia have shown this potential? Expanding this discussion would add valuable global context.
Combustion efficiency discussion: The discussion on combustion efficiency values seems underdeveloped. Is 0.94 considered high or low? Is it normal or anomalous? More CO₂ or CO affects the atmospheric carbon budget in different ways, and it would be useful to discuss air pollution and atmospheric chemistry versus greenhouse gas effects, as well as comparisons with other biomes in Brazil or other savannas globally.
Methodology: I believe it is essential to list all 69 articles reviewed. This could be done as a table or supplementary material, with details such as publication year, method, and category. It is unclear whether the 69 articles are all in the reference list or if those cited throughout the text are part of this selection.
Scientific question: The question posed—"How compiling published material on fire emissions in natural areas of the Cerrado can provide a better understanding of the placement of these emissions in the atmospheric carbon budget?"—is not adequately addressed or answered throughout the text. My impression is that the answer is "no," due to the lack of studies with a holistic approach. If that is indeed the case, a more in-depth discussion of this point is needed.
Minor review: On line 511, the term "estimate emissions" should likely be "estimate fire emissions."
Citation: https://doi.org/10.5194/egusphere-2024-2348-RC2
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