the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The response of wildfire regimes to Last Glacial Maximum carbon dioxide and climate
Iain Colin Prentice
Sandy P. Harrison
Abstract. Climate and fuel availability jointly control the incidence of wildfires. The effects of atmospheric CO2 on plant growth influence fuel availability independently of climate; but the relative importance of each in driving large-scale changes in wildfire regimes cannot easily be quantified from observations alone. Here, we use previously developed empirical models to simulate the global spatial pattern of burnt area, fire size and fire intensity for modern and Last Glacial Maximum (LGM; ~ 21,000 ka) conditions using both realistic changes in climate and CO2 and sensitivity experiments to separate their effects. Three different LGM scenarios are used to represent the range of modelled LGM climates. We show large, modelled reductions in burnt area at the LGM compared to the recent period, consistent with the sedimentary charcoal record. This reduction was predominantly driven by the effect of low CO2 on vegetation productivity. The amplitude of the reduction under low CO2 conditions was similar regardless of the LGM climate scenario and was not observed in any LGM scenario when only climate effects were considered, with one LGM climate scenario showing increased burning under these conditions. Fire intensity showed a similar sensitivity to CO2 across different climates but was also sensitive to changes in vapour pressure deficit (VPD). Modelled fire size was reduced under LGM CO2 in many regions but increased under LGM climates because of changes in wind strength, dryness (DD) and diurnal temperature range (DTR). This increase was offset under the coldest LGM climate in the northern latitudes because of a large reduction in VPD. These results emphasis the fact that the relative magnitudes of changes in different climate variables influence the wildfire regime and that different aspects of climate change can have opposing effects. The importance of CO2 effects imply that future projections of wildfire must take rising CO2 into account.
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Olivia Haas et al.
Status: open (until 20 Jun 2023)
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RC1: 'Comment on egusphere-2023-506', Anonymous Referee #1, 11 May 2023
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This study examines the combined and separate effects of climate and atmospheric CO2 on wildfire characteristics. Results are based on model estimates for the last glacial maximum and modern times. The research shows that atmospheric CO2 levels can have a significant impact on vegetation productivity, which ultimately affects the amount of fuel available for wildfires and leads to changes in fire characteristics. The scenario analysis is described and carried out properly and the results are clearly presented and discussed. I recommend this study for publication with minor revisions.
Overall comments:
1) The authors describe changes between the two time periods as anomalies. In my understanding, anomalies are deviations from the long-term mean. Since we are comparing two time periods, it is not really clear what an anomaly is and which "long-term mean" they are referring to; in fact, I think it is just the absolute difference that is being referred to here. Please clarify this at the beginning of the methods section.
2) The figures are quite small and hard to read (especially figure 3). They should be reworked (e.g. using BA, FI, FS as columns instead of lines) and provided at a higher resolution.
3) The discussion section (323-337) lacks a literature-based discussion of how VPD, DD and DTR have been observed/modeled in other studies. For example, the authors could already refer to the reference to Diffenbaugh in lines 372-374: "This work also highlights the role of VPD in promoting fuel loads and limiting fire ignition and spread, a climatic variable that has been linked to wildfire occurrence (Diffenbaugh et al., 2021)."
Minor corrections:
Line 26: missing e for emphasis
Line 66: missing space after Haas et al
Line 79: please rephrase, it sounds like Haas was an study providing observations
Line 141: "did not change dramatically". In the framework it is stated that it did not changed at all, please clarify
Figure 3: too small, legends unreadable
Line 300: rephrase " somewhat worse ".
Line 374 “Although the effect of human activity was not considered in this analysis, if reductions in burnt area do contribute to greater fuel loads, suppression policies may artificiall increase fuel loads in the same way reduced burnt area increased fuel loads under LGM conditions, suggesting resulting wildfires may be larger and more intense. “ This statement is highly speculative and oversimplifies human influence. It is also not clear how results from "past anomalies" can be extrapolated to "future anomalies". Please rephrase and elaborate on these two points or delete the statement.
Citation: https://doi.org/10.5194/egusphere-2023-506-RC1 -
AC1: 'Reply on RC1', Olivia Haas, 17 May 2023
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We thank the referee for their constructive comments on this article. The referee. was concerned about the quality of the figures. We produced the figures for this article in high resolution (300 dpi) but their quality appears to have been reduced upon upload. We have attached the high-resolution figures in this comment, though we will also re-work figures 3 and 5 to address legibility concerns (see below).
Overall comments:
- The authors describe changes between the two time periods as anomalies. In my understanding, anomalies are deviations from the long-term mean. Since we are comparing two time periods, it is not really clear what an anomaly is and which "long-term mean" they are referring to; in fact, I think it is just the absolute difference that is being referred to here. Please clarify this at the beginning of the methods section.
Within the paleoclimate community it is standard usage to employ the term anomaly when referring to the absolute difference between modern climatological averages of a variable and the climatological averages of a simulated episode of past climate for the same variable. In this study, for each grid cell and each climate variable, the long-term LGM climatology simulated by each climate model was subtracted from the long-term pre-industrial climatology simulated by the same climate model and then added to modern climate values in order to obtain a bias-corrected LGM climatology. What is referred to as “anomalies” in the resulting discussion is the difference between the simulated burnt area, fire size and fire intensity under the MOD climate/MOD CO2 experiment and the other four experiments. Since we are representing the average spatial patterns of each fire properties under each experimental condition, we believe this term to be appropriate. However, we will add text to the methods section to clarify these definitions (and the difference between the climate and the fire anomalies) as well as referring the fire anomalies as BA, FS or FI anomaly in the results and discussion section:
Line 102: “Figure 2. Latitudinal distribution of the LGM-MOD climate anomalies”
Line 115: “difference between the PI and LGM values (LGM-PI climate anomalies) were calculated and added to the MOD climatology (LGM-MOD climate anomalies) (see Figure 1). We use the term climate anomalies to refer to the difference between the MOD climatology for each climate variable and the computed bias-adjusted LGM climatology for the same variable, consistent with the PMIP4 protocol (Kageyama et al., 2017). The use of anomalies is designed to minimise the impact of systematic model biases on the derived climate. This approach provided three LGM climate scenarios, resulting in nine experiments for BA, FS and FI respectively.”
And Line 160 (break and create a new paragraph): “The resulting BA, FS and FI anomalies refer to the difference between the MOD climate/MOD CO2 experiment and the three other experiments since each experiment is considered to represent the long-term average spatial pattern for each fire property under the set experimental conditions. We used the sensitivity experiments to quantify the separate effects of CO2 and climate on BA, FS and FI independently. We then used the realistic experiments to identify which predictors were driving the largest change between MOD and the three LGM scenarios by excluding one predictor at a time from the GLM models, re-running the LGM experiments and identifying which excluded variable caused the greatest change in the BA, FS and FI MOD-LGM anomalies in each grid-cell. Comparing these results to the BA, FS and FI MOD-LGM anomalies of the full GLM models allowed us to determine if the predictor was responsible for an increase or a decrease in BA, FS and FI.”
2) The figures are quite small and hard to read (especially figure 3). They should be reworked (e.g. using BA, FI, FS as columns instead of lines) and provided at a higher resolution.
Figures were produced at higher resolution for this article but were not posted (only the figures as part of the embedded word document were uploaded). We will upload the high-resolution figures in this comment. However, we also agree with the referee that the figures could be improved and will re-work them for the revised manuscript. We will use stronger colours in Figure 2 (and move key into bottom right-hand corner to give a bit more space), we will create columns instead of rows (as suggested) for Figure 3 and 6 as well as remove Antarctica.
3) The discussion section (323-337) lacks a literature-based discussion of how VPD, DD and DTR have been observed/modeled in other studies. For example, the authors could already refer to the reference to Diffenbaugh in lines 372-374: "This work also highlights the role of VPD in promoting fuel loads and limiting fire ignition and spread, a climatic variable that has been linked to wildfire occurrence (Diffenbaugh et al., 2021)."
The calculation of all three of these variables are standard therefore we do not believe there is a need for a literature-based discussion of how VPD, DD and DTR have been observed/modeled in other studies. We believe the most crucial point is that the calculations are consistent with the methods used in Haas et al., 2022 since this was the modern data that the GLM models were built on. However, we agree with the referee that a discussion on how these variables have been shown to be important in previous observation-based studies would be beneficial. We suggest adding the following:
Line 362: “These results add to a growing body of literature highlighting the importance of considering not only changes in wildfire weather but also vegetation properties in projections of future wildfire regimes (e.g. Harrison et al., 2021; Kuhn-Régnier et al., 2021; Pausas & Keeley, 2021). The impact of rising CO2 levels will most likely enhance vegetation growth and litter accumulation, which are important controls on fuel availability, continuity, and load. However, climate and specifically VPD may have opposing effects to that of rising CO2 levels. Since VPD controls plant growth, increasing VPD can limit ecosystem productivity and tree growth, in turn reducing fuel loads (Williams et al. 2013). Nevertheless, VPD has also been shown to increase litter fall, thus increasing available dead fuel (Resco de Dios 2020, De Faria et al. 2017). As such, it is important to consider how temporal and spatial scales affect the response of vegetation to changing VPD (Grossiord et al., 2020). Although the trade-offs between future increases in CO2 and reductions in productivity due to higher temperatures and atmospheric dryness are not fully understood, this work highlights the importance of considering both. These effects will most likely not be evenly distributed across the globe (Gonsamo et al., 2021; Piao et al., 2020; van der Sleen et al., 2015) and CO2 effects may be more important in some regions than others. In fuel-limited ecosystems, CO2 fertilization could increase fuel loads and fuel continuity, increasing overall burnt area but also the potential for larger and more intense wildfires. This is particularly worrying in regions with anticipated decreases in atmospheric moisture, especially since evidence suggests rising VPD may only counteract a small proportion of CO2-induced plant growth (Y. Song et al., 2022). Increased woody thickening, for example in tropical South Asia (Kumar et al., 2021; Scheiter et al., 2020), may also alter fuel loads in regions that are likely to be vulnerable to ignition under a drier and warmer atmosphere (Clarke et al., 2022). Whilst climate variables such as DD and DTR have also shown to be strong controls of global wildfires regimes (e.g. Bistinas et al., 2014; Forkel et al., 2019; Kuhn-Régnier et al., 2021), this study highlights the importance of VPD relative to other climate variables in driving spatial patterns of BA, FS and FI. This is in line with previous studies that have highlighted the important role of VPD in promoting fuel loads and fire spread (e.g. Diffenbaugh et al., 2021; Grillakis et al., 2022; Duane et al., 2021; Balch et al., 2022).”
Balch, J.K., Abatzoglou, J.T., Joseph, M.B., Koontz, M.J., Mahood, A.L., McGlinchy, J., Cattau, M.E. and Williams, A.P., 2022. Warming weakens the night-time barrier to global fire. Nature, 602(7897), pp.442-448.
De Dios, V.R., Hedo, J., Camprubí, À.C., Thapa, P., Del Castillo, E.M., de Aragón, J.M., Bonet, J.A., Balaguer-Romano, R., Díaz-Sierra, R., Yebra, M. and Boer, M.M., 2021. Climate change induced declines in fuel moisture may turn currently fire-free Pyrenean mountain forests into fire-prone ecosystems. Science of The Total Environment, 797, p.149104.
De Faria, B.L., Brando, P.M., Macedo, M.N., Panday, P.K., Soares-Filho, B.S. and Coe, M.T., 2017. Current and future patterns of fire-induced forest degradation in Amazonia. Environmental Research Letters, 12(9), p.095005.
Duane, A., Castellnou, M. and Brotons, L., 2021. Towards a comprehensive look at global drivers of novel extreme wildfire events. Climatic Change, 165(3-4), p.43.
Grillakis, M., Voulgarakis, A., Rovithakis, A., Seiradakis, K.D., Koutroulis, A., Field, R.D., Kasoar, M., Papadopoulos, A. and Lazaridis, M., 2022. Climate drivers of global wildfire burned area. Environmental Research Letters, 17(4), p.045021.
Grossiord, C., Buckley, T.N., Cernusak, L.A., Novick, K.A., Poulter, B., Siegwolf, R.T., Sperry, J.S. and McDowell, N.G., 2020. Plant responses to rising vapor pressure deficit. New Phytologist, 226(6), pp.1550-1566.
Williams AP, Allen CD, Macalady AK, Griffin D, Woodhouse CA, Meko DM, Swetnam TW, Rauscher SA, Seager R, Grissino-Mayer HD et al. 2013. Temperature as a potent driver of regional forest drought stress and tree mortality. Nature Climate Change Change 3: 292–297.
We also suggest adding the following to the methods to make the calculations clearer:
Line 91: “The number of monthly dry days (DD) (days with ≤ 1mm of precipitation), monthly diurnal temperature range (DTR) (daily maximum temperature – daily minimum temperature) and monthly vapour pressure deficit (VPD), a function of specific humidity, temperature and pressure were all calculated following the methodology in Haas et al. (2022).”
Minor corrections:
Line 26: missing e for emphasis
This has been corrected.
Line 66: missing space after Haas et al.
This has been corrected.
Line 79: please rephrase, it sounds like Haas was an study providing observations
Line 79: “Haas et al (2002) developed empirical models of the global spatial patterns of burnt area (BA), fire size (FS) and fire intensity (FI) using generalised linear modelling (GLM) of modern observations. Here we use these models to simulate the global spatial patterns of burnt area (BA), fire size (FS) and fire intensity (FI) under four climate/CO2 scenarios (Figure 1).”
Line 141: "did not change dramatically". In the framework it is stated that it did not changed at all, please clarify
We have cut the word “dramatically”, there was no significant change.
Figure 3: too small, legends unreadable
(see above)
Line 300: rephrase " somewhat worse ".
Line 300: We have substituted “not as good”
Line 374 “Although the effect of human activity was not considered in this analysis, if reductions in burnt area do contribute to greater fuel loads, suppression policies may artificially increase fuel loads in the same way reduced burnt area increased fuel loads under LGM conditions, suggesting resulting wildfires may be larger and more intense. “This statement is highly speculative and oversimplifies human influence. It is also not clear how results from "past anomalies" can be extrapolated to "future anomalies". Please rephrase and elaborate on these two points or delete the statement.
We agree with the referee and have deleted this statement.
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AC1: 'Reply on RC1', Olivia Haas, 17 May 2023
reply
Olivia Haas et al.
Olivia Haas et al.
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