The response of wildfire regimes to Last Glacial Maximum carbon dioxide and climate
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.
Olivia Haas et al.
Status: open (until 20 Jun 2023)
RC1: 'Comment on egusphere-2023-506', Anonymous Referee #1, 11 May 2023
- AC1: 'Reply on RC1', Olivia Haas, 17 May 2023 reply
Olivia Haas et al.
Olivia Haas et al.
Viewed (geographical distribution)
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.
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)."
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.