The original records included in the database are provided by government and non-government agencies and, as described in the previous section, have already been used individually in several regional studies although not all together). We acknowledge, however, the potential limitations previously reported in national data (Belhadj-Khedher et al., 2018; Pereira et al., 2011). These data commonly face various challenges, including periods of inaccessible or poor-quality data (Mouillot and Field, 2005; Koutsias et al., 2013), difficulties in estimating burned area from field observations (Pereira et al., 2011; Short, 2015), uncertainties arising from different fire reporting protocols between countries and/or protocol changes over time (Turco et al., 2013), and high political controls on BA statistics (Kasischke et al., 2000; Belhadj-Khedher et al., 2018). Thus, even though they cannot clearly be considered error-free “ground truth”, these data represent the main source of authoritative fire histories available today, before and during the satellite era. In addition, users can evaluate the validity and accuracy of the original source for each regional dataset from its references (Table 1). It is also worth noting that here we provide BA data at a monthly scale and over a 1° grid resolution, therefore substantially reducing any uncertainties and noise that could be present at higher temporal and spatial resolutions. In addition, we provide information on the total BA, which is primarily influenced by the extent of large fires. These large fires are typically monitored due to their significant impacts (however it is worth noting that ONFIRE BA grid cells can also be affected by fires of less than 100 hectares; see e.g. Ramo et al., 2021). Also, while caution is necessary when relying solely on BA, it remains a useful tool for understanding the overall magnitude and spatial extent of fire events (Resco de Dios et al., 2022) as it provides a quantitative measure that can be easily compared across different fire events and regions.
We also followed two different approaches in order to provide a quantitative assessment of the ONFIRE data: (1) we compared it with an independent satellite-based dataset, to examine the coherence among the datasets, and (2) we make use of the ONFIRE dataset to perform the same analysis of a previous study (Jones et al., 2022), thus comparing the results. This two-step assessment process is crucial to ensure the integrity and quality of the ONFIRE data.
a) Comparison with an independent dataset
We compared the monthly BA data from the ONFIRE dataset against the remotely sensed FireCCI51 data (Lizundia-Loiola et al., 2020), available for the period 2001–2020 at a resolution of 0.25° x 0.25°. This is the most recently developed global BA dataset and complements existing global BA products using higher spatial-resolution bands of the MODIS sensor (R, red, and NIR, near-infrared). A recent comparison amongst remotely-sensed and inventory datasets for BA in Mediterranean Europe showed that FireCCI51 had the best agreement with EFFIS BA data overall (Turco et al., 2019). To perform this comparison, a series of steps were undertaken. First, we removed non-natural FireCCI51 BA data. The FireCCI51 dataset provides the sum of burned area for each land cover category. Therefore, we calculated the sum of the burned area by considering all land cover categories except for “cropland, rainfed”, “cropland, irrigated or post-flooding”, and “mosaic cropland (> 50%)”. Then, the BA data were upscaled by summing all FireCCI51 grid points at 0.25° included in the corresponding 1° grid point of the ONFIRE dataset. Then, the mean annual aggregated BA series, calculated during the overlapping period, were compared. Finally, the temporal similarity of the monthly BA series was evaluated using Spearman correlation.
There is a high degree of similarity between the mean annual area burned included in ONFIRE for U.S.A. and Canada (Fig. 2a,b). Indeed, the spatial (Spearman) correlation coefficient between the two patterns has been found to be 0.89 (p-value < 0.01). The total annual burned area in Canada averaged over the period 1986–2020 was approximately 17668 km2 based on the ONFIRE-NBAC dataset, while the ONFIRE-NFDB dataset yielded an average value of 24125 km2. This represents a difference of 27% between the two datasets, similar to what was found by Skakun et al. (2021) who report a 23% difference considering fire data from 1986 to 2018. Higher similarity appears considering the annual BA in the U.S.A. averaged over the common period of 1992–2020. Specifically, the BA estimated using the ONFIRE-FPA-FOD dataset was found to be approximately 24,651 km2/year, and the ONFIRE-MTBS dataset yielded an estimate of around 24,744 km2/year, indicating a substantial agreement in the recorded values. The comparison of these data against FireCCI51 over the period 2001–2020 provides evidence of similarities among the datasets under examination (Fig. 2c,d,e). Specifically, the correlation between ONFIRE-NBAC + ONFIRE-FPA-FOD and FireCCI51 was 0.89, and also between ONFIRE-NFDB + ONFIRE-MTBS and FireCCI51 was 0.89. The annual BA in Canada averaged over the common period 2001–2020, was quite similar among the three datasets: 16800, 22266, and 17525 km2/year for, respectively, ONFIRE-NBAC, ONFIRE-NFDB, and FireCCI51. Again, lower differences have been found over the U.S.A., with 28660, 29997, and 30552 km2/year for ONFIRE-FPA-FOD, ONFIRE-MTBS, and FireCCI51.
There was also good agreement between the annual BA averaged over the period 2001–2020, as estimated by the ONFIRE, and the BA from FireCCI51 datasets, for Australia, Chile, and Europe (Fig. 3). The spatial correlation is notably high over Australia, with a coefficient of 0.87 (p-value < 0.01), and the corresponding total annual BA values for the ONFIRE and FireCCI51 datasets are 469950 km2/year and 431892 km2/year, respectively. In Chile the level of agreement between the datasets is lower, with a spatial correlation coefficient of 0.62 (p-value < 0.01), and the averaged values of total annual BA are 1027 km2/year and 1848 km2/year for the ONFIRE and FireCCI51 datasets, respectively. In Europe, the spatial agreement is lower, with a correlation coefficient of 0.59 (p-value < 0.01), but the datasets provide similar estimated mean values, with the ONFIRE and FireCCI51 datasets yielding 3323 km2/year and 3498 km2/year, respectively of total annual BA. These differences may be attributed to various factors, including misinterpretations (remote sensing detects fires that are not fires), or possibly underreporting in national statistics. However, further investigation is required to gain a comprehensive understanding of these differences, which is beyond the scope of our analysis. Our primary goal here is to present these datasets in a standardised format, facilitating similar assessments in the future. In any case, these differences emphasise the importance of cautiously considering the underlying data sources and methodologies during the conduction of analyses with fire data. It also highlights the requirement for ongoing efforts to enhance data collection and standardisation practices.
In addition, a correlation analysis was performed to measure the temporal similarity among the monthly burned area estimated by the different databases (Figs. 4 and 5). A consistent positive significant correlation was found for the Canada region comparing ONFIRE-NBAC and ONFIRE-NFDB, and for the USA, comparing ONFIRE-FPA-FOD and ONFIRE-MTBS, both for the largest common periods and considering the 2001–2020 periods (Figs. 4a,b). The correlations between these data and the FireCCI51 are generally significant but lower. (Figs. 4c,d).
ONFIRE and FireCCI51 data are also correlated for Australia, Chile, and Europe (Fig. 5). For the Australian region (Fig. 5a), a significant and positive correlation is observed almost everywhere, particularly in the north. This can be attributed to the combination of various data collection and validation techniques, including the use of satellites. For the Chile region (Fig. 5b), a highly significant correlation is restricted to areas where surface fire activity has been recorded in both datasets, indicating the strength and direction of the relationship of the burned area between ONFIRE and FireCCI51 is significant in these areas. In the European region (Fig. 5c), a significant and high correlation is observed where a high burned area is observed, especially in the Mediterranean and Eastern Europe (compare Figs. 3e-f and Fig. 5c). Unlike the Chilean region, in Europe, even in areas with relatively low burned area values (such as Scandinavia), high correlations can be found.
b) REPRODUCTION OF EARLIER PUBLISHED RESULTS
To assess the accuracy of ONFIRE data before the MODIS-era (early 2000s) we follow a similar approach inspired by the study of Iizumi and Sakai (2020), who validated historical yields for major crops with climate data. The Canadian Fire Weather Index (FWI; Wagner 1987), a widely used index to assess the meteorological fire danger worldwide, exhibits correlation with BA over most of the globe (see e.g., Bedia et al., 2016 and Jones et al., 2022). We extracted the FWI data for the period 1979–2021 provided by Vitolo et al. (2020) and available from the Copernicus Emergency Management Service (https://cds.climate.copernicus.eu/cdsapp#!/dataset/cems-fire-historical). These FWI data are the same used by Jones et al., (2022). In order to ensure consistency between the spatial resolution of the different datasets, the FWI data are also remapped from their original resolution (0.25° x 0.25°) to the 1° × 1° fire grid with a bilinear interpolation (using Climate Data Operators, CDO, Schulzweida et al., 2019). Then, as in Jones et al. (2022), we calculate Spearman’s rank-order coefficient. The Spearman correlation coefficient was chosen due to the exponential relationship between climate (FWI) and fires (ONFIRE).
This analysis is not intended to investigate the drivers of fires or discuss the importance of the FWI but to indirectly assess the ONFIRE datasets patterns using independent climate data that prior studies have shown to be related to fire variability. Despite FWI being the most commonly applied index for meteorological rating fire danger worldwide (Flannigan et al., 2013; Field et al., 2015; Jolly et al., 2015), it may not be the best predictor for explaining BA variability (see Abatzoglou et al.,2018 and Boer et al., 2021 for more details on global climate fire drivers). Previous studies (see e.g., Bedia et al., 2016; Abatzoglou et al., 2018; Silvia et al., 2019 and Jones et al., 2022) demonstrated that the connection between BA and FWI varies depending on the geographical location. The relationship is stronger in regions with high biomass, where the main constraint to fire is fuel moisture, rather than fuel availability. In contrast, BA shows less sensitivity to FWI in xeric grasslands and shrublands since these systems are more constrained by fuel productivity. Jones et al. (2022) have shown that there is a positive and statistically significant correlation between monthly BA and monthly FWI in most regions of the world. Specifically, this relationship is particularly strong in Canadian and Alaskan forests, central Chile, the Mediterranean, north and southeast Australia, and the western U.S.A. Very similar results have been found here, as we detail in the following.
The BA provided by the ONFIRE datasets and the FWI at the monthly scale (for the period 1979–2021 or shorter, depending on the data availability; see Tables 1–2) are well correlated (Figs. 6 and 7) and mostly resemble the assessment shown by Jones et al. (2022). For the USA region, higher correlations over the western U.S.A. (of more than 0.75) are observed considering the ONFIRE-FPA-FOD (Fig. 6a) than considering the ONFIRE-MTBS data (Fig. 6b). Interestingly, Fig. 6c shows that FireCCI51 and ONFIRE data are on the same order of magnitude of correlations compared to FWI, except over the western U.S.A., where ONFIRE-FPA-FOD shows higher values. For Australia (Fig. 7a), a significant correlation between burned area (ONFIRE) and FWI data is shown in the north and southeast of the region. This may be explained as the limited availability of fuel plays a crucial role in shaping fire patterns in inland Australia. Conversely, in the forested regions of the eastern and southwestern coastlines, the dryness of the existing fuel further exacerbates the fire risk. In Chile (Fig. 6d), the ONFIRE data show a significant and high correlation practically throughout the region, with higher values in the central part of the country. Similar significant correlations are found in the European region (Fig. 5e), with particularly high values over the Iberian Peninsula, southern France, and Italy. In general, we have observed similar patterns when analysing the FireCCI51 data, albeit with slightly lower correlation values (Figs. 7d, e, f).