Identifying the shift in global wildfire weather conditions over the past four decades: an analysis based on change-points and long-term trends

Most of studies on change-point at a regional or global scale have only examined a single hydrometeorological variable and have been unable to identify any underlying explanations. In this study, we identified change-points and long-term trends of six wildfire-related variables and attempted to explain the cause of change-point from atmospheric–oceanic indices. As a result, we discovered that the main change-point dates for the precipitation, temperature, and drought codes, as well as the duff moisture code, fine fuel moisture code, and fire weather index, were 1995–2000 and 2000–2005, respectively. Furthermore, the relationship between the change-point of six variables and atmospheric–oceanic indices was discussed through the correlation coefficient. For example, the Atlantic Multidecadal Oscillation was found to dominate the precipitation in West Africa. In addition, we divided the globe into eight homogenous wildfire weather zones based on the change-point dates and long-term trends of the six variables.


Introduction
The nonstationarity in hydrometeorological variables, such as precipitation (Dhakal et al. 2015;Gu et al. 2019;Vu and Mishra, 2019), temperature (Aziz et al. 2020;Carney et al. 2020;Kärner, 2002), and drought index (Das et al. 2020;Kwon and Lall, 2016;Wu et al. 2020), has been extensively studied. Most of the existing research has focused on the long-term trends of hydrometeorological variables (Byakatonda et al. 2018;Meshram et al. 2017;Salman et al. 2017) and on the change-points in these variables (Ahmadi et al. 2018;Jiang et al. 2019;Liu et al. 2020). Such studies have discovered that, under the increasing influence of global warming, more high temperature events and extreme droughts have shown increasing trends of occurrence in the United States (Ge et al. 2016), China (Ye et al. 2019), Southeast Asia (Dong et al. 2021), Australia (Sharples et al. 2021), and Brazil (Geirinhas et al. 2021). And one of the prominent consequences is increased wildfire risk (Mukherjee et al. 2018).
The change-points of hydrometeorological variables have been widely investigated at the regional scale (e.g., Deng et al. 2018;Ouhamdouch and Bahir, 2017)), but only a few studies have examined the connections among the change-points of different variables at the global scale. Unlike the long-term trend of the external forcing caused by anthropogenic climate change, the changepoints are often assumed to be an indicator of natural climate variability (Ivancic and Shaw 2017). An abrupt change in a hydrometeorological variable has impacts on other variables, resulting in a multifaceted effect.
For example, increasing temperature and decreasing precipitation were the main drivers responsible for the change-point of drought frequency in Inner Mongolia in the 2000s (Liu et al. 2016). In addition, in the mid-1980s, an abrupt increase in large wildfires was observed in the forests of the western United States in early spring under the influence of global warming (Westerling et al. 2006). Such rapid increases in the occurrence of wildfires during the twenty-first century have also been observed in the Pacific Northwest forests of the United States driven by earlier spring snowmelt, warmer temperatures, and reduced winter precipitation (O'Dell et al. 2019). Nevertheless, the cumulative effect of shifts in global wildfire weather conditions and the causes of these shifts remain uncertain.
Compared with traditional hydrometeorological variables, wildfire variables (e.g., burned area and number of wildfires) are influenced by multiple factors, such as the fuel characteristics and weather conditions (Trollope et al. 2002). Unfortunately, previous studies did not explore this thoroughly. Therefore, since certain weather conditions are essential prerequisites for wildfires (Qu et al. 2021), it is urgently necessary to improve our understanding of the abrupt changes and long-term trend of wildfire weather. Overall, the existing studies (Liu et al. 2016;O'Dell et al. 2019;Westerling et al. 2006) have only sparingly accounted for the impact of abrupt changes in weather conditions on wildfires from a global perspective, and the mechanisms responsible for these abrupt changes have yet to be fully elucidated.
Accordingly, this paper attempts to identify the changepoints and long-term trends of multiple weather variables related to wildfires based on the Pettitt test (Pettitt 1979) and trend-free prewhitening Mann-Kendall (TFPW-MK) test (Yue and Wang 2002), respectively. Then, the causes of abrupt change are discussed through correlation analysis between atmospheric-oceanic indices and wildfire-related variables. Finally, by applying the clustering method outlined by Scrucca (Scrucca et al. 2016), grids with similar change-points in time and similar wildfire weather change trends are clustered. Consequently, maps of global wildfire weather changes are constructed, which are expected to provide a new perspective on wildfire weather conditions over the past four decades.

Materials and methods
In this paper, we selected six wildfire-related meteorological variables: daily precipitation, daily maximum temperature (Tmax), drought code, fine fuel moisture code, duff moisture code, and fire weather index. In addition, four commonly used atmospheric-oceanic indices were selected to analyze the possible causes of change-points, including the Pacific Decadal Oscillation Index (PDO), Multivariate El Niño-Southern Oscillation Index (MEI), Atlantic Multi-decadal Oscillation Index (AMO), and Dipole Mode Index (DMI). All global atmospheric-oceanic indices are on a monthly scale and come from the National Oceanic and Atmospheric Administration. Daily precipitation and Tmax data during 1979-2020 with a spatial resolution of 0.5° were obtained from the Climate Prediction Center (CPC) Global Unified Gauge-Based Analysis of Daily Precipitation (Chen et al. 2008;Xie et al. 2010Xie et al. , 2007 and the CPC global daily surface 2-m air temperature analysis (Pan et al. 2019), respectively. Drought code, fine fuel moisture code, duff moisture code, and fire weather index data during 1979-2020 with a spatial resolution of 0.5° were extracted from fire danger indices historical data from the Copernicus Emergency Management Service. Global wildfire data is from the Fire CCI v5.1 data set during 2001-2020 developed by Chuvieco et al. (2018), which is based on a hybrid approach combining the highest resolution (250 m) near-infrared band of MODIS with active wildfire information. In addition, the wildfire data were processed to a 0.5° spatial resolution consistent with wildfire-related variables. Then, we removed grids that never had a wildfire event (e.g., the Sahara desert). Of the 32,599 grids retained, those grids that did not contain at least one value per month were removed, for a total of 48 grids. As final, the total number of grids used for the calculation is 32,551. Then, for each remaining grid, we aggregated the daily values into time series of maximum monthly values to show the extreme dry/wet conditions each month. To reduce the possible impact of missing records, we extracted the maximum daily value of each month and processed the six wildfire-related variables into monthly time series. In addition, the climate reference zones (IPCC 2014) were applied to indicate different regions, as shown in Fig. 1.
For wildfire-related variables excluding precipitation and temperature, fine fuel moisture code, duff moisture code, and drought code represent moisture conditions of fine dead surface fuels, loosely compacted organic material on the forest floor, and deep organic soil layer, respectively. All three of them are unitless and larger values indicate lower fuel moisture conditions. In particular, under cover of a forest canopy, fuel moisture of forest litter fuels is represented by the fine fuel moisture code (range from 0 to 101). This index is calculated based on daily precipitation, relative air humidity, wind speed, and air temperature data. It represents moisture conditions for shaded litter fuels, the equivalent of a 16-h time-lag. The duff moisture code is calculated using daily precipitation, relative air humidity, and air temperature data taken at mid-day, indicating slow-drying surface fuels (12-day time-lag). The drought code, a unitless index, is calculated from daily precipitation and air temperature, representing the fuels with an extremely sluggish drying rate (53-day time-lag). The maximum value of the drought code is 1000, and values over 800 indicate extreme drought conditions. The fire weather index is a comprehensive fire behavior index that considers the fire spread potential and potential energy release under the premise of considering fuel conditions. It is a unitless dimensionless index rating the potential fire line intensity given the meteorological conditions for a reference fuel type. Although the fire weather index was developed for the boreal forests of Canada, it has proven to be a useful indicator in Portugal (Carvalho et al. 2008), China (Tian et al. 2011), France (Fox et al. 2018), and many parts of the world (Bedia et al. 2015).
The Pearson correlation coefficient was used to detect the correlation between the two variables. The Pearson method assesses the correlation coefficient of two conditioning factors. The correlation value is calculated by their covariance divided by the product of their standard deviations. Pearson correlations are represented in square tables called correlation matrices, which compute Pearson correlations for each column pair of a set of variables. Each matrix value represents the calculated correlation of the corresponding row and column variables (Tehrany et al. 2019). The data used for the correlation analysis were all on a monthly scale. Although different climate oscillation have different interannual or multidecades variations, stronger correlations will be detected when the pattern of this variation is similar to the periodicity of wildfire-related variables.
To evaluate abrupt changes, a nonparametric Pettitt test was utilized to detect change-points (Pettitt, 1979). The Pettitt test is one of the most widely applied tests to determine abrupt shifts in hydrometeorological time series around the world (Addisu et al. 2015;Asfaw et al. 2018;Ayamga et al. 2021;Ca 2017). Compared with other similar approaches, such as binary segmentation (Scott and Knott, 1974) (Haynes et al. 2017), the Pettitt test was found to deliver the best change-point detection results while simultaneously minimizing the number of false positives (Slater et al. 2021). After passing a significance test, the change-point date and change trend of each of the six wildfire-related meteorological variables in each grid were recorded. Beaulieu et al. (2012) used artificial time series to test the sensitivity of sequence time length on change-point testing. They found that the false alarms will be higher when the change-point appears near the beginning and end of the time series. In addition, the time series of at least 4 years can detect possible change-points. That is, mutation points detected over a 4-year-long period at the beginning and end were more prone to false alarms (Beaulieu et al. 2012). In this study, change-point dates were counted every 5 years, and this study set the time period for removal as 5 years (longer than the minimum 4-year threshold) to facilitate the presentation of statistical results. Thus, in this study, we removed the change-points detected during 1979-1983 and 2016-2020. In addition, all data were pre-whitened and detrended prior to change-point testing to avoid the influence of time series autocorrelation and long-term trend.
Next, to show the trends of the global changes in these variables more clearly, we extracted the grids that passed the significance test of the change-points and divided them into time series averaged over 27 climate reference regions for four seasons. Similarly, the data (time series of the four seasons) used for trend detection was also pre-whitened and detrended before the Mann-Kendall trend test, which is called trend-free prewhitening Mann-Kendall (TFPW-MK) test (Yue and Wang 2002). It is an improved version of the MK test proposed by Mann (Mann, 1945) and modified by Kendall (Kendall, 1948) that is widely used to analyze the change trends of hydrometeorological time series (Liu et al. 2015). The MK test requires that the data be independent. However, some hydrometeorological time series often display a serial correlation, which increases the probability that the MK test can detect a significant trend, thereby altering the estimated magnitude of the serial correlation (Yue and Wang 2002). And the TFPW-MK test can efficiently eliminate the effect of such serial correlations on the MK trend test.
Finally, the change-point date and trend of each of these six variables were adopted as input data for clustering. The Mclust toolkit (Scrucca et al. 2016) based on R was used to perform a cluster analysis on the comprehensive data set. This process comprises three elements: (1) initialization via model-based hierarchical agglomerative clustering; (2) maximum likelihood estimation via the expectation-maximization (EM) algorithm (Bradley et al. 1998;Dempster et al. 1977); and (3) selection of the Bayesian model and the number of clusters using approximate Bayes factors with a Bayesian information criterion approximation (Fraley and Raftery 2002;Fraley et al. 2012;Scrucca et al. 2016). Unlike traditional clustering methods, such as k-means clustering (MacQueen 1967) and Ward's method (Ward Jr 1963), the Mclust toolkit can solve three key problems: how many clusters there should be, which clustering method should be used, and how outliers should be handled.

Results and discussions
Before the change-points and long-term trends analysis of six wildfire-related variables, we first calculated the Pearson correlation coefficients between six wildfire-related variables and the burned area from 2001 to 2020, as shown in Fig. 2. Among the six wildfire-related variables, there was the strongest correlation between the fire weather index and burned area, especially in South America and Africa. A similar distribution of correlation coefficients was also found for the duff moisture code, but the areas with high ( Fig. 2) correlation coefficients in Africa were less than the fire weather index. The correlation coefficients of precipitation, drought code, and fine fuel moisture code also have similar distributions but with relatively small correlation coefficient values. Conversely, there was a weak correlation between temperature and burned area, even in South America and Africa. These results suggest that additional attention should be paid to areas with a high correlation to burn area (such as parts of South America and Africa) in the subsequent analysis of change-points and trends. The six variables in the weakly correlated regions should be interpreted as potential wildfire-related variables rather than wildfirerelated variables. interesting information, as all six wildfire-related variables exhibit varying characteristics in different parts of the globe. In addition, Figs. 6 and 7 summarize the proportion of change-point dates and change trend for the six variables in the 27 reference regions, respectively, to more clearly present the interesting findings obtained in this paper.
For the daily precipitation (Figs. 3a, 6a, and 7a), 1995-2000 was the period with the highest number of change-points, accounting for 11.2% of the total grid. In addition, the major change-point dates varied by region, and change trends were predominantly increasing after the change-point in SEA, TIB, NEU, and ALA regions accounting for more than 30%. However, precipitation in CGI, CAR, AMZ, WSA, and EAF regions showed decreasing trend after the change-point. Furthermore, the change zone surrounding the coast of the Peru-Chile Trench (WSA region) was the region affected by El Niño and La Niña among other climate variability phenomena (Takahashi 2004). Strong El Niño events swept this region during 1997-1998, followed by strong La Niña events during 1998-2000. The strong El Niño of 1997-1998 brought heavy precipitation to the coast of the Peru-Chile Trench but did not bring significant rainfall to the central Chilean Atacama (Kane 1999;McKay et al. 2003;Sanabria et al. 2018). After the significant increase in precipitation in 1997-1998, strong La Niña led to a sharp decrease in precipitation (Glantz 2001;Larrain et al. 2001). In addition, this change-point in precipitation was detected in our study between 2000 and 2005. Moreover, as seen from Fig. 8a-d, MEI and PDO have higher correlation coefficients than most regions of the world for the WSA southern region. These findings could verify the impact of ENSO on the sudden change in precipitation in the region, and this study also found that PDO also contributed to this precipitation variation. In addition, among the four atmosphericoceanic indices, AMO had a strongest impact on the WAF region, corresponding to the abrupt changes that occurred in this region during 2000-2005. Global warming increases water vapor in the atmosphere, resulting in enhanced precipitation over wet (ascending) regions and reduced precipitation over dry (descending) regions (Chou et al. 2013). In addition, the dependence of extreme precipitation and temperature is more significant over short durations in coastal regions (Panthou et al. 2014). Accordingly, our change trend results provide evidence that the precipitation and temperature variation characteristics along the coast of the Peru-Chile Trench are the same. The change-points of Africa varied significantly among different regions and  1990-1995, 1995-2000, 2000-2005, and 2005-2010. In addition, there were many increasing change-points in the SEA region during 1995-2000. In contrast to its impact on coastlines in the eastern Pacific, the strong El Niño during 1997-1998 brought less precipitation and high temperature to the SEA region (Tangang et al. 2017). Under the influence of global warming, this strong El Niño has further aggravated the rate of temperature rise in the SEA region. As a result, the period of 1997-1998 was a significant change-point date of temperature variability in the SEA region. From Fig. 8e-h, all four climatic drivers had an impact on the SEA region, but the location of the impact varied. Among them, PDO and MEI mainly affect temperature in the eastern part of the SEA, while AMO and DMI mainly affect temperature in the western part of the SEA. Moreover, the effects of AMO and DMI on the AMZ and NEB regions were also worthy of attention, and a large number of change-points have been detected in these two regions. Several studies have found a significant shift toward a greater annual burned area and a greater percentage of highly severe fires burning in the CNA region in 2000 (Abatzoglou and Williams 2016;Mueller et al. 2020). In comparison, our results show that the abrupt changes in fuel moisture and wildfire weather actually occurred earlier than the 2000 shift in the burned area. That is, the characteristics of fuel moisture and wildfire weather had undergone abrupt changes before 2000, which provided drier fuel and more fire-prone weather conditions for the subsequent shift in the burned area. In addition, from Figs. 9 and 10, the MEI had significant effects on the duff moisture code, fine fuel moisture code, and fire weather index in the CNA region, which suggested that El Niño-Southern Oscillation had some control over fire weather conditions in the region. For SSA region, the change-point distribution of the duff moisture code, fine fuel moisture code, and fire weather index is consistent with that of the drought code. Moreover, the increasing drought trend in this region was also reported by Damberg et al. (Damberg and AghaKouchak 2014). Ultimately, drier weather was the dominant factor affecting fuel and fire weather changes. In addition, since the early 2000s, higher-than-average fire weather has been more frequent in the SAU region, especially during the most severe season of 2002-2003(Harris and Lucas 2019. According to our results, 2000-2005 was also a concentrated period of abrupt changes in drought, fuel, and fire weather. Regarding the seasonal trends, Fig. 11 shows the change trends of the six variables over 27 climate reference regions in different seasons (only for the grids, where a change-point was detected). As for precipitation, the trends are the same for most regions in all seasons (all increasing or decreasing). The SAU area showed an increase in precipitation only in spring, with decreasing trends in all other seasons. In addition, spring precipitation showed a decline in the CAM region and an increase in winter, remaining almost constant between the summer and autumn seasons. Similar regions of inconsistent seasonal variation were found in NEU, SAU, SAS, etc. Temperatures showed a significant decreasing trend in summer and autumn in the WSA region, while the trend in spring and winter was not significant. Other regions showed a significant increasing trend or no significant change in temperature. In addition, the NEB region was the most significant warming of the 27 climate reference regions. Of the other four variables, the most significant increasing trends were for drought code in winter and duff moisture code, fine fuel moisture code, and fire weather index in autumn. A similar strong increasing trend was observed for drought code in the NEB winter and three other variables in the CEU autumn.
To better understand such a large amount of information about the change-points of different variables, we performed clustering to integrate the information (Fig. 12). A stationarity zone refers to a region, where no change-point was detected in any variable, whereas intensified drought zones and weakened drought zones denote the regions dominated by a change-point in the drought code, although the change trends of these two zones have opposite signs (increasing and decreasing, respectively). The grids where only precipitation exhibits abrupt changes are concentrated in precipitation-dominated zones. In the drought-precipitation interaction zone, abrupt changes in drought and precipitation occur in the same region but in different periods. In addition,

Conclusions
This research provides insight into the nonstationarity of wildfire weather during the past four decades by detecting the change-points and long-term trends of six variables connected to wildfires around the world. The results were also discussed within 27 global climate reference regions and an attempt was made to establish a link between change-points of these six variables and atmospheric-oceanic indices. We also did a broad  analysis of the various seasonal trends of six different variables associated with wildfires on 27 global climate reference regions. In more detail, we merged the data of the change-points with the information pertaining to the various attributes to obtain a comprehensive global map of abrupt changes in where wildfire weather has been influenced, when it has been influenced, and how it has been influenced by abrupt alterations. These findings have the potential to ultimately help increase our understanding of the impacts of anthropogenic climate change on wildfire weather. Specifically, the main conclusions of this paper are as follows: 1) The change-point of precipitation, temperature, and drought code mainly occurred in 1995-2000, while the main change-point date of duff moisture code, fine fuel moisture code, and fire weather index was 2000-2005. 2) Correlations between atmospheric-oceanic indices and six wildfire-related variables were discussed, with the most prominent combinations being: AMO-precipitation in the WAF region, MEI + DMI-temperature in AMZ and NEB regions, DMI-drought code in the SAU region, and PDO-duff moisture code + fine fuel moisture code + fire weather index in SSA region. 3) Eight sub-regions with various characteristics, such as a sub-region with a predominance of rainfall, a sub-region with a predominance of temperature, etc., were included in a newly created wildfire weather cluster map.