Global patterns of interannual climate-fire relationships

Abstract

Climate shapes geographic and seasonal patterns in global fire activity by mediating vegetation composition, productivity, and desiccation in conjunction with land-use and anthropogenic factors. Yet, the degree to which climate variability affects interannual variability in burned area across Earth is less understood. Two decades of satellite-derived burned area records across forested and nonforested areas were used to examine global interannual climate-fire relationships at ecoregion scales. Measures of fuel aridity exhibited strong positive correlations with forested burned area, with weaker relationships in climatologically drier regions. By contrast, cumulative precipitation antecedent to the fire season exhibited positive correlations to nonforested burned area, with stronger relationships in climatologically drier regions. Climate variability explained roughly one-third of the interannual variability in burned area across global ecoregions. These results highlight the importance of climate variability in enabling fire activity globally, but also identify regions where anthropogenic and other influences may facilitate weaker relationships. Empirical fire modeling efforts can complement process-based global fire models to elucidate how fire activity is likely to change amidst complex interactions among climatic, vegetation, and human factors.

Keywords: climate; ecoregions; fire; global; modeling.

Figure 1:

Average ecoregion burned area fraction per year from 1997–2016 for (a) forests, and (b) non-forests. The duration of the fire season (in months) is shown in panel (c) and ending month of the fire season shown in panel (d). Grayed out are ecoregions that comprised <0.001% of either forest or non-forested burned area or where land cover was <20% forest or non-forest. Maps are provided in an Eckert IV equal-area projection.

Figure 2:

Linear Pearson’s correlation between the base-10 logarithm of fire year burned area and 12-month accumulated precipitation ending 14-months prior to the fire season (PPT_2y), 12-month accumulated precipitation ending 2-months prior to the fire season (PPT_1y), and the Fire Weather Index (FWI_fs), and climatic water deficit (CWD_fs) over the fire season. Only statistically significant correlations (∣r∣>0.4) are colored. Ecoregions with less than 20% land cover for each vegetation class are shaded darker gray.

Figure 3:

Model generalized additive model (GAM) fit of climate correlations to burned area across global ecoregions for (left) forests and (right) non-forests as functions of spatial gradients in (top to bottom) annual average climatic water deficit (CWD), average tree density, and the Human Footprint Index. The top three panels show GAMs for correlations with concurrent CWD (r_CWD), while the bottom three panels show GAMs for correlations with antecedent precipitation (14–25 months prior to the fire season, r_P). The grey shading shows the 95% CI, black stripes along the x-axis denote the distribution of data from ecoregions, and values reported above x-axes show the percent of geographic variance explained by each GAM.

Figure 4:

Percent of interannual variability in the base-10 logarithm of fire season burned area accounted for using a linear model that includes one measure of antecedent precipitation and one measure of concurrent fuel aridity as depicted in Figure 2.