Human-driven greenhouse gas and aerosol emissions cause distinct regional impacts on extreme fire weather

Attribution studies have identified a robust anthropogenic fingerprint in increased 21st century wildfire risk. However, the risks associated with individual aspects of anthropogenic aerosol and greenhouse gases (GHG) emissions, biomass burning and land use/land cover change remain unknown. Here, we use new climate model large ensembles isolating these influences to show that GHG-driven increases in extreme fire weather conditions have been balanced by aerosol-driven cooling throughout the 20th century. This compensation is projected to disappear due to future reductions in aerosol emissions, causing unprecedented increases in extreme fire weather risk in the 21st century as GHGs continue to rise. Changes to temperature and relative humidity drive the largest shifts in extreme fire weather conditions; this is particularly apparent over the Amazon, where GHGs cause a seven-fold increase by 2080. Our results allow increased understanding of the interacting roles of anthropogenic stressors in altering the regional expression of future wildfire risk.


Supplementary
. Grey areas show less than two-thirds ensemble agreement on whether the RR is greater or less than one. Oceans, glaciers, and bare land are masked in white.  Risk ratio of extreme fire weather under different anthropogenic forcings  Less than two-thirds ensemble agreement 1.0 1.5 0.5 1.1 1.2 1.3 1.4 0.9 0.8 0.7 0.6 Risk Ratio

Supplementary Figure 2
Anthropogenic impact on extreme fire weather. Risk ratio (RR) of extreme fire weather under biomass burning and land use land cover change for 1920-1949, 1950-1979, 1980-2005 and 2006-2029. Grey areas show less than two-thirds ensemble agreement on whether the RR is greater or less than one. Oceans, glaciers, and bare land are masked in white.  1980-20052006-20291950-19791920-1949 Land Use/Land Cover Change Less than two-thirds ensemble agreement

Risk Ratio
Risk ratio of extreme fire weather under different anthropogenic forcings

Supplementary Figure 3
Changes in aerosol optical depth. Effect of industrial aerosol emissions (AER) on aerosol optical depth (AOD) for 1920-1949, 1950-1979, 1980-2005, 2006-2029, 2030-2054, and 2055-2080. The shading shows the difference of AOD between the all and the all-but-aerosol forcing ensemble-means averaged over each period. AOD values in polar regions are masked for plotting purposes.

Difference in Aerosol Optical Depth (unitless)
Difference between all-forcing and all-but-aerosol foricng Aerosol Optical Depth (AOD) 1920-1949 1950-1979 1980-2005 2006-2029 2030-2054 2055-2080 Isolating the effects of meteorological variables on extreme fire weather risk. Effect of greenhouse gas (GHG) forcing on extreme wire weather risk due to changes in maximum temperature, precipitation, relative humidity, and windspeed for 1920-1949, 1950-1979, 1980-2005, 2006-2029 and 2030-2054. The risk ratio (RR) is the probability of exceeding the 95 th percentile of the baseline daily fire weather index (FWI) distribution in the all-forcing (ALL) ensemble divided by the probability of exceeding that same threshold in the all-forcing ensemble after removing the GHG effect on each variable. Grid points masked in grey have less than twothirds ensemble agreement on whether the RR is greater or less than one. Oceans, glaciers, and bare land are masked in white. Wind 1920Wind -1949Wind 1980Wind -2005Wind 2006Wind -2029Wind 1950Wind -1979 0.5 1.0 1.5 1.4 1.3 1.2 1.1 0.9 0.8 0.7 0.6 Less than two-thirds ensemble agreement

2030-2054
Isolating greenhouse gas effects on meteorological variables of extreme fire weather risk Supplementary Figure 5 Anthropogenic effect on maximum daily temperature. Effect of aerosols (AER) and greenhouse gases (GHG) on maximum daily temperature (°C) for 1980-2005, 2006-2029, 2030-2054, and 2055-2080. The shading shows the difference between the all and the all-but-AER (all-but-GHG) ensemble-means averaged over each period.

Aerosol Emissions
Greenhouse Anthropogenic impact on relative humidity. Effect of aerosols (AER) and greenhouse gases (GHG) on relative humidity (percentage points) for 1980-2005, 2006-2029, 2030-2054, and 2055-2080. The shading shows the difference between the all and the all-but-AER (all-but-GHG) ensemble-means averaged over each period. Anthropogenic effects on surface wind speed. Effect of aerosols (AER) and greenhouse gases (GHG) on wind speed (m/s) for 1980-2005, 2006-2029, 2030-2054, and 2055-2080. The shading shows the difference between the all and the all-but-AER (all-but-GHG) ensemble-means averaged over each period. Isolated effects of meteorological variables on extreme fire weather risk. Aerosol forcing impacts on extreme fire weather risk due to changes in maximum temperature, precipitation, relative humidity and wind for 1920-1949, 1950-1979, 1980-2005, 2006-2029, and 2030-2054. The risk ratio (RR) is the probability of exceeding the 95 th percentile of the baseline daily fire weather index (FWI) distribution in the all-forcing ensemble divided by the probability of exceeding that same threshold in the all-forcing ensemble after removing the aerosol effect on each variable. Grid points masked in grey have less than two-thirds ensemble agreement on whether the RR is greater or less than one. Oceans, glaciers, and bare land are masked in white. Wind 1920Wind -1949Wind 1980Wind -2005Wind 2006Wind -2029Wind 1950Wind -1979 0.5 1.0 1.5 1.4 1.3 1.2 1.1 0.9 0.8 0.7 0.6 Less than two-thirds ensemble agreement

2030-2054
Isolating aerosol effects on meteorological variables of extreme fire weather risk

Supplementary Figure 9
Anthropogenic effects on daily precipitation. Effect of aerosols (AER) and greenhouse gases (GHG) on daily precipitation (%) for 1980-2005, 2006-2029, 2030-2054, and 2055-2080. The shading shows the percent difference between the all-forcing and the all-but-AER (all-but-GHG) ensemble-means averaged over each period.