Climate-induced variations in global wildfire danger from 1979 to 2013

Abstract

Climate strongly influences global wildfire activity, and recent wildfire surges may signal fire weather-induced pyrogeographic shifts. Here we use three daily global climate data sets and three fire danger indices to develop a simple annual metric of fire weather season length, and map spatio-temporal trends from 1979 to 2013. We show that fire weather seasons have lengthened across 29.6 million km(2) (25.3%) of the Earth's vegetated surface, resulting in an 18.7% increase in global mean fire weather season length. We also show a doubling (108.1% increase) of global burnable area affected by long fire weather seasons (>1.0 σ above the historical mean) and an increased global frequency of long fire weather seasons across 62.4 million km(2) (53.4%) during the second half of the study period. If these fire weather changes are coupled with ignition sources and available fuel, they could markedly impact global ecosystems, societies, economies and climate.

Figure 1. Long-term trends and changes in anomalous event frequency of maximum temperature, minimum relative humidity, annual rain-free days and maximum wind speed.

a,c,e,and,g show areas with significant trends in annual fire weather variables. b,d,f and h show the change in frequency of the number of years with anomalous mean annual weather conditions (>1σ above historical mean) from 1996 to 2013 compared with the number of anomalies observed from 1979 to 1996. Areas with little or no burnable vegetation are shown in grey (NB) and NC indicates areas with no significant change. Red areas indicate locations where fire weather conditions are becoming increasingly more severe or anomalously severe weather events are becoming more frequent, while blue areas indicate locations where climatic influences on fire potential are lessening or weather events are becoming less frequent.

Figure 2. Trends in global fire weather season length metrics and rain-free days.

(a) Changes in the global mean fire weather season length from 1979 to 2013 with 95% confidence limits between ensemble members (shaded area). (b) Total global annual area affected by long fire weather seasons (>1σ of historical mean). (c) Inter-annual variations in the standard anomalies of global mean rain-free days. Global mean rain-free days accounted for 49.7% of the variation in global fire weather season length and 33.8% of the variation in global long fire weather season affected area.

Figure 3. Global patterns of fire weather season length changes from 1979 to 2013.

a shows areas with significant trends in fire weather season length from 1979 to 2013. b shows regions that have experienced changes in the frequency of long fire weather seasons (>1σ above historical mean) during the second half of the study period (1996–2013) compared with the number of events observed during the first half (1979–1996). Areas with little or no burnable vegetation are shown in grey (NB) and NC indicates areas with no significant change. Reds indicate areas where fire weather seasons have lengthened or long fire weather seasons have become more frequent. Blues indicate areas where fire weather seasons have shortened or long fire weather seasons have become less frequent.

Figure 4. Examples of fire weather season length standardized anomalies during significant global fire events.

Red colours indicate areas where fire weather season anomalies are >1 s.d. from the mean, while blue areas indicate shorter-than-normal fire weather season lengths. Areas with little or no burnable vegetation are shown in grey (NB). Red circles denote regions with significant fire activity during that time period. El Niño periods often span multiple calendar years (for example, 1982–1983), so in these cases, the maximum anomaly of either year was mapped above. Maps of all years (1979–2013) are included as Supplementary Figs 1–4.