Warming weakens the night-time barrier to global fire

Night-time provides a critical window for slowing or extinguishing fires owing to the lower temperature and the lower vapour pressure deficit (VPD). However, fire danger is most often assessed based on daytime conditions(1,2), capturing what promotes fire spread rather than what impedes fire. Although it is well appreciated that changing daytime weather conditions are exacerbating fire, potential changes in night-time conditions-and their associated role as fire reducers-are less understood. Here we show that night-time fire intensity has increased, which is linked to hotter and drier nights. Our findings are based on global satellite observations of daytime and night-time fire detections and corresponding hourly climate data, from which we determine landcover-specific thresholds of VPD (VPDt), below which fire detections are very rare (less than 95 per cent modelled chance). Globally, daily minimum VPD increased by 25 per cent from 1979 to 2020. Across burnable lands, the annual number of flammable night-time hours-when VPD exceeds VPDt -increased by 110 hours, allowing five additional nights when flammability never ceases. Across nearly one-fifth of burnable lands, flammable nights increased by at least one week across this period. Globally, night fires have become 7.2 per cent more intense from 2003 to 2020, measured via a satellite record. These results reinforce the lack of night-time relief that wildfire suppression teams have experienced in recent years. We expect that continued night-time warming owing to anthropogenic climate change will promote more intense, longer-lasting and larger fires.

Automated summary

This study reveals that the intensity of night-time fires has increased due to hotter and drier nights. The global daily minimum vapour pressure deficit (VPD) has increased by 25% from 1979 to 2020. The annual number of flammable night-time hours on burnable lands has increased by 110 hours, allowing for five additional nights of continuous flammability. Across nearly one-fifth of burnable lands, flammable nights have increased by at least one week. Night fires globally have become 7.2% more intense from 2003 to 2020, as measured by satellite records. These findings highlight the lack of relief during the night for wildfire suppression teams. It is expected that continued night-time warming, driven by anthropogenic climate change, will lead to more intense, longer-lasting, and larger fires.