Journal Article

Increasing Fire Weather Season Overlap Between North America and Australia Challenges Firefighting Cooperation

Quilcaille, Yann, Zscheischler, Jakob, Taschetto, Andrea S., Richardson, Doug, Batibeniz, Fulden, Ribeiro, Andreia F. S., Pitman, Andrew J.

2025

Overview

The USA, Canada and Australia are members of an international partnership that shares firefighting resources, including equipment and personnel. This partnership is effective because fire risk between Australia and North America is historically asynchronous. However, climate change is causing longer fire seasons in both regions, increasing the likelihood of simultaneous fire risk and threatening the partnership's viability. We focus on spatially compounding fire weather as the annual number of days on which the fire seasons in Australia and North America overlap, investigating historical and future projections of fire weather season lengths. We use the Canadian Fire Weather Index and compute season length statistics using ERA5 reanalysis data together with historical and future projections from four CMIP6 single model initial‐condition large ensembles. Our analysis shows that the length of fire weather season overlap between eastern Australia and western North America has increased by approximately one day per year since 1979. The interannual variability of overlap is driven primarily by the variability in Australia, with correlations between that region's fire weather season length and the degree of overlap exceeding 0.9. Composites of ERA5 and CMIP6 sea surface temperatures suggest a link between the interannual variability of overlap and the El Niño‐Southern Oscillation, despite this climate mode's opposing relationship with fire weather in the two regions. Finally, we find that the overlap is projected to increase by ∼${\\sim} $ 4 to ∼${\\sim} $ 29 days annually by 2050. We conclude that an increasing overlap of fire seasons is expected to constrain current resource‐sharing agreements and shorten preparedness windows. Plain Language Summary The USA, Canada and Australia share firefighting resources including aircraft, and, in times of emergency, personnel. This arrangement is possible because the fire seasons in Australia and North America occur at opposite times of the year. Due to climate change, however, the fire seasons in both regions are getting longer. This means there is now a greater overlap of fire seasons, which could place greater pressure on the sharing of resources. We find that this degree of overlap has increased in recent decades. We use four climate models to project how much the overlap could change during this century. All models agree that there will be an increase, but they vary in the degree of the increase, ranging from 4 to 29 days per year by 2050. Our work highlights that current partnership arrangements could be placed under increasing pressure due to an increasing overlap of the fire seasons in North America and Australia caused by climate change. Key Points A warmer climate increases overlapping fire risk between North America and Australia, potentially compromising firefighting cooperation Variability of Australian fire weather dominates the risk of fire season overlap in boreal autumn Reanalysis data and climate model large ensembles suggest ENSO, particularly a central Pacific El Niño, is linked to overlap variability

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Australia / Climate and weather / Climate change / Collaboration / Cooperation / El Nino / Fire fighting / Fire weather / fire weather season / firefighting / Forest & brush fires / Interannual variability / North America / overlap / Partnerships / Prescribed fire / Regions / Sea surface temperature / Seasonal variations / Seasons / Southern Oscillation / spatially compounding event / Statistical analysis / Surface temperature / Trends / Weather / Weather index