Overlapping US-Australia fire seasons reduce the window of opportunity for firefighting cooperation

Wildfires are a growing global challenge. In addition to becoming more widespread and intense due to climate change, the fire seasons in many regions are becoming longer. The lengthening of fire seasons reduces the window of opportunity for preparedness (e.g. prescribed burning of dry fuels before fire season onset) and increases the likelihood of spatially compounding fire risks due to overlapping fire weather seasons. These increased risks demand efficient global cooperation in sharing firefighting resources (e.g. helicopters, planes, firefighters), and of major concern, is how well-established international arrangements may be compromised or disrupted in the near future. Here we investigate increasing fire season lengths across two distanced fire-prone regions with typically distinct fire seasons and a long-term collaboration in sharing firefighting resources, Eastern Australia (EAU) and Western North America (WNA). We aim to test the hypothesis that spatially compounding fire weather events occur due to overlapping fire weather seasons, based on the Canadian Fire Weather Index (FWI). To robustly characterize the potential overlap, we make use of CMIP6 single model initial-condition large ensembles (SMILEs) for historical and future periods, and the ERA5 reanalysis. We define Fire Weather Days (FWD) as when the FWI exceeds a climatological threshold specific to each region, and we then estimate the total number of overlapping FWD per year for different time periods. We show that these distanced regions are becoming more likely to experience periods of overlapping FWD, which can compromise the human response in terms of firefighting. Most of the overlap occurs during boreal Autumn months, coinciding with the end of the fire season in WNA and the beginning of the fire season in EAU. Correlations between the number of overlapping FWD and the length of the regional fire season suggest that the main driver of the overlapping is the increasing early start of the fire season in EAU, rather than the late offset of the fire season in WNA. Additionally, we find that overlapping FWD is expected to increase in the future in a warming climate. As fire seasons overlap, the existing international collaborations will be increasingly constrained, and the window of opportunity for firefighting will shorten.