Wildfire seasons are now overlapping worldwide
Fires have long been a seasonal scourge on the west coast of North America and the densely forested east coast of Australia.
In January 2025, flames tore through Los Angeles, destroying more than 10,000 buildings and claiming 29 lives. Just four years earlier, Australia’s 2019–2020 “Black Summer” burned over 12 million hectares – an area larger than England.
For decades, emergency crews have relied on the fact that the Northern and Southern Hemispheres reach peak fire danger at different times, allowing them to share personnel, aircraft and expertise.
New research, however, shows that this breathing space is rapidly disappearing as wildfire seasons are increasingly overlapping.
Decades of fire data
To determine how the window for mutual aid is changing, an international team of climatologists analysed daily values of the Canadian Fire Weather Index (FWI) for western North America and eastern Australia between 1979 and 2022.
The FWI combines temperature, rainfall, humidity and wind to flag “fire-weather days,” when conditions are ripe for ignition and spread.
Over the past forty years, simultaneous fire-weather days in the two regions have increased by roughly one per year. The overlap is now most pronounced from July through December, when about three-quarters of days register high wildfire risk in both places.
“This is because the fire season in eastern Australia is starting earlier in spring and overlapping with the end of the fire season on the west coast of North America,” explained Andreia Ribeiro, a climate scientist at Germany’s Helmholtz Center for Environmental Research (UFZ) and the study’s lead author.
Projections point to steeper rise
Using four state-of-the-art climate models and hundreds of ensemble simulations to capture uncertainty, the researchers projected trends to mid-century under different greenhouse-gas scenarios. Every simulation showed further convergence.
“The number of overlapping fire weather days in western North America and eastern Australia will continue to increase,” Ribeiro warned.
Depending on future emissions, the extra days could range from just four each year to nearly thirty by the 2050s – effectively adding an entire month of simultaneous high danger.
Diminishing influence of El Niño
Fire danger in the two regions has historically been tied to opposite phases of the El Niño–Southern Oscillation (ENSO). Eastern Australia usually endures severe heat and dryness during El Niño, whereas La Niña increases fire potential in coastal California, Oregon and British Columbia by priming vegetation in spring and desiccating it by late summer.
Yet the new study found an unexpected signal: “Despite these generally opposing patterns, we found that during strong fire weather overlap, El Niño conditions are especially pronounced in the Central Pacific,” Ribeiro said.
That influence, while still detectable, is likely to be overshadowed by relentless anthropogenic warming.
“Climate change is causing global temperature rises and increasing drought in some regions while the El Niño effect is expected to remain largely unchanged,” said Jakob Zscheischler, a UFZ co-author.
Increasingly overlapping wildfire seasons
Until recently, crews could count on a four- to five-month lull between California’s June–October fire season and Australia’s October–March window. Those months allowed exhausted firefighters and scarce aerial tankers to rotate across the Pacific.
“But these increasingly overlapping fire weather seasons in the US and Australia are narrowing the window for international cooperation and making it harder to respond quickly to large-scale wildfires,” said lead author Doug Richardson, a scientist at the ARC Center of Excellence for Climate Extremes at the University of New South Wales.
Existing bilateral agreements – some dating back three decades – assume non-overlapping seasons.
The study argues that both countries, along with Canada, must revisit those pacts, expand domestic training pipelines, purchase additional large air tankers that can remain on call year-round, and invest in predictive technologies that can pre-position resources when simultaneous crises loom.
Preparing for the future
Converging fire seasons ripple beyond emergency services. Insurance markets already reeling from record-breaking claims may face concurrent hits, driving premiums higher.
Public-health agencies must prepare for overlapping smoke waves that can blanket cities like Sydney, Vancouver, and San Francisco at the same time.
Wildlife managers worry that species relying on alternate recovery periods in the two hemispheres – migratory birds, for instance – could lose habitat simultaneously. Carbon-budget analysts note that overlapping megafires release pulses of greenhouse gases that undercut national climate targets in a single season.
The team urges governments to accelerate prescribed burning and forest-thinning programs during the shrinking shoulder seasons, double down on community fire-proofing, and expand early-warning networks that incorporate the latest FWI projections.
Internationally, they call for a global reserve of firefighting aircraft co-funded by nations facing rising wildfire risk, allowing rapid deployment even when regional fleets are committed.
Extreme fire weather is on the rise
The new findings illustrate how climate change can synchronize hazards that once struck at different times and places, overwhelming systems designed for isolated events.
Without deep emissions cuts, the number of days when both hemispheres simultaneously face extreme fire weather is set to climb steeply, leaving little margin for relief operations that span the Pacific.
The scientists behind the study hope their work will spur policymakers to adapt cooperation agreements – and confront the warming trend that is fanning the flames on two continents.
The study is published in the journal Earth’s Future.
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