Climate change increases risk of simultaneous wildfires

Helmholtz Centre for Environmental Research – UFZ (WeatherFarm) – An international scientific team examined how the timing of fire weather seasons is changing as a result of climate change. The west coast of North America and the east coast of Australia have been repeatedly hit hard by wildfires.

Around 75 per cent of the overlap occurred between July and December. Overall, the number of simultaneous fire weather days in eastern Australia and western North America has been increasing by one day per year for the past 40 years.

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, UFZ climate scientist.

“The number of overlapping fire weather days in western North America and eastern Australia will continue to increase,” said Ribeiro.

Depending on the climate model used and the level of future global warming, the increases could range from four to 29 days per year by mid-century.

The overlap depends partly on the variability of the El Niño Southern Oscillation. Fire weather in eastern Australia is typically linked to El Niño conditions – unusually high sea surface temperatures. This often leads to droughts and heat waves, among other phenomena.

In contrast, fire weather in western North America is more often linked to La Niña conditions – the opposite situation.

“Despite these generally opposing patterns, we found that during strong fire weather overlap, El Niño conditions are especially pronounced in the Central Pacific,” said Ribeiro.

However, the effects of El Niño are expected to be overshadowed by climate change in the medium term.

“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, UFZ climate scientist.

This poses a challenge for co-operation between U.S., Canadian, and Australian fire services, which have long supported one another by sharing personnel and aircraft during wildfire emergencies. Until recently, the fire seasons occurred at distinctly different times of the year: on the west coast of North America between June and October and in eastern Australia between October and March.

These gaps provided enough time for teams to assist one another when needed.

“But these increasingly overlapping fire weather seasons in the U.S. and Australia are narrowing the window for international co-operation and making it harder to respond quickly to large-scale wildfires,” said Doug Richardson of the University of New South Wales.

International agreements on firefighting co-operation and national firefighting capacities therefore need to be reviewed. Both Australia and the U.S. will need to strengthen their domestic firefighting capacities in order to reduce reliance on international co-operation.

The researchers used the Canadian Fire Weather Index (FWI), a meteorological index used worldwide to estimate fire risk. It considers rainfall, temperature, relative humidity, and wind speed. They used it to identify fire weather days – when there’s a high risk of wildfires. The researchers found that the fire weather days and thus the risk of wildfires have been increasingly overlapping since 1979.