Southern California is experiencing its most devastating winter fires in more than four decades.

Fires don’t usually blaze at this time of year, but specific ingredients have come together to defy the calendar in a fast and deadly manner.

Behind many of them lies human-induced climate change.

Scientists have calculated that global warming has contributed to a 172 per cent increase in areas torched by wildfires in California since the 1970s, with a further spread expected in the coming decades.

What is causing winter wildfires in California?

A cocktail of extreme weather events has fanned California’s fires.

First are the supersized Santa Ana winds whipping flames and embers at 100 mph (161 kmh) – much faster than normal – crossed with the return of extreme drought.

Added to that is weather whiplash that grew tons of plants in downpours and then the record-high temperatures that dried them out to make easy-to-burn fuel.

Then there’s a plunging and unusual jet stream, and lots of power lines flapping in the powerful gusts.

Experts say that this perilous combination is what is turning wildfires into a deadly urban conflagration.

‘The big culprit is a warming climate’

“Tiny, mighty and fast” fires have blazed through America’s west in the last couple of decades as the world warms, said University of Colorado fire scientist Jennifer Balch.

She published a study in the journal Science last October that looked at 60,000 fires since 2001 and found that the fastest-growing ones have more than doubled in frequency since 2001 and caused far more destruction than slower, larger blazes.

“Fires have gotten faster,” Balch said on Wednesday. “The big culprit we’re suspecting is a warming climate that’s making it easier to burn fuels when conditions are just right.”

Summer fires are usually bigger, but they don’t burn nearly as fast. Winter fires “are much more destructive because they happen much more quickly”, said US Geological Survey fire scientist Jon Keeley.

AccuWeather estimated damage from the latest fires could reach $57 billion (€55 billion), with the private firm’s chief meteorologist, Jonathan Porter, saying “it may become the worst wildfire in modern California history based on the number of structures burned and economic loss”.

Winds brought by jet stream have fanned the flames

“It’s really just the perfect alignment of everything in the atmosphere to give you this pattern and strong wind,” said Tim Brown, director of the Western Regional Climate Center.

Wind speed and the speed of spreading flamesare clearly linked, experts emphasise.

“The impact increases exponentially as wind speed increases,” said fire scientist Mike Flannigan of Thompson Rivers University in Canada.

If firefighters can get to the flames within 10 minutes or so, its spread can be contained, but “15 minutes, it’s too late and it’s gone. The horse has left the barn”.

There’s no sure link between Santa Ana winds – gusts from the east that come down the mountains, gain speed and hit the coast – to human-caused climate change, said Daniel Swain, climate scientist for the California Institute for Water Resources.

But a condition that led to those winds is a big plunge in the temperature of the jet stream – the river of air that moves weather systems across the globe – which helped bring cold air to the eastern two-thirds of the nation, said University of California Merced climate and fire scientist John Abatzoglou.

Other scientists have preliminarily linked those jet stream plunges to climate change.

Santa Ana winds are happening later and later in the year, moving more from the drier fall to the wetter winter, Keeley said. Normally, that would reduce fire threats, but this isn’t a normal time.

‘Clear link between climate change and dry winters’

After two soaking winters, when atmospheric rivers dumped huge amounts of water on the region causing lots of plants to grow, a fast onset of drought dried them out, providing perfect tinder, according to Swain and Abatzoglou.

Swain said this weather whiplash is happening more often.

There is a clear link between climate change and the more frequent dry falls and winters that provide fuel for fires, Swain said.

These devastating fires couldn’t happen without the dry and hot conditions, nor would they be blazing without the extreme wind speed, according to Abatzoglou and others.

California’s average temperature has risen by around 1C since 1980 causing the number of days with fire-vulnerable dry vegetation to double, fire management expert Lindon Pronto at the European Forest Institute told Irish news site RTÉ News.

‘Now we talk about fire years’

An analysis of 423 California wildfires that have grown to at least 15 square miles (39 square kilometres) since 1984 shows only four of those burned during the winter. About two-thirds of those larger fires sparked in June, July or August.

Federal data shows just six wildfires have burned more than 2 square miles (5 square kilometres) in any January in California since 1984.

Until the Palisades and Eaton fires this year, the largest had been the Viejas Fire, which burned 17.1 square miles (44.3 square kilometres) in 2001 in the mountains east of San Diego.

“Winter wildfires should be an oxymoron,” University of Colorado’s Balch said. “Well, because, you know, temperatures drop and we get precipitation. We’re supposed to get precipitation.”

Fire officials used to talk about fire seasons, said David Acuña, a battalion chief for Cal Fire: “Now we talk about fire years”.