The climate science behind the California wildfires

By Liz Stephens and Andrew Kruczkiewicz, Climate Centre science team

The immediate causes of actual fires are often associated with human error: a cigarette end tossed out of a car, broken glass magnifying the sun on a hot day, a mishandled barbecue, power lines malfunctioning, vandalism and arson. None of these are attributable to climate change; they can be very difficult to identify.

Fire weather is a different matter.

The US government last year said climate change “has been a key driver in increasing the risk and extent of wildfires in the western United States during the last two decades” through “strong direct or indirect ties” to temperature, humidity, and dried-out vegetation, if not wind.

A deep dive into the science brings up the concept of “vapour pressure deficit” (VPD) – or in plain English a measure of how thirsty the atmosphere is and how much moisture it will suck up from vegetation.

Whereas relative humidity as a percentage, at a given temperature, measures the moisture the air is holding as a proportion of what it could hold, VPD is the other side of that coin: a read of the difference between the amount of moisture in the air and what it could hold when saturated.

The principal study cited in 2021 by NOAA and the US National Integrated Drought Information System says the western US “has experienced a rapid increase of fire weather [or VPD] in recent decades, especially in the warm season”; actual observations suggest two thirds of this VPD trend is explained by human-induced warming, rising to some 90 per cent indicated for a modelled future.

A characteristically strong Santa Ana might occur in an unusually dry December and January

The 2020 “August Complex” wildfire – at that point the largest in California history that burned more than a million acres – was accompanied by “[u]nprecedentedly high VPD”, half of which was attributable to climate change, the US agencies added. 

The study showed the “western United States has passed a critical threshold since about 2000, and human-caused climate change is now the dominant contributor to the increase of wildfire risk [and] the increase of VPD since 2000 is dominated by aridification due to climate change.”

VPD will continue increasing, leading to more intense and more frequent wildfires and aridification in the western states overall, they concluded.

But if these factors might be termed the back end of wildfires, the front end is concretely measurable. Another agency, the United States Environmental Protection Agency said last June: “Multiple studies have found that climate change has already led to an increase in wildfire season length, wildfire frequency, and burned area.”

The spread of fire, and the damage that it causes, is also influenced by forest and water management, city planning and building codes, but the scariest factor in the scale of the Los Angeles wildfires has been the intense Santa Ana wind.

The gusty Santa Anas “trigger allergies, fray nerves and alarm fire-prone communities”, as the broadcaster PBS put it in 2012 (The Devil Wind: A Brief History of the Santa Anas). The name is believed to derive simply from the Santa Ana Canyon through which the wind blows, generally peaking in December.

A 2019 study supported by the NOAA found global warming may actually weaken the high-pressure systems generating Santa Anas, although less so, if at all, during the winter months. But scientists have also said California rains would most likely come later than the familiar October–April season, so a characteristically strong Santa Ana might occur in an unusually dry December and January, leading to serious wildfires.

That is precisely what happened this year.

Southern California’s double misfortune has been that in 2022 and 2023, Los Angeles got near-record amounts of rain, encouraging the growth of shrubs, trees and grasses, but then the region flipped back to aridity, making this abundant new vegetation highly combustible.

When the warm, dry Santa Ana blew in anger in the first days of this month, the fire weather combined with what was, in effect, a fuel dump, and the landscape needed only ignition to be set ablaze. A “dangerous synergy” indeed.

‘I moved my family away because I feared that our neighbourhood would burn’

Disentangling the various natural and human factors behind the cause and scale of these disastrous fires will be the focus of future studies – made all the more urgent by their scale, with the cost approaching 300 billion US dollars, according to one authoritative estimate widely quoted in the media this week.

And as firefighters begin to get the upper hand, a new danger looms at the foot of the burned slopes: landslides or “debris flows”. Experts now studying the edges of the Eaton and Palisades fires are expected shortly to issue hazard maps to help residents facing the threat of floods and landslides that will loom for a considerable time to come if and when it rains heavily, media reports said yesterday.

But meanwhile, like the hundreds of local American Red Cross volunteers who have themselves been affected by the latest California wildfire disaster, climate scientists who live (or lived) in the area have personal stories to tell.

One such scientist, Peter Kalmus, who studies heat impacts on human health and ecosystems, put it like this in the New York Times: “I moved my family away two years ago because, as California’s climate kept growing drier, hotter and more fiery, I feared that our neighbourhood would burn. But even I didn’t think fires of this scale and severity would raze it and other large areas of the city this soon.”

The bad news is that in its most recent assessment of the global climate, the IPCC said human-induced climate change has indeed caused an increase in the hot and dry conditions that drive wildfires in the west of the North American land mass (Box 14.2), adding with “high confidence” that the trend is set to continue in the coming decades.

The operations hub of the American Red Cross Los Angeles region, where Red Crossers have been “working nearly 24/7 to support the thousands of people displaced from their homes. Our priority is the people we help, and our responders make that possible,” the LA region said on X/Twitter yesterday. Thousands of people have now given blood, donated money or contributed their time to help ensure no one would experience the crisis alone, while nearly 700 evacuees woke up in Red Cross shelters Thursday, the National Society said.(Photo: @RedCrossLA via social media)