Dr. Daniel Swain: Is there a link between climate change & increasing risk/severity of wildfire in California–including the still-unfolding disaster? Yes. Is climate change the only factor at play? No, of course not.

Dr. Daniel Swain @Weather_West Jan 9, 2025, Thread Reader

Is there a link between #ClimateChange & increasing risk/severity of #wildfire in California–including the still-unfolding disaster? Yes. Is climate change the only factor at play? No, of course not. So what’s really going on? [Thread] #CAfire #CAwx #LAfires

https://iopscience.iop.org/article/10.1088/1748-9326/ab83a7

There is rarely, if ever, a singular cause of complex disasters–so don’t be fooled by disingenuous “straw man” binaries! For wildfire, encroachment into high risk areas, historical land management, & climate all matter (to contextually varying degrees).

The era of megafires: the crisis facing California and what will happen next Three scientists explain the unprecedented danger facing the western US and call for new solutions to a growing threat https://www.theguardian.com/environment/2018/aug/07/california-wildfires-megafires-future-climate-change

What is the primary link between #ClimateChange and wildfire, both in CA specifically and more broadly globally? Vegetation moisture, and therefore its flammability (which includes propensity to ignition, combustion intensity, rates of spread, and etc.).

Observed increases in extreme fire weather driven by atmospheric humidity and temperature – Nature Climate Change Climate change has led to increased fire activity in parts of the globe due to observed increases in fire weather extremes. These trends are driven predominantly by decreasing relative humidity and in… https://www.nature.com/articles/s41558-021-01224-1

With #ClimateChange, we see warming but also widespread increases in atmospheric “thirstiness” (i.e., evaporative demand/vapor pressure deficit). A warmer, thirstier atmosphere means (potentially) drier, more flammable vegetation in absence of compensating precip increases.

This increase in atmospheric “thirst” is actually caused by same underlying thermodynamic process driving increases in precip intensity w/warming. In new paper, coincidentally published today, we introduce analogy of the “Expanding Atmospheric Sponge.”

Hydroclimate volatility on a warming Earth – Nature Reviews Earth & Environment Rapid transitions between extreme wet and extreme dry conditions — ‘hydroclimate whiplash’ — have marked environmental and societal impacts. This Review outlines observed and projected changes in hydr… https://www.nature.com/articles/s43017-024-00624-z

The “Expanding Atmospheric Sponge” Effect conveys consequences of air’s increasing water vapor-holding capacity: Not only can larger sponge yield more water if saturated (& wrung), but its absorptive capacity increases *even if there’s no water to absorb.* nature.com/articles/s4301…

Hydroclimate volatility on a warming Earth – Nature Reviews Earth & Environment Rapid transitions between extreme wet and extreme dry conditions — ‘hydroclimate whiplash’ — have marked environmental and societal impacts. This Review outlines observed and projected changes in hydr… https://www.nature.com/articles/s43017-024-00624-z

Critically, this means the “Expanding Atmospheric Sponge” can increase vegetation aridity, and hence flammability, *even in absence of regional precipitation decreases*. In fact, this can occur (at least episodically) even in places that see mean precipitation *increases*!

What about CA specifically? Previous research finds large increases in summer & autumn conditions favorable for extreme wildfire spread (i.e., a doubling between 1980 and 2020) due to temperature, humidity, & autumn precip trends.

https://iopscience.iop.org/article/10.1088/1748-9326/ab83a7

Now, it’s absolutely true that many distinct ecological & fire regimes exist within California. Fire season is generally much longer in southern CA vs. northern CA (& can sometimes extend into winter). Also, there’s greater proportion of grass & brush fires in SoCal (vs NorCal).

In fact, in coastal southern California, where grass and brush (including chaparral) are predominant vegetation types, there is actually a historical relationship between wetter winters and *increased* fire activity in following fire season. Why is this?

https://esajournals.onlinelibrary.wiley.com/doi/10.1890/07-1183.1

Many ecosystems in arid and semi-arid regions are effectively “vegetation/biomass-limited” when it comes to wildfire–in other words, the abundance of grass and brush varies according to moisture availability year to year. More moisture = more biomass (potential wildfire fuel).

Hence, there is literally more fuel for wildfires following wetter periods with lots of vegetation growth in grasslands/shrubland ecosystems (as opposed to forests, which are usually more “moisture limited” and burn more extensively/intensely during droughts). HOWEVER…

In a warming climate, we are seeing broad global increases in what we term “#HydroclimateWhiplash”–and thus increasingly frequent and/or intense swings between extremely wet and extremely dry conditions. Why does that matter in ths context of wildfire?

Hydroclimate volatility on a warming Earth – Nature Reviews Earth & Environment Rapid transitions between extreme wet and extreme dry conditions — ‘hydroclimate whiplash’ — have marked environmental and societal impacts. This Review outlines observed and projected changes in hydr… https://www.nature.com/articles/s43017-024-00624-z

Well, in grassy and brushy settings, wet antecedent conditions favor increased growth & fire fuel–but *transitions* from unusually wet to dry (& hotter/”thirstier atmosphere”) conditions favor increased growth *and then increased drying* of said extra-abundant vegetation.

And so, in many settings, the “worst climate for wildfire” is not always continuous drying, but is instead an increased cycling between wet (vegetation growth) and dry (vegetation desiccation) periods amid a broad environment with increased evaporative demand (air thirstiness)!

In 2024, Southern California experienced an exceptional episode of wet-to-dry hydroclimate whiplash. Very wet conditions in winters of 2023 and 2024 gave way to a record-dry start to 2025 wet season (with periods of record warmth and evaporative demand in between). #CAwx #CAfire

This exact temporal sequence of events, as observed in southern California in 2024, is something we use as a prototypical example of practical consequences of wet-to-dry “hydroclimate whiplash” events–which we show are increasing due to #ClimateChange.

Hydroclimate volatility on a warming Earth – Nature Reviews Earth & Environment Rapid transitions between extreme wet and extreme dry conditions — ‘hydroclimate whiplash’ — have marked environmental and societal impacts. This Review outlines observed and projected changes in hydr… https://www.nature.com/articles/s43017-024-00624-z

Additionally, in coastal southern California, most of the largest/most destructive wildfires are driven by strong, dry, downslope “Santa Ana” winds. Those mainly occur during Oct-Jan; usually, seasonal rains arrive by Nov or Dec (“short circuiting” fire risk associated w/them.)

This year, the seasonal rains have failed to arrive. It’s bone dry–with a record-dry start to wet season in some cases–throughout SoCal. Is there a climate change connection here too? #CAwx #CAfire

Well, climate change is not expected to increase the frequency or intensity of Santa Ana winds. It may even decrease them, eventually, though that would mainly occur in Jan-Feb at a high level of warming and be less relevant to fire risk (vs Oct-Dec).

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL080261

However, that’s not primarily what matters here! The main issue is the overlap between “critically dry vegetation season” and “strong dry wind season.” Due to decreases in autumn precipitation, increases in overall evaporative demand, that overlap is expected to increase.

The season for large fires in Southern California is projected to lengthen in a changing climate – Communications Earth & Environment More frequent large wildfires and a lengthening of the fire season in southern California are projected under future emission and climate scenarios, according to machine learning and climate model pro… https://www.nature.com/articles/s43247-022-00344-6

In fact: In a 2021 piece, I discussed how there is already evidence that this “dry & windy” overlap has already increased from autumn and (episodically) into early winter–especially across Southern California. #CAwx #CAfire

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021gl092843

TL;DR version: #ClimateChange, while certainly not only relevant factor, has indeed increased risk of extreme CA #wildfires. The main reason is increasing temperatures & atmospheric “thirstiness,” but increased precipitation “whiplash”/seasonal shifts is also an emerging factor.

Ryan Katz-Rosene, PhD@ryankatzrosene:

Last year I spent 10 months working on a 160 page massively-overpriced academic book which will be seen by like 200 people after it’s published this spring

Last night I spent 10 seconds writing a flippant 160 character tweet which has been seen by 2.6 million people in <20hrs

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