Thread by @Weather_West

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 iopscience.iop.org/article/10.108…

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). theguardian.com/environment/20…

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.). nature.com/articles/s4155…

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." nature.com/articles/s4301…

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…

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. iopscience.iop.org/article/10.108…

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?
esajournals.onlinelibrary.wiley.com/doi/10.1890/07…

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? nature.com/articles/s4301…

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. nature.com/articles/s4301…

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). agupubs.onlinelibrary.wiley.com/doi/full/10.10…

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. nature.com/articles/s4324…

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 agupubs.onlinelibrary.wiley.com/doi/full/10.10…

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.