California Wildfires and Climate Change
It’s challenging to make an empirical connection between climate change and wildfires because the climate signal is mixed with other factors like building practices and fire suppression strategies. But intuitively, it just makes sense that climate change will make fire-prone areas hotter and drier, leading to more frequent, larger fires.
I found Williams et al. (2019) a helpful starting point for thinking about the connection between California wildfires and climate change. They focus on the historical impact of climate change on fires, but give a nice overview of the different types of California fires and their drivers:
Summer forest fires (mostly in Northern California) are driven by temperature and humidity, so it’s relatively easy to detect a climate signal. Williams et al conclude: “The large increase in California's annual forest-fire area over the past several decades is very likely linked to anthropogenic warming.”
Summer non-forest fires depend on interannual swings in precipitation. A particularly wet year drives a lot of vegetation growth (fuel), which burns if the following year is particularly dry. Changes in these interannual precipitation swings are a more subtle climate response than temperature and humidity, so it’s harder to detect a climate signal.
Fall fires are driven by strong offshore winds (the Santa Anas) and dry fuels. They’re also sensitive to the onset of winter precipitation: the earlier the winter precipitation, the less chance of bad wildfires.
So fall fires, like the ones we’re experiencing now, are harder to tie to climate change. Guzman-Morales and Gershunov (2019) find reductions in the frequency and (to a lesser extent) intensity of Santa Ana winds in future warming simulations, though the smallest reductions are in Nov-Dec-Jan (for an earlier study see also Hughes et al. (2011)). This response makes sense: Santa Anas are driven by the pressure gradient between a high pressure system over a cold land surface and a low pressure system over a relatively warm ocean. We expect land surfaces to warm more than ocean surface temperatures under climate change, so this pressure gradient should weaken (though things might be more subtle at the scale of a Sant Ana, explaining the relatively weak signal).
In terms of precipitation, Swain et al. (2018) find increasing volatility in California precipitation, potentially with more wet-to-dry interannual swings. They also see evidence of a later winter precipitation onset (their Figure 5, see also Mahoney et al. (2021)). These results are generally consistent with California precipitation becoming less frequent but more intense under climate change.
So the evidence suggests a slight decrease in Santa Anas is offset by less/later precipitation and generally warmer and drier conditions – probably more fall fires. But a caveat to any model projection of wildfire changes is that Simpson et al (2023) found that observed vapor pressure trends in the southwest are weakly negative, whereas models would have predicted positive trends. This suggests models might be underestimating future drying and underestimating future wildfire risk – another example of the importance of resolving model-observation discrepancies: if models can’t simulate important historical signals, it puts their future projections into doubt [My guess is this discrepancy is related to the models’ issue simulating cooling trends in the Eastern Pacific – La Ninas tend to drive drier conditions in the southwest.]
