This autumn has been very dry in California and this has undoubtedly increased the chance of occurrence of the deadly wildfires that the state is seeing.
When assessing the influence of global warming (from human burning of fossil fuels) on these fires, it is relevant to look at climate model projections of extremely dry autumn conditions in California. Below is an animation that uses climate models to calculate the odds that any given November in California will be extremely dry.
Here, extremely dry is defined as a California statewide November that is characterized by soil moisture content three standard deviations below the mean, where the mean and standard deviation is defined over the period 1860-1900.
We can see that these extremely dry Novembers in California go from being exceptionally rare early in the period (by definition), to being more likely now (~1% chance), and much more likely by the end of the century (~7% chance).
In terms of an odds ratio, this would indicate that “extremely dry” conditions are approximately 7 times more likely now than they were at the end of the 19th century and that these “extremely dry” conditions would be approximately 50 times more likely at the end of the century under an RCP8.5 scenario.
*chance is calculated by looking at the frequency of California Novembers below the 3 standard deviation threshold across all CMIP5 ensemble members (70) and using a moving window of 40 years.