Along with the fairly recent popularization of terms such as atmospheric river and bomb cyclone, we increasingly hear the term “vapor pressure deficit” (VPD). At any given temperature, the term refers to how much moisture is in the air compared to how much moisture could be in the air. The higher the deficit, the dryer the air.

A recent article in Cal Matters describes how a high vapor pressure deficit in Los Angeles caused winds to have greater drying power, removing moisture from living plants, making fires “almost unstoppable once they start.”

The article goes on to cite a UCLA paper from 2021 that claims that “compared to the 1980s and 1990s, the number of days with an extreme vapor pressure deficit nearly doubled in the first two decades of this century.”

For everyone who wants fires to stop immolating cities, whether they believe the cause is primarily climate change or land mismanagement, adaptation is an urgent priority. The debate is over how to adapt. Everyone may also agree that for the foreseeable future, prioritizing EV mandates and subsidizing solar farms will have zero impact on wildfires. But what if decreasing humidity and higher temperatures in Southern California are not mostly the product of increasing CO2 content in the atmosphere, but something else entirely? How would that affect how we should adapt?