Wildfires are a natural part of many ecosystems, but in recent decades, they have become more extreme in the western United States. One concern is the rise in high-severity fires, which can kill most or all trees in a burned area and make forest recovery harder. Scientists know that hotter and drier conditions lead to more forest fire area overall, but it’s less clear how these conditions affect fire severity—in other words, how damaging the fires are to ecosystems. In this study, we created maps of fire severity across all wildfires in California’s forests from 1985 to 2023. We analyzed over 14,000 fires to understand what influences the severity of fire. We found that, with respect to annual forest fire area, the proportion of high-severity fire has increased significantly, especially in forested regions like the Sierra Nevada. These increases are linked to hotter, drier conditions, including higher vapor pressure deficit, a measure of atmospheric dryness. Our findings show that climate change is not only increasing the amount of land burned but also changing the type of fire that occurs, with important consequences for forests and their ability to recover after fire.