California now has more than 100 million dead trees, mostly in the central and southern Sierra. This has implications for water supply, wildfire management, local economies, and many other issues. The die-off is a symptom of unsustainable forest conditions. Drought is part of the California landscape, but why are we seeing such profound mortality? The drought caused more trees to compete for less water and increased infestations of bark beetles, which kill trees. Droughts are getting warmer, which stresses the trees more. But the underlying cause is unsustainably dense forests. Forest management in the past century increased forest density by removing the most common ecosystem process that once thinned the forest: fire. We need to reduce tree density in Sierra forests so they are more resilient to drought.