Understanding how exposure to extreme heat changes over time is vital to helping humanity minimize heat deaths. Here, we divide past changes in heat exposure into components from overall population increases, population movement, overall climate warming, and local climate warming. Globally, heat wave exposure has nearly doubled in the past 30 years, with overall population growth contributing half of this growth. Population movement and global climate changes, meanwhile, have each contributed a roughly 25% increase in exposure, while local climate change has decreased exposure because the largest increases in heat waves have occurred in less populated areas. In the world’s three most populous countries the relevant factors differ, with total population growth being dominant in India, climate changes being dominant in China, and both being important in the United States. These results indicate that each country must understand the drivers of their exposure to climate to effectively minimize damages.