Extreme heat is responsible for more mortalities in the United States than any other weather-related event. Cities are more vulnerable to heat extremes than rural areas because of the preexisting effects of urban heat island. In addition, cities have the highest concentration of people, industry, and infrastructure. Therefore, the implications of extreme heat events for human health and energy consumption are magnified in urban areas. Anticipating changes in exposure to future heat extremes and the potential of mitigation strategies in rolling back their effects is critical for urban resilience and sustainability.

Here we use a suite of high-resolution (1.5 km) regional climate simulations driven by global climate models and enhanced by satellite-based information and an urban canopy model (UCM) to project the future of heat extremes in urban regions of California for the mid twenty-first century. We use a spatially explicit population projection to assess the changes in the population exposure to the heat extremes. We find that population exposure to extreme heat days in California increases by three- to fivefold across major urban counties and that climate change, population growth, and their compounding effects all play significant roles in driving this outcome. We further show that broad implementation of cool roofs, an urban heat mitigation strategy, can meaningfully offset a significant percentage of future increases in frequency, intensity, and duration of extreme heat events in urban areas and growing population exposure to these conditions.